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Phosphides and Simple Compounds
Chapter
2
PHOSPHIDES AND SIMPLE COMPOUNDS
2.1
THE
ELEMENT
About 60 years ago three major allotropic modifications of elemental phosphorus had been recognised : white (or yellow when impure ) , red and black. These allotropes, which can be interconverted, represent successively more dense and chemically less reactive forms. In addition, gaseous and liquid states, and many other solid forms of the element are now known, although many of these can be regarded as sub-varieties of the three main allotropes. White phosphorus corresponds to the original pyrophoric form of the element prepared by Bfiand in 1669. The red allotrope was obtained in 1847 by Von Sdh&ofctQJi, and the black variety by
Bsiidgman in 1914.
White Phosphorus White phosphorus, the longest known form, is the most volatile and reactive form of the solid, and is closely related to the liquid and vapour phases of the element into which it readily transforms. White phosphorus is soft and waxy and readily soluble in many org anic solvents such as carbon disulphide and benzene. Solubilities (25°C) per 100g solvent are ; 1.27g in CC1 4 , 1.39g in Et 2 0 , 3.7g in C 6 H 6 , 0.30g in acetone and > 1000g in CS 2 . Its molecular weight in solvents corresponds to P 4 , as it is in the liquid and vapour states (see below). The density is 1.83g/cc at 20°C, mp = 44.1°C, bp = 280°C and vapour pressure = 0.173 mm The P^molecule forms a tetrahedron (see below). White phosphorus was first obtained industrially by carbon reduction of phosphoric acid, the latter having been produced by the action of sulphuric acid on bones (Chapter 3 ) . Since about 1890, white phosphorus has been obtained commercially, in about 90 % yield by heating a mixture of phosphate rock, silica (sand) and coke in an electric furnace (Fig 2.1). The mechanism of this reduction is Feed chutes
Steel casing
\
,
Gaseous P. + CO
Carbon electrodes
Carbon lining
Figure 2.1.
Phosphorus Electric Furnace (simplified).
2.1
46
quite complex, but the overall reaction is generally represented by equation (1). In spite of the heat of combustion of the coke, the reaction is strongly endothermic and requires an electrical imput of about 12,000 kW hours per ton of phosphorus produced. An 'acid displacement' mechanism is considered the most likely course of reaction (2)(3). 2CaQ(P0 )
+
6SiO
2Ca 3 (P0 4 ) 2
+
6Si0 2
P
4°10
+
+
IOC
> 6CaSiOQ > >
10C
6CaSi0 3 P
4
+
+ +
10C
P
10CO + 4
P
(1) (2)
°10
(3)
°
The phosphorus vapour is taken from the top of the furnace, condensed and collected under watert The yellow product is liable to contain As and Sb in quantities up to ^ lOOppm, as well as some Si,C,Fe and F. Decolorisation and partial purification can be effec ted by filtering the liquid element through active carbon while steam distillation will reduce the arsenic content to (\, 2ppm. The fluorine from the fluorapatite is evolved mainly as gas eous silicon tetrafluoride which is removed by scrubbers according to equation (4). If treated with caustic potash, the fluorine is recoverable as a concentrated solution of potassium fluoride (5). SSiF^ 4 H0SiF^ Δ
b
+ +
2H O Δ 6K0H
>
2H SiF„ 2 6
+
^
6KF
Si0o
+
Si0 o 2 2
(4)
+
4H O
(5)
2
M o l t e n c a l c i u m s i l i c a t e s l a g i s t a p p e d o f f from t i m e t o t i m e d u r i n g c o n t i n u o u s f u r n a c e o p e r a t i o n . Some p h o s p h o r u s combines w i t h i r o n i m p u r i t i e s t o form ' f e r r o p h o s p h o r u s ' , a p r o d u c t which c a n a l s o be t a p p e d o f f and which h a s m e t a l l u r g i c a l a p p l i c a t i o n s . The c a l c i u m s i l i c a t e s l a g c a n b e u s e d a s a low g r a d e r o a d m e t a l and a s a c o n c r e t e a g g r e g a t e . Owing t o a s m a l l d e g r e e o f n a t u r a l r a d i o a c t i v i t y , h o w e v e r , i t i s n o t u s e d i n d o m e s t i c b u i l d i n g . F o r e v e r y t o n of p h o s p h o r u s e x t r a c t e d , r o u g h l y 4 t o n s of s l a g and 0 . 3 t o n s of f e r r o p h o s p h o r u s a r e o b t a i n e d . Major o r e components a r e t y p i c a l l y : wt
P
F l o r i d a USA
2°5 34.4
K h o u r i b g a Morocco
37.2
CaO
Fe
49.3
2°3 1.2
54.2
0.1
A1
2°3 0.95 0.39
Si0 2
C0
4.21
3.8
0.97
2.64
4.2
The a n a l y s i s o f a t y p i c a l p h o s p h o r u s f u r n a c e s l a g i s a s f o l l o w s
(wt%) :
CaO
48.5
K20
1.1
F
2.8
F e ^
0.2
SiO_
40.3
S0„
0.6
P
0.5
MnO
0.2
4.9
MgO
0.5
Nao0
0.4
2
Al 0 o
F
2 3.12
«J
* E a r l y f a c t o r y workers with the element s u f f e r e d from a dangerous and f a t a l c o n d i t i o n known as n e c r o s i s of the jaw - "phossy jaw". The d i s e a s e i s now v i r t u a l l y unknown as a r e s u l t of g e n e r a l i n d u s t r i a l p r e c a u t i o n s , and t h e use of white phosphorus in match compositions was banned many y e a r s ago.
47
2.1 Phosphate rock is a potentially valuable source of fluorine and there is
currently much interest in the economic recovery of the elementfrom both the electric furnace process and the wet phosphoric acid process (Chapter 2 ) . It is estimated that the quantity of fluorine present in the annual total of mined phosphate rock exceeds that presently mined as fluorospar, CaF Most apatite contains 0.01^O.03 % uranium, but its direct recovery is not economic. Extraction from wet process phosphoric acid is more feasible (Chapter 3.1) Q
Since the world output of phosphate rock exceeds 10
tons p.a., a concentration of 4 only 0.01 % uranium represents a potential supply of over 10 tons of the heavy element. Most of the uranium in the earths crust is beleived to be associated with Apatite. Some Apatites contain relatively high concentrations (~0.01 %) of chromium and vanadium.
Elemental phosphorus can be obtained by the reduction of other minerals such as the aluminous phosphates, but, owing to tech nical difficulties, these have not yet assumed any importance in commercial processes. Aluminium phosphate can be reduced by heating with coke and lime at 1600°C (6). The byproduct calcium aluminate has a potential use a hydraulic cement, but unless the raw materials have a very low iron content, phosphorus is lost as ferrophosphorus. The process is not economic for high yields of both phosphorus and calcium aluminate. If apatite is treated with lead chloride solution it forms pyromorphite (Chapter 3 ) . This pyromorphite (or lead orthophosphate) can then be reduced by hydrogen at the comparatively low temperature of 500°C (7). 4A1P0
+
2CaO
+
2C
>·
P
+
2CaO.Al 0
+ 2C0
(6)
2Pb ^(POJ Cl + 50 H > 3Ρ„ + 20Pb + 4HC1 + 48H 0 (7) 10 4 o 2. Δ 4 2 Some electric furnace phosphorus is converted directly to phosphorus chemicals, but more than 80 % of production is 'burnt' to the pentoxide which in turn is converted to phosphoric acid (Chapter 3 ) . Ultrapure white phosphorus can be obtained by thermal decomposition of a suitable metal phosphide (Section 2.2). White phosphorus oxidises spontaneously in air, often bursting into flame.* It will burn in both oxygen and carbon dioxide to give the pentoxide (8)(9). In a restricted supply of oxygen, lower oxides are produced. Dense white smoke can be obtained in reaction (8) and the element has application as a smoke generator. Another military P4
+
P4
+ *
5 02 10C0 2
■
>
P4010
>
P4010
(8) +
10C0
The pyrophoric properties of white phosphorus were utilised in the earliest form of matches, which consisted of strips of paper tipped with the element and sealed in glass tubes. When broken, the paper would catch fire.
(9)
2.1
48
application is as a self-igniting agent in incendiary shells and tracer bullets. World War II 'Molotov cocktails' were bottles con taining white phosphorus dissolved in benzene or gasoline. Under special conditions, a cold greenish phosphorescent glow is associated with this form of the element, and it is due to a slow oxidation of the vapour emitted. This glow has been the sub ject of numerous investigations and is still not fully understood. Various chain reactions have been proposed, and spectroscopic anal ysis indicates that various molecular species are involved (Chapter 12). It has been established that the oxidation is a gas-phase re action which can take place with very low concentrations of P 4 vapour. At room temperatures the glow has a maximum intensity when the partial oxygen pressure is about 300 mm. The glow becomes weak er and finally vanishes when the partial pressure is either incre ased or decreased. The limits are influenced by vessel shape, impur ities, traces of water etc. A little ozone is also formed. The element will combine vigorously with halogens, with sul phur and many metals. It is a reducing agent and with concentrated alkalies, phosphine and hydrogen are produced (10)(11). P
+
3K0H
+
3H 0
>
PH
+
3KH PO
(10)
P
+
4K0H
+
4H 0
>· 2H
+
4KH PO
(11)
White phosphorus will precipitate copper and lead from aqueous solutions of their salts. Lumps of white phosphorus, if placed in copper sulphate solution, will rapidly become coated with black copper phosphide, which is in turn reduced to metallic copper. Sulphur chloride is reduced to sulphur (12), and thionyl chloride and potassium iodate are also reduced (13)(14). P.
+
6S 0 C1 0
>
4PC1 Q
P4
+
12KI0 3
>
4K
P4
+
8S0C1 2
>
4PC1
3P04 3
+
12S
+
6I
+
4S0
(12) +
2
+
2
10
°2
(13)
2S
2C12
(14)
White phosphorus is virtually insoluble in water (<0.0005 %) and it can be stored, fused and transported quite safely under an aqueous layer. Water dissolves in white phosphorus, however, to the extent of 0.1 mg / g of Pu at about 30°C. If it is exposed to both air and water, under conditions in which it will not inflame, a complex mixture of oxyacids is slowly produced. At elevated temper atures and pressures, water vapour will react with white phosphorus to form various products. Above 200°C in a sealed tube, phosphine and phosphorus acid are the main products (15), but above 1000°C steam will oxidise phosphorus vapour to give the pentoxide (16). P4
+
6H 2 0
>
2H
3P03
P4
+
10H 2 0
>
P4010
+
2PH
+
3
10H 2
(15)
(16)
49
2.1
White phosphorus is a strong poison and as little as 50 mg can be fatal to humans. At normal temperatures white phosphorus is cubic and has a density of 1.82 g/cc (α-form), and appears as glistening polyhedra of various kinds if grown by slow sublimation in sealed tubes. At -77°C the cubic form transforms to a hexagonal (3) form with a den sity of 1.88 g/cc. The transition point is raised to +64 C under a pressure of 11,600 atmospheres. Both solid forms of white phosphorus contain tetrahedral P4 molecules similar to those which exist in the liquid and vapour states (Fig 2.2). Electron diffraction measurements on the vapour indicate P — P = 2.21 A and interbond angles of 60°. In the cubic a form free rotation of these tetrahedra probably occurs, but in the low temperature form this freedom may be lost. White phosphorus transforms to red under the action of heat, light or X-radiation. Black Phosphorus Black phosphorus is thermodynamically the most stable form of the element and exists in three known crystalline modifications as well as in an amorphous form. Unlike white phosphorus, the black forms are highly polymeric, insoluble, practically non-inflammable and have comparatively low vapour pressures. The crystalline black varieties represent the densest and chemically the least reactive of all the forms of the element. Orthorhombic black phosphorus was originally produced by the action of high pressures on the white or red forms. It was later made by the action of heat on white mixed with mercury and in the presence of a seed crystal of black. This form of the element has a continuous double layer structure in which each P atom forms three bonds of length 2.23 A, pyramidally disposed at mutual angles of 100° (Fig 2.2). It is a semiconductor and exhibits a flakiness similar to mica and graphite which also have layer structures. Under very high pressures, orthorhombic black phosphorus under goes further (reversible) transitions to produce more dense rhombo-
white P„
rhombohedral black P n
orthorhombic black P
cubic black P n
2.1
50
hedral and cubic forms. In the rhombohedral form the simple hexag onal layers are not as folded as in the orthorhombic form, and in the cubic form each atom has an octahedral environment (Fig 2.2). Amorphous black phosphorus is made from white by the applicat ion of somewhat lower temperatures and pressures than are needed to make the crystalline varieties, and it represents a transition structure. Vitreous grey phosphorus, distinct from amorphous black, can be obtained as an intermediate product when making amorphous black. This form is dark grey, amorphous, hard and brittle, and exhibits conchoidal fracture. It can ignite on impact and is an electrical insulator. Red Phosphorus Red phosphorus is a term used to describe a variety of differ ent forms, some crystalline and all of which are more or less red in colour. They show a range of densities from 2.0 to 2.4 g/cc, and melting points in the range 585-610°C. The stabilities and reactiv ities of these red forms lie between those of the white and black forms, although they resemble the latter more closely. The vapour pressure of the red is much less than that of the white (Table 2.1). Samples of red phosphorus usually vapourise at about 450°C under atmospheric pressure. TABLE 2-1 Comparison of white and Red Phosphorus White Crystalline, waxy, translucent mp = 41.1°C Vapour pressure - high Density 1.83 g7cc Soluble in organic solvents Highly toxic Heat of sublimation 13.4 k cal/mol Chemiluminescent Spontaneous ignition near room temp Characteristic smell Spontaneously ignites in chlorine Produces phosphine v/ith alkali aqueous Contains discrete Fu molecules
Red Amorphous or crystalline, opaque mp = 585-6lO°C Vapour pressure - very lov/ Density 2.0-2.4 g/cc Insoluble in organic solvents Non-toxic (or almost so) Heat of sublimation 30.0 k cal/mol(am) Non-chemiluminescent Ignites only above 260°C. No smell Heat necessary for ignition in chlorine No reaction with aqueous alkali Highly polymeric P
Red phosphorus i s very i n s o l u b l e . It behaves as a high polymer and, unlike the white form, i t does not inflame, although i t can be more e a s i l y i g n i t e d than black. Red phosphorus i s almost non-toxic and for t h i s reason i t has replaced the white v a r i e t y in match compositions. Finely divided red P can explode when i g n i t e d in a i r . *
The cubic form i s derived from the rhombohedral form by reducing the interbond angles in the l a t t e r from 100 t o 90°, and bringing the layers c l o s e r t o g e t h e r .
2.1
51
Red can be made by heating white at 260°C, amorphous black at 125°C, or crystalline black at 550°C (Fig 2.3). Red phosphorus, like the white form, will combine directly with oxygen,sulphur, halogens and metals, although the reactions are generally less vig orous. There is no reaction with aqueous alkali , however. Red phos phorus undergoes some useful reactions with carbon compounds and is used in general chemical manufacture. Safety matches were first introduced by LuLyidU>t/l0m in 1855. The match heads contain an oxidising agent, sulphur, ground glass and glue, while the striking surface contains the red phosphorus.* Toy pistol caps contain potassium chlorate and red phosphorus in separate compartments, which,when struck together will explode. On exposure to damp air, red phosphorus will undergo a very slow oxidation to orthophosphoric acid. The oxidation of red phos phorus using concentrated nitric or sulphuric acids can be repres ented approximately by equations (17)(18). P
+
5HN0
>
HΗ ο Ρ0„ P
4P
+
8H SO
>
4Η
3 °4
0 4Hο3ΡP0 „4
++
5N0 o 5N0
++
HH o 0
2°
(17)
2H 200 7S0 ++ 2H ++ Ss ++ 7S0
(18)
Commercial red phosphorus, which is largely amorphous, is made by heating white in closed vessels at about 280-350°C for 48 hours. The product is wet milled, boiled with alkali to remove traces of white, then filtered and washed. The commercial red form exhibits conchoidal fracture and can show a range of colours from pale yellowish-red to dark violet-red. The colours obtained by heat ing pure white at various temperatures in the range 300-610°C under laboratory conditions can vary from dark red to light orange-red. Such differences can arise from particle size variations, the paler coloured forms generally being more reactive because of smaller particle size. A very fine pale red form can be obtained by expos ing a carbon disulphide solution of white to UV or visible light. On the other hand, comparatively large violet particles can some times be obtained by systematic levigation of commercial red with water. Violet phosphorus or W/JUtoh^ & phosphorus, made by recrystallisation of the element from molten lead, and Sch&nck1A scarlet phosphorus, made by heating a solution of white in phosphorus tribromide, are known to be chemically impure. The many colour varia tions reported for red phosphorus can often be associated with either particle size or surface impurity effects. *
Typical formulae are : match head KCIO 3 K
Cr
2 2°7 Mn0 2 S Fe
2°3
match bo x (wt%)
(wt %) ground glass
8.5
red P
49.5
3.5
kieselghur
1.0
Sb
27.6
4.7
ZnO
0.5
Fe
3.5
glue
5.0
3.8
water
37.0
S
2 3
2°3 gum
water
1.2 20.9
2.1
52
X-ray diffraction, optical microscopy and DTA have neverthe less established the existence of several different crystalline 'red' varieties of the pure element, in addition to the amorphous form. These crystalline varieties are generally made by heating the amorphous form to various temperatures below the melting point. Hittorf's violet phosphorus is a complex three-dimensional polymer in which each P atom has a pyramidal arrangement of three bonds linking it to neighbouring P atoms (Fig 2.2). It seems likely that all forms of red phosphorus (like black) are built from pyram idally linked atoms, the different crystalline varieties represent ing different kinds of ordered framework, or differing degrees of polymerisation.
"V W ΐ ί P
I
P
X
P I
I
P
^P I
νΛ W
Ί P
P
I
X
P I
l
^
P
\
I
ΑΡ
V,p \
v ^
/ N—p'
(19)
A,-
^* P --P'
\
It is likely that in most samples polymer growth is terminated by occluded 'impurity' atoms such as halogen, oxygen or hydroxyl. Amorphous samples probably consist of entirely random networks of atoms (19). Brown phosphorus has been obtained by condensing phosphorus vapour containing P 2 molecules on to a surface at liquid nitrogen temperatures. Above 0°C it changes to a mixture of red and white. Phosphorus Vapour When heated under normal pressures, all allotropic forms of phosphorus will produce a vapour consisting of tetrahedral P 4 mol ecules (Fig 2.2). The interbond angles of 60° represent a highly strained arrangement for which hybridised pd 2 orbitale have been proposed. The tetrahedral P 4 type structure is very unusual but not quite unique in chemistry. Elements of the same group adopt it, i.e. As 4 , Sb 4 and perhaps Bi 4 (see below). The existence of isoelectronic S i 4 4 , and Ge 4 4 ~, Sn u 4 ~and Pb 4 4 ~ anions has recently been established. At temperatures above 800°C dissociation of the tetraatomic vapour occurs. This is appreciable at 900°C, and at 1700°C roughly equal numbers of P 4 and P 2 molecules are in equilibrium. Emission spectra indicate that the P 2 molecule contains a triple bond with P = = P = 1.875 A. The equilibrium ^ ^ Λ_ _M „ , H p ^ ^ 2P - 54.5 k cals /mol lies to the left hand side except for high temperatures or low pre ssures. Further dissociation by the reaction P
^
± 2P
-
116 k cals/mol P
is very small at 1700°C, but it has been calculated to reach about 8 % at 3000°C. If any solid form of phosphorus is vapourised and then condensed at low pressure, a red variety is obtained. At low
53
2.1
pressure red phosphorus depolymerises directly to P2 molecules which recombine to form Pn molecules of white phosphorus. Under non-equilibrium conditions, the red form on vapourisation may yield other species such as P 3 , P 6 , P 8 etc. When red phosphorus is slow ly precipitated from solutions of white in CS 2 or PBr 3 , it probab ly involves photochemical dissociation into P 2 molecules which then polymerise to form the red variety. Liquid Phosphorus It is probable that all forms of phosphorus can be melted to form what is the same colourless liquid. Liquid phosphorus can be readily supercooled to a state from which the rate of crystallisat ion of the white form is extremely rapid. The Raman spectra of the solid and solution states of white phosphorus resemble that of the liquid, indicating that the latter also contains tetrahedral P4 molecules.
T n c l i nci r ed (14)
[ Rhombohedral black
Cubic r e d -
U) Cubic black
Figure 2.3
Relationships Between Forms of Elemental Phosphorus.
(1) High vapour pressure at room temperatures,(2) heat at 540°C,(3) heat at 550CC,(4) heat at 600°C, (5) heat at 125°C, (6) heat at 400°C, (7) heat at 550"C, (8) heat at 300°C at 8000 atm, (9) heat at 380°C with Hg or above 250°C at 12 kb, (10) heat at 400°C with Hg for days, (11) heat at 200°C at 12000 atm, (12) heat at 200*C at 15000 atm, (13) heat at 200°C at 12000 atm (14) reversi ble transition 50-100 kb, (15) reversible transition 110 kb, (16) recrystallize from molten Pb, (17) heat a Pßr^ solution, (18) reversible transition at 900°C, (19) reversible transition at 1700"C, (20) reversible transition at low pressure (21) reversible transition at 44.1ÖC (but can supercool), (22) reversible trans ition at -77°C or +64f,C under 1200 atm, (23) sublime under vacuum, (24)heat at 220°C at 12 kb, (25) irradiate with UV at -190°C, (26) condensation of P2 vapour at -196°C, (27) heat above -100°C, (28) heat at low pressure, (29) boils at 280 °C, (30)heat at 300°C or expose to light or X-rays, (31) melt about 600°C.
2.1
54
Phosphorus Ions Alciiough there remains some doubt about the existence of monatomic P3"" anions, the existence of a large number of polyphosphorus anions has been confirmed in recent years. These anions are found in metal phosphides and involve catenated atoms which form chains, rings, cages etc(Section 2.2). Phosphorus cations have been observed as unstable species in the mass spectrometer (Chapter 12). Comparisons of Pnictide Elements Nitrogen is unique amongst the pnictide elements because it exists solely as N 2 molecules in the gaseous, liquid and solid states. In all these forms the element is colourless, odorless, non-inflammable and non-toxic. Nitrogen, Ν = Ξ Ν , forms the most inert diatomic molecule known. The remaining pnictide elements, because of their comparative reluctance to form multiple bonds, all prefer to catenate and consequently exist in forms more highly pol ymerised than diatomic nitrogen. Elevated temperatures are required to break them down into diatomic species. Arsenic and antimony exist in yellow crystalline forms which contain tetrahedral As 4 or Sbu units similar to the Pu units of white phosphorus. These tetrahedral molecules, which pack together into cubic lattices in the solid state, persist in CS 2 solution and if the elements are melted or vapourised. On heating to higher temp eratures, the tetraatomic vapours dissociate to diatomic species and eventually to single atoms. This dissociation takes place more readily with increasing pnictide atomic weight (arsenic is complete ly dissociated to A s 2 at 1700°C). Arsenic exists in a black and a grey form, and antimony and bismuth both exist in 'metallic' forms, all of which are highly polymerised. Like the red and black forms of phosphorus, they show insolubility, greater densities, higher melting points and lower reactivities than their tetraatomic polymorphs. The black form of arsenic has the same crystal structure as orthorhombic black phosphorus. The grey form of arsenic and the grey (metallic) forms of antimony and bismuth all have the same layer type structure which is similar to that of rhombohedral black phosphorus. Although the latter appears only under pressure, the structure becomes more stable as the atomic weight increases. The rhombohedral forms of As, Sb, and Bi are the most stable forms of these elements and they can all be obtained by condensation of their tetraatomic vapours. The tetraatomic forms or the vapours from any allotropes of P, As or Sb are all extremely toxic either by inhalation or ingestion. The red, black or metallic forms, unless vapourised are, on the other hand,much less toxic.
t
heat
P
4
As,
Sb 4 -^
Asr
Sb n -^
(Bi4) decreasing thermal stability Bi n increasing thermal stability
2.1
55
The differences between the interatomic and the inter sheet distances in the rhombohedral layer structures of the pnictide elements become progressively less as the atomic weight increases: 3 closest in same layer
P 2.13
As 2.51
Sb 2.91
Bi 3.10 A
3 closest in parallel layer
2.83
3.15
3.36
3.47
This c o n s t i t u t e s an approach towards a s y m m e t r i c a l o c t a h e d r a l e n v i r onment f o r e a c h p n i c t i d e atom and i s i n a c c o r d w i t h i n c r e a s i n g met a l l i c c h a r a c t e r . This m e t a l l i c c h a r a c t e r i s s u g g e s t e d by t h e chang i n g appearance and i n c r e a s i n g v a l u e s of d e n s i t y , c o e f f i c i e n t s of e x p a n s i o n , e l e c t r i c a l and thermal c o n d u c t i v i t y which are o b s e r v e d on moving from phosphorus t o bismuth ( T a b l e 2 . 2 ) . TABLE 2-2 Physical P r o p e r t i e s of
Pnictide
Elements P
As
yellowish red
Colour
various
Crystal system Density
(g/cc)
Sb
s t e e l grey
s i l v e r white
rhombohedral
rhombohedral
rhombohedral
2.31
5.73
6.67 631
273
1380
1560
88
40
13
102
383
204
Melting point
(° C)
589 (^3 atm)
8l8 (36 atm)
Boiling point
(° C)
416 (sub)
633 (sub)
Latent heat of fusion (cals/g)
Bi reddish white
122
Latent heat of vapourisation
9.80
If data were available, a more precise comparison could be made with the isostructural black rhombohedral form of P. There is, however, a remarkable lack of fundamental physical data on even the red form.
Although the highly polymerised pnictide structures represent the least reactive forms of these elements, red P and the metallic forms of As, Sb and Bi can all be made to burn in air and they will combine directly with oxygen, sulphur and the halogens. They are all attacked by concentrated nitric and sulphuric acids and by hyd rogen chloride in the presence of oxygen. The elements are attack ed either very slowly by moist air at room temperature or more rapidly by steam at higher temperatures (Table 2.3). TABLE
2-3
Chemical Properties of Pnictide Elements P Combustion in air
P
Ignition in C0 2
P^O
Steam at high temps
H°10 Q
P^O +H~
As
Sb
As^Og (+A S l | 0 1 0 )
sb^o6
B1
ksH06
S
Bi
As
C+AS^Q)
i | O 6 + H 2 ( + A s l4°10 )
Cone HN0 o
H3POi|
H^AsO^
Hot cone H2SOi|
H^PO^
<*iPe
V6
Bi
H°6
H°6
Sbjj06+H2
Bi^0 6 +H 2
Sb
Bi(N0 3 ) 3
H°6
Sb^SO^),
Bi 2 (SO l 4 ) 3
2.2
56
Interpnictide Compounds The pnictide vapours have similar tetrahedral structure and the vapours from mixed liquid phases of phosphorus and arsenic have been shown spectroscopically to contain stable interpnictide molec ules P 3 As, P 2 As 2 , and PAs 3 , all based on tetrahedral units. Mass spectra of the vapours from various As/Sb/Bi mixtures at 300-750°C and 10~"5 atm also indicate that many stable interpnictide species exist e.g. BiSb 3 , Bi 2 Sb 2 , Bi 3 Sb, Sb 3 As, SbAs 3 etc. Orthorhombic black phosphorus will take up arsenic in solid solution and it forms a definite compound AsP. The latter can also be made by reaction (20). Arsenic phosphide resembles orthorhombic black phosphorus in physical properties and very probably has a similar puckered layer structure. AsH 3
+
PC1 3
2Quc
>
AsP
+
3HC1
(20)
Continuous series of solid solutions are formed between the common highly polymerised forms of adjacent pnictide elements i.e. P/As, As/Sb and Sb/Bi. Solid solution formation between remaining combinations i.e. P/Sb, P/Bi and As/Bi appears to be very limited and lie below 1 % of one element in the other. Nitrogen does not dissolve to any appreciable extent in either Sb or Bi, although the mononitrides SbN and BiN have been reported to exist. Nitrogen does, however,form several phosphorus nitrides (Section 2.6), and arsenic analogues probably also exist.
2,2
METALLIC
PHOSPHIDES
Almost all metals form phosphides and over 150 different binary compounds are known (Table 2.4). In addition there are many mixed-metal phosphides, phosphide sulphides and phosphide selenides. Many of the binary phosphides lie within the composition range M 3 P to MP 3 with some metals such as nickel forming several phases : Ni 3 P, Ni 5 P 2 , N i 1 2 P 5 , Ni 2 P, NiP, NiP 2 while others such as aluminium form only one : AIP. It is only since about 1960 that most of the metal phosphides have been properly characterised and obtained in a sufficiently high state of purity to enable reliable measurements of their prop erties to be made. These compounds show a wide range of crystal structures and differ greatly from one another in their physical and chemical properties. Bond type is often uncertain. The majority of metal phosphides have a metal arsenide analogue which they usually resemble closely in properties and structure (Table 2.5). Metal phosphides, arsenides and nitrides not infrequently exhibit properties similar to those of metal carbides, silicides and germanides.
2.2 TABLE
57 2-4
Conposition of Well Characterised Binary Phosphides
LUP
Be 3 P 2
B
IAPC
BeP 2
BP
UP?
AIP
Na^P
GaP
Na2P5
InP
Na3Pn Ti P
V P 3
Cr P
Mn P
FeP
Co 2 P
Ni P
CaP
Ti 5 P 3
VP
CrP
Mn 2 P
Fe 2 P
CoP
Ni 5 P 2
CaP 3
T
VP2
CrP^
MnP
FeP
CoP
Ni12P5
MnP^
FeP 2
Ni 2 P
FeP^
Ni 5 P 4
¥5
Ca 3 P 2
KP15 K
4P6
ScP
V3
K3P7
TiP
^4
Cr
KP
TiP 2
V3 V7
CrP 2
Rb 2 P 5
SrP
^4Ρ6
Sr3Pl4
RbP ?
SrP 3
YP
12P7
V12P7
NiP 2
V2P
NiP
Nb 3 P
Mo~P
Ru 2 P
Rh 2 P
Pd
ZrP 2
Nb5P3
M 0l| P 3
RuP
Rh n P 3
Pd 7 P 3
NbgP 5
MoP
RuP 2
RhP 2
PdP 2
NbP
MoP 2
Re
6P17
RuP^
RhP 3
PdP 3
Nb 2 P 5
MoP^
Re
6P13
Ba 3 P 2
LaP
Hf 2 P
Ta 2 P
Cs
BaP 3
LaP^
Hf 3 P 2
HfP
3Pl4
TlPc
BaP 1 Q
LaP
CeP
PrP
LaP 7
CeP 2
PrP 2
UP
Re 2 P
OsP 2
Ir 2 P
pt 5 P 2
TaP
3P WP
R^H
OsP^
IrP 2
PtP 2
TaP 2
WP2
ReP
Th3Pl4
U P 3 4
ih2pn
UP2
NdP
Νρ 3 Ρ 4
3P
ReP W
HfP 2
ThP
3
ZrP
CsP ?
Ba
3
NiP
NbP 2
3 P ll
13P2
PuP
IrP 3 Zn^P 2
SiP
s^?5
SiP 2
SnP
SmP
CiuP
ZnP 2
GdP
Cu 2 P
Cd^P 2
TbP
CuP 2
cd 6 P 7
r^yP
A g 3 P n CdP 2
HoP
AgP 2
CdP^
ErP
Au2P3
cd7P10
SnP
GeP GeP
3 GeP c
3
2.2
58
TABLE
2-5
Phosphides and Isostructural
Metallides
Li 3 P, Na 3 P E e 3 P 2 , Mg 3 P 2 Zn 3 P 2 , Cd 3 P 2 Cu 3 P, Cr 3 P,Mn 3 P, Fe 3 P, Ni 3 P Γ4ο3Ρ, W 3 P Ti 3 P, V 3 P , Fe 0 37B0 63, Nb 3 P, Zr 3 P, Ta 3 P Mn 5 PB 2 , Fe 5 PB 2 , Co 5 PB 2 Pd 3 P Mn 2 P, Fe 2 P, Ni 2 P Co 2 P, Ru 2 P, Re 2 P, ZrFeP, TiFeP Ta 2 P, Hf 2 P, V 3 PC, V 3 PN, Cr 3 PC, Cr 3 PN V 2 PC Li 9 TiP u Ba 3 P 2 Hf 3 P 2 U2N2P Rh u P 3 Ti5P3, V5P2N
Li 3 As, Na 3 As, K 3 As, IrSi 3 Mg 3 As 2 Zn 3 As 2 , Cd3As 2 Cu 3 As Pd 3 As aV 3 S Ti 3 Si, Zr 3 Si, Nb 3 Si, Ta 3 Si, Zr 3 As
BP, AIP, GaP, InP MgGeP 2 , CuSi 2 P 3 , CuGe 2 P 3 Zn 3 PI 3 ZnSiP 2 , ZnGeP 2 , CdGeP2 B12BP2 Rh 2 P, Ir 2 P LiMgP, LiZnP, LiyVP, Li 5 SiP 3 , Li 5 GeP 3 , Li 5 TiP 3 Li 3 AlP 2 ScP, YP, ZrP, LaP, CeP, PrP TbP, ErP, DyP, HoP, NdP, SmP, ThP, UP, PuP
8-MbP, ß-TaP TiP, ZrP, HfP
MoP VP
CrP, MnP, FeP, CoP, RuP, WP Th 3 P t ,, u 3 p u , Np 3 P L», Ti u P 3 GeP, SnP CuPS,, CuPSe , AgPS3, AsPSe Re 3 P t l SiP 2
UP2
IvbP2., WP 2 FeP 2 ., RuP 2 , 0sP 2 PtP 2i , NiP 2 , SiP 2 CoP 3 ,, NiP 3 , RhP 3 , PdP 3 , IrP NiP 2 ., RhP 2 , PdP 2 , IrP 2 , ZnP 2 VP 2 , NbP 2 , 'TaP 2 , WP 2 TiP 2 .• ZrP 2 , HfP 2
Cr 5 B 3 , Mo 5 SiB 2 , Nb 5 Si 3 , Ta 5 Si 3 , V 5 SiB 2 , Fe 5 SiB 2 , Mn 5 SiB 2 Pd 3 As, Pd 3 Si, Pd 3 B, Ni 3 B, Co 3 B, Fe 3 C, Mn 3 C Pt 2 Si, Pd 2 Si, Ni 6 Si 2 B, ß-Co 2 As, Pd 2 As 6-Rh2As, Rh 2 Si, Co 2 Si, Ru 2 Si, Ir 2 Si, 6-Ni2Si, ZrAs 2 Ti 2 S, Zr 2 S, Ti 2 Se, Zr2Se V 3 AsN, Cr 3 AsC, V 3 AsC, Re 3 B V 2 AsC Li 9 TiAsu, Li 3 Bi Ce 2 S 3 Cr 3 C 2 Ce 2 0 2 S Ni,B 3 Mn 5 Si 3 , Sc 5 Si 3 , Y 5 S i 3 , V 5 S i 3 , Mo 5 Si 3 , Zn 5 Si 3 , Nb 5 Si 3 Ta 5 Si 3 , Ti 5 As 3 , Cr 5 Si 3 , Hf 5 Si 3 , Fe 5 Si 3 AlAs, GaAs, InAs, BAs, SiC, ZnS, Si, C ZnSnAs2 Zn 3 AsI 3 ZnSiAs 2 , ZnGeAs 2 , CdGeAs 2 , CdSnAs 2 Bi 2 AsP 2 , Bi 2 Si 3 , B i 2 B C 2 , B i 2 C 3 , B i 2 S 3 Mg 2 Si, Be 2 B, Be 2 C LiMgAs, LiZnAs, Li 7 VAs u , LiMgN, LiZnN Li 5 SiAs 3 , Li 5 GeAs 3 , Li 5 TiAs 3 , Li 2 AlAs 2 YAs, LaAs, CeAs, PrAs, NdAs, SmAs, VAs, PuAs, PaC, PuN LaS, CdS, PrS, ErS, ,NdS, YbS, ThS, US, PuC, HfC, NbC, UC, SiC, HfB, ZrB, PuB, CrC, TiC, ZrC, VC, TaC, LaN, CeN, ThC, UB NbAs, TaAs MoC, TiAs, ZrAs, NbN, HfAs WC NiAs, TiAs CrAs, MnAs, FeAs, CoAs, PdSi, PtSi, VAs, HfSi, IrSi Th 3 As u , U 3 A s 4 , U 3 B i u , U 3 Sb u , C e 3 S u , P u 3 S 4 , N d 3 S 4 , Sm 3 S u G d 3 S u , La u Ge 3 , Ce u Bi 3 , GeAs, SiAs CuBS, CuBSe, AgBSe Fe 3 Se u , Cr 3 S4, Cr 2 NiS u , Ce 3 Se u SiAs 2 , GeAs 2 UAs 2 ZnSi 2 FeAs 2 , NiAs2, FeS 2 , CrFeAs u , LaC 2 FeS 2 , MnS2i PdAs 2 , CaC 2 , PtAs 2 , PdSb 2 , PtSb 2 , PtBi 2 CoAs 3 , IrAs 3 CoAs2, RhAs 2 , IrAs 2 , FeAsS ZnAs 2 VAs 2 , MoAs 2 , W A s 2 , TaSb 2 , NbSb 2 , NbAs 2 , TaAs 2 ZrAs 2 , HfAs 2 , ThS 2 , U S 2
59
2.2
Metal phosphides can, in general, be made by direct union of the elements in vacuo or protective atmosphere under conditions which prevent undue loss of phosphorus, as e.g. (21)(22). 3Li
Li
Ge
GeP
(21)
3P
(22)
In special cases they may be made by reaction of phosphine with an oxide (23), or a metal chloride (24), or by reduction under the correct conditions, of a phosphate with carbon (25) or hydrogen (26). 2PH
Ga
PH.
3ZnCl
->
2°3 2
Ca
3(P°4)2
8C
Fe
2 P 2°7
7H„
2GaP
^ -»
3 2 Ca
3P2
-> 2FeP
3H 2 0
(23)
3HC1
(24)
8C0
(25)
7H 2 0
(26)
Other methods include the reaction of calcium phosphide with an appropriate metal powder at about 1200°C (M = Ti,V,Mn,Co,Cr,Nb, Ta,Mo, or W) (27) or with a metal chloride (28). The high temperat ure electrolysis of a metal oxide-alkali phosphate melt can be used as well as simply heating a higher phosphide (29). Ca
3P2
2Ta
-> 2TaP
3Ca
(27)
Ca
3P2
2CrCl,
->2CrP
3CaCl,
(28)
4RhP.
->
(29)
4RhPr
Metal phosphides, except those with a high phosphorus content, usually have melting points above 1000°C (Table 2.6). TABLE 2 -6 Melting Points of Metal Phosphides (°C) Fe3P Cu3P Pd3P Ni3P Mn3P
1166 1023 10^7 970 1229
Fe2P Co2P Ni2P Mn2P Ti2P
1365 1386 1110 1327 1920
Rh2P Ca3P2 MnP CrP VP
1500 1600 1193 1800 1315
TIP AIP GaP NbP ReP
1100 1800 1522 1729 1204
The chemical purity of phosphides is important since impur ities can have drastic effects on their properties, particularly electrical. Many phosphides are high melting point materials which are produced by sintering processes. Specimens can consequently exhibit large variations of porosity - a factor which also affects their physical properties. Densities in particular are liable to be somewhat less than the theoretically possible values. However, the densest of all phosphorus compounds are found amongst the metal phosphides (Table 2.7). These high densities arise from the
2.2
60 TABLE 2 -7 Densities of _Metal Phosphides g/cc g/cc Fe Ni 8.90 7.87 FeP 6.92 N1 P 7-90 3 Fe2P Ni2P 6.90 6.30 FeP NIP 6.07 5.85 FeP2 NiP2 ^.58 5.11
Mo Mo^P MoP MoP2 MoP^
g/cc 10.22 9.07 7.20 5.35 3.88
g/cc Ir 22.4 Ir2P 15.6 IrP2 9.33 Pd 12.02 Pd?P3 9.53
presence of the heavy metal atoms and the efficiency with which they pack with the phosphorus atoms in the crystal lattice. A simple but highly symmetrical scheme of coordination of both the metal atoms and the phosphorus atoms is found in many phosphide structures (see below). Phosphides, in general, tend to have lower melting points and lower stabilities, and to be less hard than silicides, borides and other metallides. Metal-rich Phosphides
MxPy
(x>y)
The metal-rich transition metal phosphides (< 60 % P) are dark coloured and insoluble in water, they have high chemical and thermal stability, they are dense, hard and brittle and have high thermal and electrical conductivities. These properties they have in common with transition metal borides and silicides (and in some cases carbides and nitrides) to which they are often structurally related. With few exceptions, the transition metal phosphides borides and silicides are not attacked by dilute acids and bases and may remain unaffected by hot concentrated mineral acids.
(b)
(c) Figure 2.4
C r y s t a l S t r u c t u r e s of P h o s p h i d e s .
(b) Fe 2 P f r a g m e n t , ( c )
Rh P u n i t c e l l ,
(a) Fe~P f r a g m e n t ,
(d) ZrP u n i t c e l l ,
(unfilled circles = P).
A common s t r u c t u r a l f e a t u r e of almost a l l t r a n s i t i o n metal phosphides of the m e t a l - r i c h category i s the n i n e - f o l d ( t e t r a k a i d e c a h e d r a l ) arrangement of metal atoms around the phosphorus atom
2.2
61
(Fig 2.4a). Such tetrakaidecahedra contain metal-phosphorus distan ces expected for covalent bonds, and are arranged in various patter ns in which the metal-metal distances are greater than in the corre sponding pure metals. There are no covalent P — P linkages. In some metal-rich phosphides, one or more of the equatorial arms of the tetrakaidecahedron may be missing, resulting in only 8,7 or 6-fold coordination of the P atom by metal atoms. The 6-fold trigonal prismatic coordination (Fig 2.4b) occurs in some monophosphides (see below). Non-stoichiometry or 'extended homogeniety ranges' are found in most of the transition metal phosphides with formulae M 3 P and M 2 P . Palladium phosphide, Pd 3 P, e.g. can have any composition in the range P d 3 P 0 # 7 5 - Pd3 P 1 > 0 0 and cobalt phosphide in the range Co 2 P - C o 1 # 7 5 P . These phenomena are believed to be associated with vacant lattice sites where P or metal atoms are missing. Both boron & silicon have appreciable solid solution in some metal phosphides, where random substitution for P atoms occurs. Close structural relationships exist between some borides, suicides and phosphides. Phosphorus will dissolve to the extent of about one or two percent in some metals such as iron and copper, without detectable compound formation. This has little effect on mechanical properties and produces only slight changes of unit cell dimensions from the original metal. Small amounts of phosphorus may, under some circum stances, appear as metal phosphides which form separate phases at the grain boundaries of the metal. In this case the consequent mod ifications of mechanical properties of the metal may then be consid erable . The strength and corrosion resistance of iron is increased by the presence of small quantities of phosphorus. Grades of wrought iron and cast iron frequently contain 0.1 -0.2 % P, some of which is present as phosphide. Phosphorus is nearly always present as an unavoidable impurity in commercial steels. A maximum of 0.05 % is usually allowed, since it otherwise reduces ductility, although machinability is improved. The metal phosphide which is produced in largest quantity commercially, is ferrophosphorus, which contains Fe 3 P and Fe 2 P. This material is formed as a byproduct in the production of element al phosphorus by the electric furnace method (see above). Until recently, ferrophosphorus was added to steel furnace melts, where, together with the phosphorus already present in the iron ore, it reactedwith the basic furnace lining to form basic slags. The latter is a useful fertilizer. Phosphor-bronze contains copper, tin and about 0.1-0.5 % P, the latter element being added for de-oxidising purposes. The phos phorus is incorporated with the copper before the addition of the tin, in order to prevent the formation of tin dioxide which is more difficult to reduce than oxides of copper (the oxygen is removed from the melts as volatile phosphorus pentoxide). Phosphorus remain ing in excess of that needed for de-oxidation, increases the ten sile strength, hardness and corrosion resistance. Phosphor-bronze finds use in springs, electrical contacts and non-corrodible fixing
2.2
62
lugs in building construction. De-oxidised and 'phosphorised' copper contains 0.01 -0.05 % P and has improved hot and cold working characteristics although its electrical conductivity is less than that of the purer metal. It is used for gas and water pipes. The addition of larger quantit ies of phosphorus to copper results in the separation of Cu 3 P as a separate phase. The master alloy known as 'phosphor copper'contains 10-15 % phosphorus, much of it as Cu 3 P. This is used for de-oxidat ion and for adding phosphorus to alloy melts. Copper phosphide, CU3P, is hard, dense, brittle and electric ally conducting. In contrast to most M 3 P and M 2 P transition metal phosphides, Rh 2 P and Ir2P are very hard and inert, and the latter has been used for fountain pen tips. Schreibesite, (Fe,Ni)3P, has been detected in meteorites and lunar samples and it constitutes a rare example of a naturally occurring reduced phosphate mineral. Phosphides of this type may be present in the earth's core. Vitreous Metals So-called 'vitreous metals', 'amorphous alloys' or 'metallic glasses' can be obtained by extremely rapid chilling of molten mixtures of one or more metals with boron, carbon or phosphorus. Most compositions contain phosphorus e.g. : Fe
80 P 13 C 7
Fe
80 P 14 B 6
Fe
Ni
79 P 21
Fe
70 C r iO P 13 C 7
Fe
80
Pt
60 N i 15 P 25
M
°60 R e i6 P 10 B 10
Fe
32Ni36Cri4Pl2B6
40
N1
P
40P14B6
16C3B
Disordered alloys of this kind have high strength and corros ion resistance, extreme hardness and wear resistance/and compare favourably with conventional crystalline metal alloys. Some amorph ous alloys have unique magnetic properties with low hysteresis losses, low permeability etc . Related to these alloys are metal/ P/S and metal/P/Se glasses which can be used as electrical conduct ors or as optical fibres (Section 2.4). Monophosphides MP The monophosphides, MP, where M = Ti,V,Cr,Mn,Fe,Co,Ru,W,Mo,Zr, Nb, or Ta, are greyish black with metallic lustre and they have very high melting points and densities. Three types of hexagonal structure which are found amongst this group of compounds are indic ated in Fig 2.5abd. Trigonal prismatic coordination of the phosphor us by the metal atoms is mostly present, and the bonding is probably part metallic and part covalent. Most of these compounds are hard, chemically inert and resistant to oxidation at high temperatures. Tantalum molybdenum and tungsten monophosphides have been used as nose-cone materials in space rockets. The monophosphides MP, where M = B,Al,Ga or In, form an important group of phosphides in which each atom is tetrahedrally coordinated by atoms of the opposite kind in a cubic zinc blendtype structure. Their structures are also similar to those of
63
2.2
Figure
2.5
Structures of Monophosphides.-
diamond, silicon and boron nitride (Fig 2.6). These monophosphides are hard high melting-point compounds which have important semi conductor properties. The isomorphous arsenides have similar semi conductor properties and the system GaP-GaAs has been much studied in this connection. The value of the energy gap can be controlled in solid solutions of the type GaAs x Pi_ x (Table 2.8). The compound GaAsP is utilised in light-emitting diodes. Difficulties in obtain ing samples with the necessary high chemical purity and freedom from lattice defects have so far limited the wide-scale commercial use of these materials.
(a) Figure 2.6
(b)
(c)
(d)
C r y s t a l S t r u c t u r e s of Diamond Type P h o s p h i d e s .
(a) AIP, (b) BN, ( c ) Z n S i P 2 ,
(d) diamond ( o r s i l i c o n ) .
Various mixed phosphides such as MgGeP2, CuSi 2 P 3 and isomorph ous compounds a r e semiconductors w i t h c r y s t a l s t r u c t u r e s r e l a t e d t o
64
2.2
TABLE
2-8
Physical Properties of Monophosphides and Isostructural Pnictides Unit cell (A)
Density
g/cc
mp (°C)
Energy gap (eV)
6.0 2.5
BP AIP GaP
4.538
2.9
2500
5.451 5.450
2.85 4.13
1497 1477
InP
5.869
1057
1.27
AlAs
5.662
4.79 3.81
1598
2.16
GaAs InAs
5.653 6.058
5.32
1298
1.35
5.66
1942
0.36
AlSb GaSb
6.135
4.22
1057
1.60
6.095
5.62
0.67
InSb
6.479
5.77
707 525
2.24
0.16
those of the simple monophosphides in Fig 2.6. Small quantities of P atoms ( ^0.001 %) when substituted for Si atoms in the silicon lattice enhance its semiconductor properties . There are two well-characterised boron phosphides, BP and B 1 3 P 2 . The monophosphide exists as an amorphous brown powder or as red brown cubic crystals with the zinc blende structure. The second phosphide has a structure based on the complex icosahedral frame work of elemental boron and is believed to be isostructural with the compound B 1 2 C 3 (Fig 2.7).
Figure
2.7
Structure of B
P .
Open circles represent end views of P-B-P chains which link together B cages.
The monophosphide is made by heating boron with red phosphorus at about 900°C in sealed tubes. It is stable at ordinary temperature and up to 2500°C under pressure. Heating in vacuo induces decompo sition to the icosahedral boride (30). Thermal decomposition of some boron trihalide addition compounds will yield the monophosphide (31) and displacement from another metal phosphide may also be used (32) (33). B
red
H 3 P.BC1 3 BC1 3
*
->
BP
->» BP AIP -
-> BP , -
">
B
13 P 2
3HC1 AlClr — 3
white
(30) (31) (32)
The B/P ratio is slightly variable and the formula is sometimes quoted as BgP
65
2.2 Zn P
+
^
2B
2BP
+
3Zn
(33)
The crystalline monophosphide is inert, it is harder than most metal borides and is as hard as silicon carbide and nearly as hard as boron nitride. It resists oxidation up to 800°C and is not dissolved by boiling mineral acids or cold concentrated alkali. Boiling with the latter produces phosphine and with steam above 400°C, some phosphine and boric acid are formed. Boron phosphide reacts on heating with halogens to form addition complexes. When heated to high temperature in an atmosphere of ammonia, cubic boron nitride and phosphine are formed (34). BP
+
NH
2A1P
+
3H
2
> S0
4
>
BN 2PH
+ 3
+
PH A1
(34)
2(S°4)3
(35)
Aluminium phosphide, AIP, can be made by reacting Zn 3 P 2 or red P with excess aluminium above 900°C. Whereas boron phosphide is not attacked by water, aluminium phosphide is slowly hydrolysed in moist air, but more violently by water or dilute acid (35). Alumin ium phosphide tablets are used for grain fumigation since they will slowly release phosphine under storage conditions. Decomposition by water is a property of ionic phosphides (see below), but highly purified AIP is reported to be water stable. The monophosphides of the elements Ca,Sr,Y,Sc, and the lanthanides La to Th all crystallise with ionic-type structures, thus suggesting they may contain assemblies of anions and cations. Uranium phosphide, UP, mp = 2610°C, has a rocksalt-type struc ture (Fig 2.4d). It slowly dissolves in dilute acids evolving phos phine, but the sintered variety is inert to boiling water. It does not react with U 0 2 below 2500 C and is of interest as a nuclear power material. The monophosphides SiP, GeP and SnP are inert high temperat ure materials which can be made by direct combination of the elem ents. They exist in a variety of crystalline forms containing Si—Si and S i — P type bonds. Ionic Phosphides The phosphides of the alkali and alkaline earth metals prob ably all contain ionic bonds. Those with formulae M 3 P (M = Li to Rb) and M 3 P 2 (M = Be to Ba) liberate phosphine on contact with water and may all contain the simple phosphide ion P 3 ~ (Fig 2.5c). Metal phosphides such as LaP, CeP,Th 3 P 4 Zn 3 P 2 , LiMgP, Li 3 AlP 2 and others (see above) also liberate phosphine on contact with water or dilute acid. In general they exhibit salt-like character and ionic-type crystal structures such as rocksalt (LaP) or fluorite (LiMgP) (Fig 2.4c,d). P
~P—P"
~P—P—P~
~P—P—P—P~
¥t
P—P4-P+-P—P
(36)
The existence of polymerised phosphide ions (36) is now firmly established. Both the monophosphides CaP and SrP consist of an
2.2
66 assembly of cations and P 2 4 anions. On contact with water they liberate diphosphine (Section 2.5), which, like the anion P 2 4 T contains a covalent P — P linkage (37). 4H 2 0
2CaP
Ca 3 (PH 2 ) 2 .5NH 3
■>
H2P-PH2
CaP
">
(37)
2Ca(0H), Ca
(38)
3P2
Pure calcium monophosphide is best prepared by passing phos phine into calcium dispersed in liquid ammonia. The addition comp ound which is formed can then be heated at 150°C to give the monophosphide which above 600°C converts to a lower phosphide with loss of phosphorus (38). The lower phosphide liberates phosphine on con tact with water (39). It is made commercially by heating quicklime in phosphorus vapour, and together with Mg 3 P 2 , it finds applicat ion in sea flares. Sea flares are spontaneously inflammable due to the presence of small quantities of diphosphine which arise from impurity CaP. Reaction (37) can be compared with the action of water on calcium carbide which liberates acetylene, HC==CH . 2Ca
6H 2 0
3P2
-^ 2PH.
(39)
3Ca(0H),
The triphosphide, P 3 and pentaphosphide P5 ions have been identified in black crystalline LaP2 and the tetraphosphide P 4 6 ion is present in CeP 2 . Infinite polyphosphide P n n ~ chains exist in crystalline NaP and KP. All these compounds hydrolyse in water to PHq P n H n etc. give mixtures of various polyphosphines
Figure
2.8
Phosphide Anions
. (a) Pr
(b) P
11
(c) P
16
The interesting cage anion P 7 (Fig 2.8a), is of similar shape to P/+S3 (Section 2.4 below). It occurs in red S r 3 P 1 4 , Ba 3 P 1 4 and the highly coloured alkali phosphides M 3 P 7 where M = Li to Cs. Hydrolysis of these compounds yields mainly a hydride of composition P7H3 (Section 2.5 below). The alkali phosphide Li 3 P 7 can be obtained in 95 % yield by reaction (40). The special geometry of the P 7 3 anion makes possible valence bond tautomerism (Chapter 1.4) 3P.
6LiPH
2Li
3P7
4PH
(40)
Another cage anion of unusual shape is P 11 3 ~which is found in Na3Pi:L(Fig 2.8b) On hydrolysis this yields P U H 3 . If the sodium salt of (40) is heated with tetraphenyl phosphonium chloride, the
2.2
67
main product is (Ph u P) 2 P 16 which contains the anion (Fig 2.8c). These cage anions are all built from 3-linked P— and twolinked PC atoms, the latter bearing a formal negative charge. Arsenide analogues are known. p^P-p1
1_ 1 Ρ ·^Ρ^Ρ
p>-P^p
1-
i
II
P^P^P
p^P^p I I 11P^P^P
p^P^p1
1-
1
1-
P^P^P
ll
P
-P--
1-
P
P-
|
P
~
1
P ^ P - ^P
(41)
Two other phosphides attacked by water are K U P 6 and Rb u P 6 . These compounds both contain flat hexagonal P 6 4 ~ring anions in which all the bonds have the same rather short lengths of 2.15 A. Some multibond (π) character and a uniform spread of the four formal negative charges around the ring is indicated for these anions. In terms of conventional valence bond structures, contribu ting forms may be as (41). In contrast to the phosphides already mentioned, there are a whole series of alkali and alkaline earth phosphides which contain a relatively large proportion of phosphorus and which are insoluble and not easily attacked by water. Compounds of this kind contain highly polymerised chain, sheet or three-dimensional anions in which comparatively few P atoms form only two linkages. The two-linked (b)
(c) Figure (a)
2.9 KP15
Chain Phosphide Ion Structures (b) TIP 5
(c) BaP 3
(d) RbP ?
P atoms are associated with a formal negative charge although appar ently not conferring the same ionic properties as possessed by the lower phosphorus content compounds just discussed. Examples of in soluble chain anion structures are found in LiP 5 , LiP 7 , K P 1 5 , BaP 3 LaP 7 , ZnPbP 14 (Fig 2.9). RbP^ TIP. The infinite chains in KP 1 5 resemble those in Hittorf phos phorus (Fig 2.2). Many of these compounds can be regarded as derived from a basic polymerised phosphorus network in which a few of the P atoms are linked to two rather than to three other P atoms. The formal negative charges so introduced have to be balanced by the introduction of a suitable number of cations into the structure. There are probably many more phosphides of this type which are capable of synthesis.
2.2
68 Phosphorus-rich Phosphides
MxPy
(χ < y)
Phosphorus-rich phosphides of most metals other than alkali or alkaline earths (above) contain polymerised P atoms, but cannot be satisfactorily represented by ionic formulae. In their structure each P atom is covalently linked to at least one other P atom and up to three metal atoms in a tetrahedral configuration. On heating, these compounds lose phosphorus and usually revert to monophosphide or a metal-rich phosphide. Semiconductor properties are frequently found amongst these compounds. ^Ni -Ρς—Ni
I
I
-P-
I
V I
-Ir
•SIL
/ V
(b)
(a)
I
I
I
^Pv
^Sn
I x NP/ N> \P
/P
\>
(42) (c)
Dimeric covalent P — P units are found in PtP2 FeP 2 , 0sP 2 , and RuP 2 (42a). Square four-membered rings of P atoms occur in MP 3 type phosphides where M = Co,Ni,Rh,Pd or Ir (42b). Chains of polymerised P atoms are found in PdP2 ,NiP2 ,ZnP2, TiP2 , and CdP 2 . and sheets in CuP2.CdP4 , and in SnP 3 . The last compound has a layer structure similar to that of orthorhombic black phos phorus in which every fourth P atom is replaced by an Sn atom (42c) Recently InP3 has been found to be isostructural with SnP 3 , but A1P3 remains an unknown compound.
Figure
2.10
^r
Structural
Units found in Re_P,_. 6 13 (Open circles P atoms)
The crystalline compound Re 6 P 1 3 is remarkable in that it contains P 2 dimers, P4 chains and P6 rings (Fig 2.10).The ternary phosphide Cu 4 SnP 1 0 , prepared by direct synthesis from the elements, contains the highly symmetrical adamantane-type P 10 cage unit (43).
.A/
v
'_/
V-
P'
(43)
2.3
69
A number of phosphide-sulphides and phosphide-selenides are known e.g. CuPS, CuPSe, AgPS , PdPS, PdPSe, NbPS and TaPS. The compounds MPS (M = Rh,Co,Ni,Ir) are semiconductors. Compounds MPS 3 contain the P2S£~anion and are also of interest for their electric al properties (Chapter 7 ) . Metal phosphide-nitrides can be obtained by the reaction of P 3 N 5 (Section 2.6) with lithium (43A) and magnesium (43B) nitrides. The compounds obtained are formally salts of phosphenimidic nitride HN=P.^N, and phosphenodiimidic amide (HN) 2 P—NH 2 respectively. o P N + Li N 800_C ^ 3LiPN (43A)
+
P3N5
2M
g3N2 '
3M
I^IS—^
e 2 PN 3
(43B)
The lithium compound contains PN 4 tetrahedra which link to gether by sharing all four of their corners to build up a contin uous 3-dimensional cristobalite type structure (Chapter 3.2), which contains the lithium (cations) in the cavities.
2,3
OXIDES 0
ϋ^\ / 0 - -P.
rC/Xf
r^>
i i^r P^T jkzr u^r 0
(a) (b) (c) (d) (e) Five oxides of phosphorus form a series of molecules which are based on a tetrahedral arrangement of atoms (44). The two end members of the series correspond to the well-known trioxide, P 4 0 6 (P 2 0 3 ), and the pentoxide P u 0 1 0 ( P 2 0 5 ) . Their symmetrical dimeric oxide structures have been confirmed by vapour density, electron diffraction and x-ray diffraction studies. The structures may be regarded as derived from a Pz* tetrahedron by adding oxygen atoms to the centres of the edges, and then to each of the corners as well. Phosphorus tetroxide, Ρ20** , consists of various proportions of mol ecules with structures (44b-d), which contain P atoms in two diff erent valency states. In all these structures the terminal P — 0 bonds have multiple character and are considerably shorter than the remaining P — 0 — ( P ) bonds in these molecules. These oxide struct ures have many analogues in chemistry, as e.g. in the phosphorus sulphides (below). (Table 2.10). Phosphorus Pentoxide Phosphorus pentoxide, Ρ 4 0 1 0 , is obtained when phosphorus is burnt in an excess of dry air or oxygen. On the commercial scale,
2.3
70
phosphorus vapour is burnt in a specially designed burner, in a current of air which has been dried by refrigeration or other methods. The phosphoric oxide vapor so formed passes to a cooling chamber where it condenses to the familiar white powder. It may be purified from possible traces of the lower oxides by sublimation in an oxygen atmosphere. Phosphorus pentoxide is an exceedingly hygroscopic white pow der, density 2.30 g/cc, which sublimes at 359°C, but if heated rap idly melts at about 423°C. It combines avidly with water, forming orthophosphoric acid (45). The oxide finds much use as a desiccant, P 0 4 10
6H
2°
(45)
4H 3 P0 4
but has the great disadvantage that it 'skins' with a mixture of metaphosphoric acids, formed as intermediate products in reaction (45). This effect may be alleviated to some extent by spreading the oxide over a large surface such as glass wool. Industrial uses are many and varied. Worthy of note are de hydration in methyl methacrylate manufacture, the preparation of triethyl phosphate (6-18) and phosphorus oxychloride (53)(54), and for raising the softening point of asphalt by inducing cross-linking. -10° C + 4HP0. (46) P 4HN0 4°10 ■> 2 N 2°5
2H2S04
+
P
4HC1CK 4
+
P
+
P
C0NH2 CONH
4°10 4°10 4°10
cnr
^COOH
-> ">
2S0 o 2C1
2°7
CN
(47)
4HPC)
(48)
4HP0.
(49)
4HPC)
(50)
CN
c=oo 4 10
+
4HP0^
^c=co
Phosphorus pentoxide will dehydrate acids, forming metaphos phoric acid and the acid anhydride (46)-(48). It will also remove water from organic compounds as for example amides and carboxylic acids (49)(50). Under controlled conditions of hydrolysis, the cage-like mol ecule of Ρ 4 0 1 0 will disintegrate by rupture of successive P — 0 — P linkages to form a variety of products. The main process entails the rupture of two P — 0 — P linkages to give cyclic tetrametaphosphoric acid as indicated in (51). Ice-cold alkaline hydrolysis can be used to prepare the sodium salt of this acid in nearly 100 % yield. The next stage of hydrolysis results in breakage of the ring to give linear tetraphosphoric acid, which may then hydrolyse further by either of the two routes indicated. In a parallel process, a small proportion of cage molecules hydrolyse by breaking a ring bond first, giving rise to trimetaphosphoric acid and isotetraphosphoric acid. In all cases, however, the final hydrolysis product is orthophosphoric acid.
2.3
71
/1\
H20
^
i
•s^ Λ
/
0=F^-OH 0
\
^O-f-O ÖH
I
°vj _^——f~* 0 H
N
\H20
OH
+
0
OH
I
I
OH
OH
OH /2H.O
1
1
OH
OH OH
OH
OH
1
I OH
I
OH OH
OH
|
HO—P=0 — ^
HO—P=0 11 OH
1
1 OH
I
HO—P=0
OH
OH
1
°T° OH
OH
0
II
+
\>H
I 0— P — 0— P = 0 II I
OH -
+
j /
N 7°
1
0—P —0— P = 0
o
Ηθ" > 0 ^
OH
1
I 0 = P —OH I on
I 0=P—
0=P—0— P=0
x
o'
I ■ 0 — P — 0 — PI = r O 0=P— OH OH OH
^
0 — P— 0—P=rO
HOv
I
OH 1
•OH ' 0H 07-
4
HO-P=0
\
0~P—OH
"\
OH
0=P-OH
0
(51)
\
!
(T 0 H X 0
^
main < \ P " . ° tl
^0 OH 0 0=P-0H χ ^ \
... OH
2
Ό
HO-P=0 0 = PP-OH - O H 0 HO-1
YQ P==o K> —
> OR
,0
I
4 HO— P = 0
OH
I
OH
Phosphorus pentoxide reacts with dry or wet ammonia to produce a variety of both linear and cyclic amido derivatives and ammonium salts. In addition, bridge oxygen atoms may also be replaced, giving P — N H — P linkages. The course of low temperature ammonolysis is probably similar to that of hydrolysis (52). With sodamide, mono and diamido salts are obtained directly (Chapter 5 ) . Ethyl alcohol reacts with the pentoxide to give a mixture of ethyl phosphoric acids (Chapter 6 ) . Phosphoryl halides are obtained from phosphorus pentahalides
o
2NH
3
y
ό
\£
0 ^
H
2
N
X
p
,PC
ONH
NH 2
.
o
P H 4 N0
2
V
v
t--
0NH,
^°
NH 4 0
^•p
H
^*Q
H2N
0
,NH4
N
Λ
->
ΌΝΗ. / 2NH 3
V
2NH„
I
% / 0 ^
N
NH 2
H 0 N — P — 0 — P —NH 2
I
I
ONH,
ONH.
(52)
2.3
72
(53) or hydrogen halides (54), but with hydrogen fluoride the pro duct is hexafluorophosphoric acid (216) or phosphorofluoridic acid. (245). P
+
4°10
P 0
+
6PC1
5
^
10P0C1 3
>
3HC1
P0C1
(53) +
3
3HP0
<54>
3
At 500°C phosphoryl chloride can be obtained by heating the pentoxide with rocksalt (55), and if calcium fluoride is added, mixed phosphoryl halides are produced together with calcium metaphosphate (56). With calcium fluoride alone, phosphorus pentafluoride is ontained (181). Metaphosphates are also produced by heating with sodium carbonate (57). P 0 4P 0 T;
3P
+ JL U
4°10
6NaCl
+ 6NaCl + 3CaF +
6Na
> 2P0C1 £
2P0F Cl + 2P0FC1 ci
2C°3
^
+ £
2Na PO
(55)
+ 3/n{Ca(P0 ) } ό
4(NaP0 3 ) 3
+
£
n
6C0 2
(56) (57)
At high temperatures phosphorus pentoxide attacks fused silica ware and many silica-containing ceramic materials. Above 400°C it is reduced by carbon (58). The reduction of Ρ 4 0 10 (obtained from the dehydration of phosphoric acid (45)) by carbon;formed the basis of the earliest commercial preparation of elemental phosphorus (Sect ion 2.1) . P 0
+
IOC
^ 10CO
+
P
(58)
Phosphorus pentoxide exists in at least four polymorphic forms as well as a glass. The common variety of laboratory and commerce already discussed, consists of cage molecules (44a) arranged in a hexagonal (rhombohedral) crystal lattice with only weak van der waal-type forces between the cages. This is sometimes known as the 'Η' form or 'Form I' of phosphorus pentoxide. There are also two orthorhombic forms which contain highly polymerised arrangements ( P 2 0 5 ) n . The 0' form or 'Form II' has a sheet structure built from rings of six P 0 4 tetrahedra (59b), and the 0 form (Form III) is built from puckered rings of ten P0 4 tetrahedra which are linked laterally to form a three-dimensional structure. Another solid form I
I
1
?
9
9
^p^
Λ^
/Pv
/°\
I
I
0=P — 0 — P = 0
0
0
.
ό
/k
0
X
(b) 0
<59>
2.3
73
of unknown structure exists at high pressures. The vapour is stable up to at least 1400°C, but the existence of dimeric species such as (59a) may be possible. The t h r e e forms which a r e s t a b l e at ordinary p r e s s u r e s have i n t e r e s t i n g d i f f e r e n c e s in p r o p e r t i e s which can be r e l a t e d t o t h e i r c r y s t a l s t r u c t u r e s . The highly polymerised 0 and 0 f forms have higher melting p o i n t s and higher d e n s i t i e s than t h e H form. On h e a t i n g in a closed system, the H form transforms f i r s t i n t o t h e 0 form which then changes i n t o the more s t a b l e sheet 0 ' form. All t h r e e forms can be vapourised to produce the same cage molecules as e x i s t i n t h e H form, but t h e r e are d i f f e r e n c e s in the l i q u i d s produced by m e l t i n g . The H form f i r s t melts a t 420°C t o produce a m e t a s t a b l e l i q u i d with a high vapour p r e s s u r e and c o n s i s t i n g of d i s c r e t e P u 0 1 0 u n i t s . This l i q u i d then r a p i d l y polymerises t o form a g l a s s c o n t a i n i n g some c r y s t a l s of the 0 form. The 0 and 0 ' forms melt at 562°C and 580°C r e s p e c t i v e l y , to give viscous l i q u i d s with much lower vapour p r e s s u r e s than t h e l i q u i d H form. The l i q u i d orthorhombic forms presumably contain r e l a t i v e l y large fragments of the o r i g i n a l polymers and a r e consequently not e a s i l y v o l a t a l i s e d to P u 0 1 0 m o l e c u l e s . Both orthorhombic forms a r e considerably l e s s d e l i q u e s c e n t than the rhombohedral form. The H form r e a c t s v i o l e n t l y with water evolving much h e a t , whereas the 0 ' form r e a c t s much more slowly, evolving l e s s heat and forming a s t i f f gel which slowly d i s a p p e a r s in s o l u t i o n . This gel probably contains fragments of the sheet s t r u c t u r e in various s i z e s and s t a t e s of h y d r a t i o n , the f u r t h e r a c t i o n of the water then causing more breakdown and eventual s o l u t i o n . The 0 form d i s s o l v e s in water very slowly, even at 90°C, and t h i s i s because of t h e g r e a t e r d i f f i c u l t y of p e n e t r a t i o n of t h e t h r e e dimensional s t r u c t u r e by t h e water molecules. The h y d r o l y s i s products from t h e orthorhombic forms a r e i n i t i a l l y much higher molecular s p e c i e s than those obtained from t h e common H form. Unlike the l a t t e r however, t h e i r path of eventual breakdown t o phosph o r i c acid i s not properly known.
Phosphorus T r i o x i d e P h o s p h o r u s t r i o x i d e , P 4 0 6 , c a n be made by b u r n i n g p h o s p h o r u s i n a r e s t r i c t e d s u p p l y of o x y g e n . I t i s a c o l o u r l e s s c r y s t a l l i n e m a t e r i a l w i t h a m e l t i n g p o i n t of 2 3 . 8 ° C and a b o i l i n g p o i n t of 1 7 5 . 4 ° C and i t c a n be c r y s t a l l i s e d from c a r b o n d i s u l p h i d e s o l u t i o n . The s o l i d i s b u i l t from t e t r a h e d r a l u n i t s ( 4 4 e ) w h i c h a r e s i m i l a r t o t h o s e e s t a b l i s h e d i n t h e v a p o u r by e l e c t r o n d i f f r a c t i o n and vapour d e n s i t y measurements. T h i s o x i d e h a s an u n p l e a s a n t s m e l l and i s v e r y p o i s o n o u s . I t o x i d i s e s r a p i d l y i n a i r t o t h e p e n t o x i d e and t a k e s f i r e i f h e a t e d . The v a p o u r i s c o n s i d e r a b l y l e s s s t a b l e t h a n t h a t of t h e p e n t o x i d e , and i f t h e s o l i d i s h e a t e d s t r o n g l y i n t h e a b s e n c e of a i r , i t d e composes t o t h e t e t r o x i d e and r e d p h o s p h o r u s ( 6 0 ) . 200° C 2P40g > 3P204 + 2P (60) P
4°6
+
6H
2°
>4H3P°3
( 6 1 )
W h i t e p h o s p h o r u s w i l l d i s s o l v e i n P 4 0 6 t o t h e e x t e n t of 1.7 g p e r 100 g of P 4 0 6 . Each m o l e c u l e r e t a i n s i t s i d e n t i t y b u t t h e p h o s p h o r u s c a n b e removed by c o n v e r s i o n t o t h e r e d form by UV l i g h t . I t c a n t h e n be s e p a r a t e d from t h e o x i d e by s o l u t i o n of t h e l a t t e r i n
2.3
74
CS 2 . With an excess of cold water the hydrolysis product is phosph orous acid (61). With hot water the process is more complex, and the products include phosphoric acid, phosphine and phosphorus. The cold water hydrolysis may proceed in a manner analogous to that of the pentoxide (62).
°"V° P-0
\
0'
0
/
/ 0
-» P-OH 2H 0 \ ^
0
^ 0 (62)
HO-P /
OHO P^
Phosphorus trioxide reacts violently with chlorine or bromine to produce the corresponding phosphoryl halides (63). With hydrogen chloride, phosphorous acid is obtained (150). In carbon disulphide under pressure the di-iodide is formed (64). Addition of sulphur readily takes place to give an oxysulphide, P u 0 6 S 4 (65), and with ammonia, phosphonic diamide is obtained (5-23). Phosphorus trioxide forms various addition complexes using its lone-pair electrons to complete a tetrahedral configuration. With diborane it forms P 4 0 6 nBH 3 , and with nickel carbonyl it forms Ρ^Οβ nNi(C0) 4 , where n = 1-4 (Chapter 10). Phosphorus is obtained on heating phosphorus trioxide with arsenic or antimony in a sealed tube (66). 4 6 5P„0^ 4 6 4 6 4 6
Br
2
8I
2
4S 4Sb
->
P0Br„
■> ^
4P I 2 4
■> ■*
P
4°6 S 4
Sb
4°6
(63) +
3P 0 i n 4 10
(64) (65)
(66)
Phosphorus Tetroxide Phosphorus tetroxide, f P 2 0 4 f , which can be made by thermal decomposition of the trioxide as in (60), forms white crystals which sublime at about 180°C. At about 350°C oxidation to the pen toxide takes place in air. The oxide dissolves in water with cons iderable evolution of heat to give a mixture of phosphorous and phosphoric acids. There are two crystalline forms of this oxide. The rhombohedral α-form contains P 4 0 8 and P 4 0 9 molecules in varying proportions to give an average composition in the range P u 0 8 m λ -P.0 portions which cover a composition range P 4 0 7 7 -Ρ 4 0 8 . 0 · T n e u n i t cell dimensions of these forms remain almost constant, but their crystal densities vary with composition. Completely pure samples of the three types of molecule (44b-d) are difficult to obtain, but
75
2.3 there is evidence that Pu0 8 exists in the amorphous state.
Miscellaneous Oxides Molecules or radicals such as PO, P 0 2 , P0 3 2 ~have been obser ved spectroscopically . The diatomic molecule PO has an interatomic distance of 1.447 A corresponding to a multiple bond. Early workers described the existence of several solid yellow or orange sub-oxides to which various empirical formulae such as P 2 0 and P u 0 were assigned. A stable brown solid, insoluble in wat er, with empirical formula PO can be made by the electrolysis of anhydrous phosphoryl chloride at 0°C (67)(68). P0C1 3nP0Cl
> +
>
P0C1
2
(PO)
+
C1
+
(67)
~
2nP0Cl
+
+
2nd"
(68)
These amorphous unreactive and ill-characterised solids prob ably belong to a class of polymeric phosphorus networks to which various terminal groups such as H or OH may be attached. A contin uous range of composition between P u 0 6 and P 4 may be possible, with colours ranging from white through yellow, orange and brown to red. A violet solid of composition P 2 0 6 > phosphorus peroxide, is formed by condensing a mixture of oxygen and pentoxide vapour at low pressure under an electric discharge. The peroxide reacts with water to form peroxydiphosphoric acid (Chapter 3 ) . Pnictide Oxides The oxides of nitrogen bear little resemblance to those of the other pnictides, but arsenic, antimony and bismuth form oxides with the same empirical formulae as those of phosphorus : P
2°3
P
2°4
P
2°5
AS
2°3
AS
2°4
AS
2°5
Sb
2°3
Sb
2°4
Sb
2°5
Bi
2°3
(Bi
2 0 5>
The vapours of P 4 0 6 , A s 4 0 6 and Sb 4 0 6 all have the same molec ular structure (44e) and these structural units persist in the solid states where they pack into similar cubic lattices, and in solution in organic solvents. Above 800°C dissociation of A s 4 0 6 occurs and at 1800°C only As 2 0 3 molecules are present. On the other hand S b 4 0 6 is more stable and persists up to at least 1560°C. The least stable trioxide seems to be P 4 0 6 which decomposes above 210°C. The trioxides can all be obtained by reacting the elements or their sulphides in air. They show increasing thermal stability and basic character, but reluctance to oxidise to the pentavalent state on progressing from P to Bi. Although less soluble in water than its phosphorus analogue (Table 2.9), As 4 0 6 eventually produces arsenous acid As(OH) . Unlike phosphorus acid, however, the latter compound
2.3
76 TABLE
2- 9
Properties of Pnlctide Trloxides
P
4°6
mp
bp
(°C)
(°C)
23.8
175.4
solubility
density
g/100g H 2 0
g/cc
vs, d
2.13
acidic
2.04
3.7
weakly acidic
As406
218
-
S
\°6
655
1425
0.002
5.67
airphoteric
Bi20^
817
1900
vss
8.9
weakly basic
does not exist in tetrahedral form with an A s — H linkage (Chapter 3) In addition to the cubic forms based on discrete AsuOe and S b 4 0 6 molecules, there are alternative crystalline forms of ( A s 2 0 3 ) n and ( S b 2 0 3 ) n which are highly polymerised structures. In all of these, the pnictide atoms form pyramidal configurations of three bonds to 0 atoms, which are linked to give two-dimensional sheet structures as in ( 6 9 ) .
?1
,
|
J
i1
As
f
J1
1
As ^As^
I
^
1
1
X
As^
Is 1
^
^As
1
1 01 I
As
^As
(69)
0
1
^As
X 0 0^
M.
^As
1 The double oxide As 2 0 3 .P1 2 0 5 , sometimes described as arsenic phosphate, AsP0 4 , contains a network of ASO3 pyramida and P0 4 tetrahedra which share all their corner 0 atoms to give a continuous structure . The arrangement is similar to that adopted by ars enic tetroxide, AS2O4, which can be formulated as As 2 0 3 .As 2 0 5 . Among the pentavalent oxides, P 2 0 5 is thermally the most stable and Bi 2 0 5 the least. Whereas P^Oiois produced on heating Pu0 6 in oxygen, the arsenic analogue cannot be made by this method. On heating in air, As 4 0 10 loses oxygen. Although P u 0 l o and As u 0 10 both dissolve in water to produce similar ortho acids H 3 P0 4 and H 3 As0 4 , their solid state structures are quite different. In contrast to discrete Ρ^0 1 0 molecules,
2.4
77
c r y s t a l l i n e a r s e n i c p e n t o x i d e i s b u i l t from As0 6 o c t a h e d r a and AsOu t e t r a h e d r a which s h a r e c o r n e r s t o p r o d u c e a c o n t i n u o u s t h r e e - d i m e n s i o n a l s t r u c t u r e . O c t a h e d r a l c o o r d i n a t i o n of p h o s p h o r u s by oxygen does not o c c u r i n p h o s p h a t e s t r u c t u r e s (Chapter 3 ) . TABIE 2-10 Adamantane Type Structures A Sjj Se 6
Ρ
P Se
As^fWe)^
p
ti°6 s H
B
Pl4(NMe)6
(W?)H(Cti2)6
p
*s604
^ 1 0
P(CH) 3 (CH 2 ) 3 O s
(CH) 1) (CH ? ) 6
P
As
(CH)„S6
Ve10
Ga Se
(SiH) l 4 (SiH ? ) 6
P^NMe
In^S108-
P
H°G 4 6
4°6
As^Sg
As,4(NMe)g
Λθ
S
i| S 10
Ga
i) 10
3-
/) S 10 "
n io8
The P u 0 l o and PU 0 6 structures were the first examples found in phosphorus chemistry of the adamantane, (CH) u (CH 2 ) 6 or hexamethylene tetramine, (NH 2 ) u (CH 2 ) 6 type tetrahedral structures. Many examples of this structure type are now established (Table 2.10).
2Λ
SULPHIDES
AND
SELENIDES
The phosphorus sulphides can be prepared by heating mixtures of red phosphorus and sulphur in an inert atmosphere, or by react ing white phosphorus with sulphur in a high boiling point solvent. The phosphorus sulphides, which are formed above lOCPc, are all sol uble in carbon disulphide but are generally less stable than the oxides. They dissolve in water only with decomposition.
Pse
KΓ^ ^ v h \ r
» ! i-
p-
Liquid
\
\
\
\
Γ Γ
\
P4
^^\
af ♦ liquid
\
^/p
*
p-
/
I 1"
1 /
/
\
Mixture of < and fl h
♦ liquid
\ \
/
1
.
1 1 1 40 60 Atom-·/, of phosphorus
l_
, , 1
Figure 2.11 The System Phosphorus-Sulphur -below the reaction temperature.
2.4
78
If white phosphorus and sulphur are mixed together at temper atures below 100°C, solid solutions are formed as indicated in the phase diagram (Fig 2.11). The α-phase has the crystal structure of orthorhombic sulphur built from Ss rings with Pu molecules in solid solution. The ß -phase, on the other hand, has the structure of white phosphorus, with Se ring molecules in solid solution. The properties of elastic sulphur, Sy , can be stabilised by adding a few percent of phosphorus. A vulcanisation process occurs in which crosslinking is produced via the P atoms, and as more phosphorus is added, the product becomes more brittle and finally vitreous when crosslinking is excessive. The firmly established phosphorus sulphide molecules P4S3, P4S4 , P4S5 , P4S7 and Pi+Sxo are all based on P 4 cage structures which in some cases are related to those of the oxides (Fig 2.12). These have been confirmed by X-ray and electron diffraction, and by NMR studies. The structures of P 4 S 9 and P 4 S 1 0 are analogous to those of the oxides. The remainder are almost unique in chemistry except for the following pnictide chalcogenides which are known to form isostructural molecules : P
P
4S3
α
4 4
4Se3
P 7 3As 7 3-
4 N
P S
4 4
„ ,i
4
„4S „4 4
As3S4+ + As 3 Se 4
B
P S, i „
„4S *5
P
4S7
P
^4 S e c5
P
4Se7
P,S i A 4 10 p
„Se_ 4 10
s> i N ~ E
4 5
4 4 4
As
P
„4S e „4
AS
4S4
AS
4S5
AS
P
4S10
4S9N"
As 4 Se 3 The industrially important phosphorus sulphides are P 4 S 3 and P4S10 Phosphorus selenides have been much less studied than the sulphides. Reported compounds, additional to those listed above, include P 4 Se, P 2 Se and P 2 Se 3 . The best defined telluride is P 2 Te 3 . Phosphorus selenides form glassy phases more readily than phosphorus sulphides. During the last 100 years the existence of numerous phosphor us sulphides has been claimed, with compositions ranging from P 4 S to P4S24 . It seems likely that some of these e.g. P 4 S 2 do in fact exist, in addition to the confirmed ones mentioned above. The exist ence of P4S6 has been persistently reported, and molecules of this composition are probably present in a sulphur-deficient form of P 4 S 7 (below). At least five types of phosphorus-sulphur anions exist (Chapter 7 ) . Some properties of the four longest-established phosphorus sulphides are listed in Table 2.11.
2.4
Figure 2 . 12
79
Molecular Structures of Oxides and Sulphides of Phosphorus
2.4
80 TABLE 2-11 Properties of the Common Phosphorus Sulphides P4O7
305-310 523 2.19 0.029 decomp nearly white Monoclinic
Melting point °C 171-172 170-220 Boiling point °C 407-^08 Density g/cc 2.03 2.17 10 Solubility in CS2/100g 100 Action in air slow oxidation Colour yellow yellow Crystal system Orthorhombic Monoclinic s
s s
i00
s s
I 1 · 86
s
1103 ^v£0
P
s
'feH
2.235
1124
11.91 ^Pv^.io S 109S S 1110
"^v
'^
(70)
*^43^ (d)
(c)
(b)
(a)
P4S10
286-290 513-515 2.09 0.222 slow decomp yellow Triclinic
Phosphorus sulphide melts with low P/S ratios contain mainly the established cage molecules in Fig 2.12, but with high sulphur contents, long chain polymers are also present. An insoluble yellow powder of composition (PS) n can be made by reacting magnesium with thiophosphoryl bromide (71). 2PSBr
3Mg
2/
n
(PS) n
3MgBrr
(71)
Sulphur dichloride will combine with phosphorotetrathioic acid with the elimination of hydrogen chloride (72). Further react ion of the remaining SH groups enables three dimensional networks of general composition (PS ) x to be built up. HS^ S-^P—SH + Cl.S. HS
,SH \ S-^P~S — S — S — P ~ S + 2HC1 HS SH
(72)
Purification of P4S3 and P 4 S 1 0 can be effected by recrystallisation fromcarbon disulphide, but the less soluble P 4 S 7 is best purified by solvent extraction of the impurities. The order of thermal stabilities is : P4S3
>
P^S7
>
P4S10 >
P4S5
Tetraphosphorus trisulphide (also known as phosphorus sesquisulphide) and the heptasulphide show no appreciable decomposition up to 700°C, but the pentasulphide decomposes at its melting point to give tri and hepta sulphide (73). Similarly the disulphide, mp = 47°C, decomposes according to reversible reaction (74), which can also be used for its preparation 2P„Sc 4 5
4 7
(73)
81
2.4 3P 4 S 2
> 2P 4 S 3
<
2P 4
+
(74)
Tetraphosphorus decasulphide (still known often as 'pentasulphide') decomposes a few degrees above its boiling point, but this decomposition is reversible since the compound can be purified by distillation. If this sulphide is melted and re-solidified, some decomposition occurs and the product consists of a mass of crystall ine P 4 S 1 0 embedded in a mixture of amorphous P^Sc, and sulphur. Vapour density and mass spectra indicate the vapour contains P 2 S 5 molecules which seem to be a good deal more stable than their poss ible oxy analogue (59b). Free radicals may be produced in the de composition of the decasulphide since the vapour can be condensed on a cold surface to give a green solid. In carbon disulphide solution, P 4 S 3 readily adds on sulphur and undergoes rearrangement to give a -P4S5 . In view of the low thermal stability of the latter, this is the best method for its preparation. On cooling a CS 2 solution, this compound rearranges to g i v e ß P 4 S 5 . if a CS 2 solution of P 4 S 3 is allowed to oxidise in air, an amorphous pale yellow precipitate of composition P 4 S 3 0 u is formed. The sesquisulphide, P 4 S 3 , is used together with potassium chlorate, powdered glass and glue, to make 'strike anywhere' match es, which were first introduced by SdV£n
+
18S
P
>
4 S in
+
8FeS
(75)
The heptasulphide exists in two crystalline forms a and ß . The 3 form is composed of wholly symmetrical molecules of the type indicated in Fig 2.12. The a form however, has an approximate comp osition P 4 S 6 - 5 and is believed to contain a small proportion of molecules with the composition P 4 S 6 or P4S5 corresponding to either one or two sulphur atoms missing from terminal positions in the P 4 S 7 molecule. The nonasulphide, P 4 S 9 , can be made by heating appropriate proportions of hepta and decasulphide (76). Alternatively it can be made by reacting the decasulphide with either PCI 3 or PPh 3 in a sealed tube or in C S 2 solution (77). P
4S7
+
2P
4S10
VlO
+
PPh
> >
3
3P
P
4S9
4S9
(76)
+
Ph
3PS
(77)
Like a P4S7, the nonasulphide has a variable composition and may appear with a sulphur deficiency corresponding to P4S8.5. This *
A typical formula is
: KC103 P
S
4 3
Fe
2°3
20 %
ground glass ZnO
9
glue
11 Vi'ater
14 % 7 10
2.4
82
might arise from the presence of a sulphur analogue of P 4 0 8 . Two forms of the nonasulphide are known, a with a mp = 240-270°C, and 3 with mp = 250-259°C. At 255°C the nonasulphide disproportionates to P4S7 and P|+S10 . The phosphorus sulphides differ in their hydrolytic stabili ties. The sesquisulphide reacts only slowly with cold dilute HC1, whereas the heptasulphide is readily attacked by atmospheric moist ure. The order of stability is :
p4s3
>
P4S10
>
>
PUS9
PUS7
These hydrolyses are complex processes involving the initial rupture of P — S — P followed by the P — P linkages (where present), and the eventual replacement of S by 0. There have been conflicting reports about the nature of the products which depend on pH, temperature and other factors. P
4 S 10
+
16H
2°
^
4H
3P°4
+
10H
2S
(?8)
Acid hydrolysis of P 4 S 1 0 at 100°C quickly produces only orthophosphate (78). On the other hand if cautious alkaline hydrolysis is carried out, various thioated ortho ions (Chapter 7) can be found as intermediate products. These later react under oxidative condit ions to give mostly orthophosphate P 0 4 3 ~ together with some phosph orous HP0 3 2 ~and hypophosphorous H2P02~ions.In addition, traces of phosphine, PH 3 may be produced, and those sulphides containing P — P linkages can give rise to hypophosphate P20^~ions. The decasulphide is very reactive with halogens. It reacts with PCI3 in a sealed tube at 150°C to give thiophosphoryl chloride (79). Anhydrous HF or PF 3 under pressure will give thiophosphoryl fluoride (80). A reaction also occurs with carbon tetrachloride (Chapter 7 ) . Air combustion of P4S 1 0 produces Pi,0lo and S0 2 . P S^ 4 10 P4S
+
>
6PC1
10PSC1
5 +
12HF
(79) o
>
4PSF 3
+
6H S
(80)
The decasulphide undergoes nucleophilic attack by sodium fluoride in acetonitrile, to give ions (81), which can be isolated as their n-propyl ammonium salts. By reacting the decasulphide with F
S" >>
S^ yS~ F— P— S—P— F
S^ ΐ—Ρ—S—S—P—
SF
(81)
sodium azide in acetonitrile and treating the product with n-propyl ammonium bromide, a salt containing the (N 3 ) 2 PS 2 ~ion can be isolated. Reaction of this latter salt with more decasulphide will give (Pr^N) (P 4 S g N) which contains an anion which is the structural analogue of the decasulphide, except that one terminal S atom is replaced by N. Potassium phosphorothioate is produced in a reaction with KC1 or K S (82), and with KCNS a large cage molecule is formed (7-162).
2.4 P
83 6K 2 S
4 S 10
4K 3 PS 4
(82)
Grignard reagents will react with P 4 S 3 , P4S7 or P4S 1 0 to give products in which one, two or three alkyl groups are attached to the P atom (83)(84). With alcohols, phenols and thiols, dithio est ers are the predominant products (85) (Chapter 7 ) . Primary amines 12RMgX
VlO PS
+
9RMgX
+
-> 4R 3 PS 3H 0
6MgS
GMgX^
-^3R (H)P + R P + 3MgX 2
3
2
P„S^ 4 10
+
8R0H
->
4(R0) P(S)SH
PS 4 10
+
8RNHr
->
4(RNH) P(S)SH
P4S10
+
12RNH2
->
4(RNH) PS
+
+2MgS +3MgOHX (84)
2H S +
(83)
2H 0 S
6H 2 S
(85) (86) (87)
react with P 4 S 1 0 to yield either thiophosphoric (phosphorothioic) diamides or the triamides, depending on the conditions (86)(87). Tetraphosphorus decasulphide is used in organic chemistry to convert OH, C = 0 , COOH or C0NH 2 groups into their sulphur analogues. This sulphide is an important intermediate in the manufacture of insecticides (Chapter 6) and a large industrial use is in the manu facture of zinc dialkyl or diaryl dithiophosphates for oil additives (Chapter 7 ) . Some of the reactions of P 4 S 1 0 are summarised in Fig 2.13.
PSCl
^VP12S12N14>
RNH ^
Na3PS202
Figure 2.13
+
Na3PS30
Reactions of P.S -. ^
.
·* (RO) PSSH 2 (RNH) 2 PSSH
2.4
84
The phosphorus sulphides react readily with ammonia. At -33°C in liquid ammonia, the trisulphide and pentasulphide form solid compounds with formulae P 4 S 3 .4NH 3 and P 4 S 5 .6NH 3 respectively. These compounds are in fact ammonium salts and should be formulated as (NH 4 ) 2 P 4 S 3 (NH 2 ) 2 and (NH 4 ) 5 P 4 S 5 (NH 2 ) 3 . The hepta and deca sulph ides react with ammonia to give a variety of ammonium salts and and amino-substituted ions including (NH 4 ) 3 PS U and (NH^) 2 (PS 3 NH 2 ). At -78°C liquid ammonia reacts with both Pi+Sg and P 4 S 1 0 to give cyclophosphorothioate anions (7-129). On heating with ammonium chloride, the decasulphide forms polymeric (PSN) n . The sesquisulphide reacts with iodine to form P 4 S 3 I 2 , a cage-like molecule which converts to an isomer on heating to 125°C. These isomers react with (Me3Sn)2S to give the a and 3 forms of P n S 4 (88).
P
4S3
I fI \/PI
\l/ PI I
IP
f I
(Me 3 Sn) 2 S
(88)
2Me SnI
IV? 1
The reaction between P4S3 and iodine can be made to go further (89), and zinc phosphide can be obtained with zinc (90). 7P
P
4S3
4S3
241,
16PI
3P
9Zn
3ZnS
2Zn
(89)
4S7
(90)
3P2
Phosphorus - Sulphur Glasses Various phosphorus sulphide glasses have been prepared. When P/S = 1, nearly 60 % of the total phosphorus is present as P 4 S 3 according to NMR data. Solid solutions and glasses are formed in the system P U S 3 P 4 Se 3 . Mass spectra and NMR studies indicate all species (91) can be present.
£N
1
V
/
i
S
Se S
Se
ρ 1
P
\p'—
P
S Se
/ ρI
φ
/
Se 1
P
N \
Se Se (91)
/
'
Suitable thermal treatment of P, S^ As 4 S 3 mixtures leads to _ x Xi,J Asx~3 compositions of the type P u4-x xSc »where ^ ^ ~x - -1-3. ". Mass spectra and NMR studies indicate the occurrence of six different structures in which the P atoms occupy either apical or basal cage positions
85
2.5
S
I
S
/
S !
S^
S
S
As-^As
1 /
·
Sx 1
S
S
/
1
1
/
P s / vs \ s
'
(92)
As-·!—·— As
(92). Products in the system P/S/Ge have applications as semicond uctors or in optical fibre technology, and glasses in the system P/S/Li/I are electrical conductors. The heptasulphide P 4 S 7 , reacts with bromine in CS 2 solution to give two compounds, P 2 S 5 Br 4 , mp = 90°C, and P 2 S 6 Br 2 mp = 118°C. The latter has the ring structure (93).
y^
/p\
(93)
Four oxysulphides are known : P406S4
P4S604
P4S304
P6010S5
The first of these, mp = 110°C, can be made by heating the trioxide with sulphur (65) or a sulphide-oxide mixture (94). The structure 6P
+
4°10
4P
4S10
10P
>
4°6S4
(94)
of this oxysulphide in the vapour and solid states is similar to that of P 4 0 1 0 , except that the terminal 0 atoms in the latter are replaced by S. Controlled hydrolysis of this compound will yield initially the species (95). S
s—P—o—P5-O_
">P—0-,"PVS-
(95)
The structure of the cage compound P 4 S 6 0 4 , mp = 290-295°C, is similar to that of P^S^Og with the S and 0 atoms reversed (Fig 2.12) The compound can be made by reaction (96). There is spectroscopic evidence that PuS 6 0 3 (70d) can be obtained by heating P S and P u 0 1 0 together to high temperatures. 4P0C1 3
+
6(Me 3 Si) 2 S
2.5
HYDRIDES
>>
P
4°4 S 6
+
12Me
3
SiC1
< 96 >
There are six possible hydrides based on a single P atom (97) H
H-(<Ü H
H
- H H
H /?X H
HA
f
' -
2.5
86
are well Phosphine, PH 3 , and the phosphonium cation PH characterised and the existence of the PH 2 anion has also been established. Pentaphosphorane, PH 5 , and the hexaphosphoride anion PH 6 , based on outer valence shells of 10 and 12 electrons respect ively, remain hypothetical compounds, although halides and many other derivatives are known. Long established and well recognised are the nitrogen analog ues, ammonia, NH 3 , the ammonium NH 4 , amide NH 2 and imide NH ions. Established hydrides of the remaining pnictide elements are the AsH2 and SbH2 anions, arsene, AsH 3 , stibine, SbH 3 , and bismuthine, BiH 3 . The arsonium cation AsH 4 + , has been detected spectroscopically at low temperatures, but it does not form simple salts stable at room temperature. The cations SbH 4 + and BiH u + do not appear to exist and penta or hexacoordinated hydrides of these elements are also unknown. Unlike nitrogen, which forms only NH3 and N 2 H 4 , phosphorus forms a very large number of hydrides based on more than one P atom. While the simpler compounds are gases or liquids, others are highly polymeric, amorphous, insoluble and highly coloured solids Phosphine, PH3 , diphosphine, P 2 H 4 , and triphosphine, P 3 H , are the first three members of the series of composition P n H n + 2 . Other well characterised phosphorus hydrides (phosphanes) include cyclopentaphosphine, P 5 H 5 , representing the series P n H n , and the cage hydride P 7 H 3 , which is a member of a whole series of compounds that are now known to exist (98). Condensed phosphorus H /?\ > _ p
ά
H
H
P
P
ά
/
"~P\
ι
H p'
- v\ H—P
H
U
/
H ^P
\\
H
~HP
HHE>
-PH
,ΛΩΧ
/P~H
P
1 H γ—H hydride anions such as P 7 H 2 , P 7 H , P 5 H 2 and P 9 H 2 are also known. Various arsenic and antimony hydrides with empirical composit ions As 2 H 4 , As 3 H 5 , As 2 H, As 2 H 2 , As 2 H 5 , Sb 2 H 2 , Sb 2 H 4 , Sb 2 H 5 etc, are have been made. Although somewhat less precisely characterised, they are believed to belong to series analogous to those formed by the phosphides (see below). No phosphorus analogues of multiply bonded nitrogen species such as diazene, HN=NH, tetrazene, H 2 N-N=N-NH 2 , and hydrazoic acid HN=N-N have been isolated. As far as the hydrides are concerned, those of phosphorus and the heavier pnictides are , as yet of little more than academic significance. They stand in complete contrast to NH3 and NH4 which are of overwhelming industrial, biochemical and environmental impor tance. Phosphine is virtually absent from biological processes (Chapter 11). Simple XH3 Hydrides The gaseous XH3 molecules (X = N,P,As,Sb,Bi) all have symmet rical pyramidal (C3v) configurations which have been established by
2.5
87
/ ζ \ 1 . 14 3 7
\ \ 1 .015
4 bp(°C)
Η^,-VVH
H
-73
-88
χ Ά δ Ό -519
K2y-n
^ ^ v1 . 7 0 7
/Bi\
H^AJ
-62
+22
-18
(99) v(X-H)
cm"1
3337
y (D)
(NH )
2327
1.45
(PH )
2122
(AsH
)
0.55
numerous infrared, microwave, electron diffraction and nuclear mag netic resonance studies (99). In pyramidal XH 3 pnictide molecules, the central X atom osc illates from one side of the plane of H atoms to the other. The inv ersion frequency is about 10 3 -10 u times less in PH 3 than it is with NH 3 , and this frequency decreases progressively with increase of molecular weight of X. The calculated inversion times are : -11
NH
2.5
xlO
PH
1.1
xi0~7
AsH
1.4
sees
years.
Phosphine has a smaller dipole moment than ammonia (99).This arises from the increased electron drift twards the H atoms and the smaller polarity of the bond in the case of PH 3 . Bond stretching frequencies become lower with increasing pnictide weight (99). The shapes of these molecules and much of their chemistry can be interpreted in terms of sp 3 hybridisation, and d orbitals are not likely to be much involved. The progressive reduction of interbond angle with increasing molecular weight can be associated with a change from nearly pure sp 3 hybridisation in NH 3 , to nearly pure p bonding in SbH 3 as the angle approaches 90°. Mainly p orbitals are involved with PH 3 but some s character predominates in the lone pair electrons. Owing to the smaller electronegativity differences involved (xp^ x H = 0, x^ ^ XJJ =0.9) , hydrogen bonding between XH 3 molecules other than NH 3 is likely to be very weak. The available evidence indicates that intermolecular association in liquid PH 3 and the heavier hydrides is indeed very slight or non-existant. The lack of internal association is indicated by the anomalous position of NH 3 with regard to its melting and boiling points (Chapter 12.1). Phosphine PHg Phosphine, PH 3 , bp = -88°C, mp = -133.8°C, is the best known hydride of phosphorus. At ordinary temperatures it is a colourless and very poisonous gas which has a characteristically unpleasant garlic like odour. (Appendix III).
^ i SΊ \t "
H'
η
|
n
h = 0.764 A ° 50 ' β = 57° 30«
α = 93
C3V
P—H = 1.42 A
2.5
88 PH
+
3
2
°2
^
H
3P°4
(100)
Pure phosphine ignites in air at about 150°C and burns to produce phosphoric acid (100). When impure, the gas is spontaneous ly inflammable at room temperature, and this is usually attributed to traces of diphosphine, P2H4, or possibly P^ . A slow oxidation of phosphine can occur by a branched chain reaction, and, like the slow oxidation of white phosphorus, there appear to be critical pressure limits for the reaction. Phosphine solidifies at -133.8°C (triple point) under its own vapour pressure of 27.3 mm Hg, and there are at least four differ ent crystalline forms existing at lower temperatures. Association in liquid phosphine is negligible compared to that in liquid ammon ia where there is extensive hydrogen bonding (above). The gas is only very slightly soluble in water, to give a neutral solution (26 ccs of gas dissolve in 100 ccs at 20°C). It is somewhat more soluble in organic solvents such as cyclohexane and carbon disulphide. An aqueous solution of phosphine gradually decomposes forming phosphorus, hydrogen and a yellow solid of approximate composition P2H.
Phosphine is both a weak acid and a weak base (101)(102). It is however more strongly acidic but much more weakly basic than NH3 . PH 3
+
H20
PH
+
H20
^
PH 2 " > PH4+
%
+
H30+
(101)
+
OH"
(102)
+
3
Protonation to form PHj,. (i.e 3p ^ sp ) involves greater hybridis ation changes than in the case of the change NH 3 — * N H 4 + , and the base strength of PH 3 is accordingly much lower than that of NH 3 . Phosphine acts as a weak donor towards protons and Lewis acids. The dissociation of phosphine is negligible unless heated to several hundred degrees. It is thermally more stable than AsH3 but less so than NH 3 . Photodissociation also occurs (Chapter 12.5 ) . At 0°C and atmospheric pressure, activated charcoal (1 cc ) absorbs considerably less PH 3 (69ccs) than NH 3 (170 ccs). Phosphine can be made by the action of water or dilute acids on certain metal phosphides (35)(39)(103). The hydrolysis of white phosphorus is a viable commercial method (104)and the gas may also be made by the action of caustic potash on phosphonium iodide (105) or by heating dry phosphorous acid ( 106). Phosphine and nitrogen are obtained by the interaction of phosphorus vapour and ammonia at red heat (107) and the gas is liberated in many other reactions of phosphorus compounds. Mg P + 6H + 3 2 2° " * 2PH3 3Mg(0H) 2 (103) *
The faint flickering light sometimes seen in marshes, *will-of-the-wisp' ,
has been attributed to the spontaneous ignition of impure phosphine, formed by the bio reduction of phosphate esters.
89
2.5 P
4
PH I 4 4H
3K0H
+
+
3H20_
KOH
+
*N-
s
3P03
2P
->
2NH 3
PH
3
+
V
3H 3 P0 4
Sfc
2PH 3
7
+
3KH PC) 2 4
y
+ KI
PH 0 3 +
HO 2
PH3
+
(105) (106)
N2
+
(104)
(107)
Phosphine is a strong reducing agent and it will e.g reduce many metal salts to free metal and pentahalides to trihalides (108). 3PC1
+
PH
>
4PC1
+
3HC1
(108)
When heated with sulphur, hydrogen sulphide and a mixture of phosphorus sulphides are produced. Direct union of phosphine with a hydrogen halide produces a phosphonium halide (109). PH
+
HI
>
PH I
(109)
Phosphine dissolves in liquid ammonia to give a salt-like compound NHz*+ PH 2 , and it also reacts with lithium aluminium hydride to give a salt of the PH 2 anion, which is soluble only in ammonia (110). This compound is analogous to the amide LiAl(NH 2 ) u . Phosphine is liberated on contact with water (111). LiAlH
+
LiAKPH )
+
4PH
>
LiAl(PH )
4H 0
^
LiAl(OH)
+ +
4H 4PH
(HO) (111)
At low temperatures phosphine reacts with perchloric acid to produce explosive crystals of phosphonium perchlorate, PHu + ClOu". Phosphine readily forms addition complexes in which a metal-phosph orus bond is present e.g. PH 3 .A1C1 3 , PH 3 .TiCl 4 , Cr(CO) 3 (PH 3 ) 3 and Co(NO)(C0)2PH3 (Chapter 10). Borane complexes are also known (Chapter 9 ) . Important reactions of phosphine are with formaldehyde to form THPC (4-348 ) and with olefins to produce trialkyl phosphines, Phosphonium Salts Phosphonium salts containing the tetrahedral PHu cation are generally less stable than the corresponding NH 4 salts and dissoc iate more easily. The chloride and bromide easily form gases at room temperature and only the iodide, PH 4 I , mp = 18.5°C, is crys talline. Phosphonium halides are produced by direct union of phos phine and hydrogen halide (109) or acid. Phosphonium salts of the PH 4 cation have few uses but their derivatives are important (Chapter 4 ) . Phosphide Anion PH 2 The phosphide anion, PH2 , is obtained from phosphine by reaction with the amide (112) or lithium aluminium hydride (110). PH
+
KNH
>
KPH
+
NH
(112)
2.5
90
The PH molecule does not exist at room temperature, but can be detected spectroscopically in reactions between hydrogen and phosphorus vapour at higher temperatures. H
^p_p-^-H (113) H ^*^H Diphosphine, P 2 H U , bp ~ 52°C, mp = -99°C, unlike hydrazine N 2 H 4 / ignites spontaneously in air and has no basic properties. It can be made by the action of water on calcium monophosphide (37). Decomposition of diphosphine yields triphosphine, P 3 H 5 together with higher members of the series, and a yellow solid of approxim ate composition P 2 H . Diphosphine
Polyphosphines During the last two decades a large number of phosphorus hyd rides (phosphanes) have been detected in the thermolysis products from diphosphine, or amongst the volatile hydrolysis products from calcium and other metal phosphides. These phosphanes have been characterised largely by mass spectra and NMR studiesand they form various series : P H ,P H ,P H P H n n+2 n n n n-2 n n-14 Members of the first series are the open chain compounds PH 3 , P 2 H 4 , P 3 H 5 etc. The second series are cyclic, one member being P 5 H 5 ( 98b) and the remaining compounds (e.g. P7H3 (98c)) are based on condensed networks of P atoms, only some of which may have H atoms directly attached. The majority of these compounds have only been prepared and studied as mixtures because, in many cases there is a very close similarity in properties, or they have a very marked tendency to disproportionate. Apart fromthe long established phosphine, PH 3 and diphosphine, P 2 H4 ; only the three phosphanes already cited (98) have so far been prepared in pure form. Diphosphine under the correct conditions of thermolysis will decompose mainly according to (114), and the desired triphosphine can be concentrated by fractionation. Triphosphine is a colourless 2P
2H4
P
>
3H5
+
PH
(114)
3
liquid, soluble in diphosphine, stable at -80°C, but turning yellow at room temperature giving eventually solid products. If liquid diphosphine and triphosphine are heated, tetraphosphine PnH 6 , and cyclopentaphosphine P 5 H 5 are among the products which can be obtain ed. Both n- (115a) and iso- (115b) forms of tetraphosphine have been characterised, moreover the n derivative exists in diastereoisomeric forms (Chapter 12). II
H
/p\
\„/ \ 1
H
H
H 1
p1
1 H
H
(a)
\P/H
I \p/P\p^H I I H
H
(115) (b)
2.6
91
The phosphorus-rich heptaphosphide, P 7 H 3 (98c) is obtained by methanolysis of (Me 3 Si) 3 P 7 (9-130) or hydrolysis of Ba3Pi4 (Section 2.2), and is amongst the thermolysis products from diphosphine. This compound is an amorphous white powder which is stable up to about 300°C and is insoluble in diphosphine and most common solvents. If potassium phosphide, KPH 2 is reacted with white phosphorus in dimethylformamide solution, a deep red amorphous compound with composition KP 5 H 2 is obtained. Formula (116a) has been proposed on the basis of molecular weight and NMR data. More highly polymerised ions such as P 9 H 2 (116b) also exist.
κ+ "Xj^P—PH 2 P
K+ X J ^ P — p \ l / p — P H 2 P
(a)
P
(116)
(b)
and Lithium salts containing the heptaphosphide anions P 7 H P H 2 can be obtained by reacting the heptaphosphane with Li 3 P 7 or LiPH2 at -78°C (117) . P?H3
+
2,6
^Li2HP7
2LiPH 2
+
2PH
3
(117)
NITRIDES
A whole series of amorphous polymeric materials appear to ex ist within the composition range P 3 N 3 to P 3 N 5 . They are white, yellow or brown, chemically very inert, they have high melting points and upon heating evolve gaseous PN molecules. The structures of these compositions are believed to be based on random networks of P — P and P — N linkages (118), although multiply-bonded units such as — P = N may be involved. Only P3N5 has been obtained cryst alline and this presumably has a regular structure of some kind.
I
1
I
I
-N
l
'N'PNV/IV 1
/P
\
/p
1
1
/P
\
\/
\ Ν /-Γ i Ϊ?-- Ν ^ ~ .p—ή /
},
(a)
!
,
(us)
pP—
; _N>-P-
(b)
Spectroscopic studies of gaseous PN molecules indicate multi ple bonding with P ^ N = 1.491 A . Dissociation occurs above 800 C and the bond is weaker than N = N (Table 1.20) An explosive colourless oil of composition P 3 N 2 i is an azide based on the phosphazene ring (Chapter 5 ) . Phosphorus triazide, P ( N 3 ) 3 , and the penta azide, P ( N 3 ) 5 have been prepared by reacting sodium azide with PC1 3 and PC1 5 respect-
2.6
92
ively (119)(120). Reaction of the latter with tetraphenylphosphonium chloride gives an explosive hexa-azidophosphate (hexa azidophosphoride) (121) (Chapter 5 ) . PC1 3
+
3NaN 3
P(N
>
> + 5NaN 0 5 o P(N 0 )_ + NaN + PhPCl 3 5 3 4 The penta azide decomposes to the phosphate hydrolyses according to PCI
P(N3)5
M£CN
Ph P + P(N.)_4
^
+
P(N
3)3
3NaCl
(119)
P(N0)„ + 5NaCl (120) Jo > Ph / P(NJ " + NaCl (121) 4 3 6 triazide (122) and the hexa azido (123). +
3>3
2HO
>
Ρη
Δ
o b
+
3N
„Ρ+
PO
(122)
2
o( N o>o"
4
z
+ 4HN
d z
< 123 >
o J
The earliest effective method of producing P 3 N was that due to Stock and Ho^mann who in 1903, heated P 4 S 10 with ammonia (124) 3P
4 S 10
+
80NH
>
3
4P
3N5
+
30(NH 4 ) 2 S
(124)
Amorphous material is obtained by this method, but on heating to 800°C it is converted to the crystalline variety. Another method of synthesis is by heating a diaminophosphazene (5-247). Triphosphorus pentanitride is insoluble in water and organic solvents, and it is not attacked by dilute acids or alkalis. Decomposition occurs at about 800°C according to (125). P3N5
^ ^
>
PN
£20_£
^
3PN
g
ip 4
+ +
N2
(125)
iN2
(126)
Monomeric PN is stable in the gaseous phase at 450-800 C. At higher temperatures it starts to decompose to the elements (which may then appropriately polymerise)(126), and at lower temperatures it is deposited as an amorphous polymer (PN) n . Polymeric phosphorus nitride is obtained when phosphine and nitrogen react at 80-290°C. This can be represented approximately as (127). 2PH 3
+
N2
>
2/n (PN) n
+
3H 2
(127)
No reaction occurs with P3N5 in hot water, but hydrolysis can be effected by steam at 800 C to give ammonia and phosphoric acid (128). Ammonium phosphates can be obtained if the reaction is carr ied out at 250°C under pressure. P3N5
+
12H 2 0
>*
3H
3P04
+
5NH
3
(128)
When heated in nitrogen, P 3 N 5 decomposes according to (125): in hydrogen, phosphorus and ammonia are obtained, and in air the final residue is polymeric (P 2 0 5 ) n · Triphosphorus pentanitride is decomposed on heating with concentrated nitric acid or by fusion with alkalis.
93
2.6
A series of somewhat ill-defined compounds containing Ν,Η &Ρ can be obtained from reactions between ammonia and the phosphorus halides. Some of these materials, e.g. phospham, P 2 NH and phosphor us amide imide, P(NH)NH2 are inert, insoluble and highly polymer ised (129). (129)
P — -NHNHr
L NH In
When phospham, made by reacting NH 3 with PC1 5 , is heated to 500 C, network rearrangement occurs with the elimination of H to give P 3 N 5 , which itself changes to P 3 N 3 at high temperatures (130) PN(NH)
P
P N (NH ) 3 3 2 6 2P red
->
-4
■>
3N5
4NH
l/n{(NPNH) 3 ) n 450°c
>
(130)
(PN) +
3NH
2PN(NH)
+
(131) 5H
(132)
The cyclic trimer, (PNC1 2 ) 3 (Chapter 5 ) will react with liq uid ammonia in a sealed tube to give the fully ammoniated derivat ive {PN(NH2)2}3 . The action of heat on this compound first prod uces a form of phospham in which the trimeric rings probably remain intact (131), but this eventually loses more ammonia and decomposes along the route (130). The normal form of phospham, made from PC1 5 and NH3, probably has a random network structure (133), like that obtained from reaction (132). 1
/^
NH
N H N
-<
(133)
NH
p «N^V V - k N / Polymeric phosphorus amide imide is produced directly by re acting phosphorus trichloride with ammonia in ethereal solution at -20°C (134). If PC1 3 is added to a saturated solution of NH 3 in chloroform at -78 C, phosphorus triamide is produced. Phosphorus triamide will lose ammonia, then hydrogen and nitrogen to form first the amide imide and eventually (PN) n (135). PCI. PCI,
-> HN.PNH
5NH +NH
3
>
P(NH
3NH Cl 4
-NHc
2}3
->
(134)
P(NH)NH„ (135)
-NHC P2(NH)3
->
P
4N6
*
(PN)
n
^
P
+
N
2.6
94
Numerous compounds based on N,P and H are possible in princi ple. Those containing a multiple bond and based on a single P atom include : H—P—NH iminophosphine
2
N
phosphenimidic nitride
phosphenimidous amide /
H
HN=P=N
H N-P=NH
- %
N H
2
NSP.
NH
2 phosphonitrilic amide
phosphenodiimidic amide Ηχ^ΝΗ
H
W
Η
^Η
N
^NH
NHn 2 phosphinimidic amide
phosphine imine
phosphazyne
H2NN^NH N H N H 2 phosphonimidic diamide
H
2
N
H
2
N /
\p^
N H
X N H
2
phosphorimidic triamide.
Most of these monomeric hydrides remain hypothetical compounds but H-substituted derivatives are in some cases known (2-43A)(2-43B) (see Chapter 5 ) . Amorphous polymeric materials with compositions (PON) and (PSN) can be obtained from reactions of NH 3 with P0C1 3 and PSC1 3 respectively. With more limited reactions, phosphoryl triamide (136) or thiophosphoryl triamide (137) can be made. These compounds are colourless crystalline solids which are soluble in water, but the action of heat will eventually transform them into (PON) n and (PSN) n . The oxynitride can be made crystalline by heating to 700°C under 45 kb pressure. Another route to the polymeric sulphur comp ound is to heat P 4 S 1 0 with ammonium chloride (138). P0C1
6NH
+
liq
PSC1.
6NH
P
4NH Cl 4
4S10
OP(NH 2 ) 3 SP(NH 2 ) 3
+
3NH Cl 4 3NH4C1
-> 4/n (PSN) + 6H S + 4HC1 n 2
(136) (137) (138)
On heating with dry HC1, reaction (139) takes place and with chlorine, polymeric chlorophosphazenes are obtained (140)(Chapter 5) PSN
+
4HC1
SPClr
2PSN
+
3C1„
2/n(PNCl 2 ) n
NH Cl 4 +
S2C12
(139) (140)
The insoluble compound P 2 0 3 N 3 H 5 , obtained from pyrophosphoryl chloride and liquid ammonia, probably has the structure (141) 0
0
?_0_?■f NH
NH
-NH-
(141)
2.7
95
Phosphoryl trihydrazide, Ο Ρ ( Ν Η — N H 2 > 3 , can be made by react ing hydrazine with phosphoryl chloride in anhydrous ether at -12°C (142). The compound forms colourless hygroscopic needles which are more stable than phosphoryl triamide, 0P(NH 2 >3 , in aqueous sol ution, but slowly decompose to give hydrazine N2H4 . 0PClo
+
2.7
6H N-NH
>
OP(NH—NH
)
+
3Ν H Cl
(142)
HALIDES Well-characterised phosphorus halide species include : PF 3
PC133 PCI
PBr 3
PF r
P PCU CU 5 5
PBr r
5 P
2F4
PF*
5
PP CC11
22 44
p
V4
1+
PC1," ci„ "
PBr +
4
4 PCI 4~
4 ( PBr 4 ~ )
PF ß
PCI6"
( PBr " )
6 POF PO3F"
Pig
6
POCI33
P0C1
6
POBr
( PO3CI~")
P
°2F2"
P
°2 C 1 2 _
P
2°3F4~
P
2°3 C1 4~
Trihalides The phosphorus halides are generally very reactive compounds which can cause both acute and chronic poisoning. They are strong electron donors by virtue of the lone-pair electrons on the P atom.
/ p \\i.57o F
X
F
F
U(D) 1 Ό 3
y^N^.oua X
C1^°C1 C1 0.80
^A\V2-220 Br^oiBrXBr 0.61
2 , U 6
^A\ ΙΛθ2Ι X I 0
(143)
The trivalent halides, P X 3 , are well known (143). All exist as pyramidal molecules with X/P/X angles of about 100 . Bond lengths in the trifluoride are abnormally short and this may indicate that, unlike other trihalides, some π-bonding may be present. Physical properties are listed in Table 2.12. Each trihalide may be made (although not necessarily most
2.7
96
conveniently) by direct union of the elements. They will undergo atmospheric oxidation to the oxyhalide, add sulphur to form the thiohalide, hydrolyse under acid conditions to form phosphorous acid, readliy add more halogen to form the pentahalide, and form addition complexes with various metals (Fig 2.14) The trifluoride, PF3 , is a colourless gas, odourless in tox ic concentrations, which burns in air in the presence of nitric oxide as a catalyst. It is best made by fluorination of the chlor ide with arsenic or zinc fluorides, or potassium fluoride dissolved in liquid SO2 .It may also be made by the action of HF on red phos phorus at elevated temperatures (144), or by reacting copper phos phide with lead fluoride (145). 2P red
+
6HF
2Cu P
+
3PbF
>
2PF 2PF
^
o
+ +
3
3H
6Cu
+
(144) 3Pb
(145)
The trifluoride has the largest dipole moment, corresponding to the largest phosphorus-halogen electronegativity difference in the series (143). The fluoride is slow to hydrolyse compared to the other trihalides (it can be washed with water during its preparat ion) , but the ultimate products under acid conditions are phosphor ous and hydrofluoric acids (146). Under controlled alkaline conditPF
+
>
3H 0
Η
·3 Ρ0 Τ
+
3HF
(146)
ions of hydrolysis with KOH, the product is potassium phosphite K2HPO3 , whereas if KHC03 is used the product is potassium fluorophosphite, KFHPO3, (Chapter 3 ) . Phosphorus trifluoride will react with carbon at high temperatures to produce tetrafluoroethylene, and at temperatures above 500°C it will attack silica (in glass), producing substantial quantities of S1F4 . Various metals react with PF3 at high temperatures to produce fluorides and phosphides. Phosphorus trifluoride forms a complex with arsenic pentafluoride below -78 C, but above this temperature it is fluorinated by the latter (147). PF
3'ASF5 ^ 7 ^ C
1
PF
3
+
ASF
>-7^CV
5
?F
5
+
AsF
3
(147)
The trichloride is made commercially by the direct action of dry chlorine gas on red phosphorus suspended in PC1 3 . White phosph orus will give a purer product. Commercial material is usually 99.5 % purity, with P0C13 as the most likely contaminant. Alternat ively the trichloride may be made by the reduction of oxychloride by passage over red hot coke (148), by reaction of the element with certain halides such as HgCl2 , CuCl2 or S0 2 C1 2 (149), or by the action of hydrogen chloride on phosphorus trioxide (150). P0C1
+
C
2P
+
3S0 Cl
> ^
PCI 2PC1
+ 3
+
CO 3S0
2
(148) (149)
2.7
97 6HC1
4 6
2H
-»
(150)
2PC1
3P03
The lone pair 3s electrons, the highly polar nature of the P — C l linkage and its donor or acceptor capacity, means the tri chloride will participate in many chemical reactions (Fig 2.14). Although acid hydrolysis yields phosphorous acid as in the case of the fluoride (146), controlled conditions of pH can give other products such as pyrophosphite and hypophosphate (Chapter 3)
Ni(PCV4
«
3
*
BBi
2 ^
5
f 3
λ
CH_?C1Q A l C l " ά ό 4
GT
^
ί
P0C1
er /*
/ /v
0
PSCI„
7*
**0Λ RPC1
ν
T
Cl PNP0C12
\
^
.H3P°3
* * > . <0*
H
}
(RS)3P^.
RSH
VC\r
2° - > PC1C
Cl,
(RO) 3 P «r~ * ° t t
P(NCO) PCl2Br + PBrCl2
P(CN) P C 1
P(NH2)3
5
F i g u r e 2 . 1 4 R e a c t i o n s of P h o s p h o r u s T r i c h l o r i d e
Phosphorus t r i c h l o r i d e w i l l reduce many o x i d e s , some v i o l e n t l y , i n r e a c t i o n s such as ( 1 5 1 ) - ( 1 5 3 ) , but i s i t s e l f reduced by ant imony, a r s e n i c or a r s e n e ( 1 5 4 ) ( 1 5 5 ) . PCI3
+
5PC13
+
PC1„
+
PCI3
+
PCI
+
2CrC>
- >
P0C1 3
2C1CL
->
4P0C1
sc)
->
POCI3
+
Sb
->
SbC1
+
>
AsP
AsH
+
Cr 0 2 3 +
PCI
3
3
+
0ft 2
(151) (152)
5
+
so 2 P 3HC1
(153) (154) (155)
2.7
98
Phosphorus trichloride is said to react rapidly with pure oxygen, even at low temperatures. It appears that various trace impurities can inhibit this reaction, although it is the commercial route to POCI3 . With ammonia various amides are formed (134)(135) and with nitrogen compounds the reaction products include dialkyl amino-substituted phosphonous halides, phosphazenes and cage comp ounds (Chapter 5 ) . Phosphines may be obtained by reaction with Grignard reagents (4-24), phosphites from reactions with alcohols (4-281,291-294)and phosphonic and phosphinic halides from reactions with metal alkyls (4-126)(4-137) or other organic compounds (4-129)(4-130)(4-132) (4-135). The trichloride is used in general organic chemistry to con vert carboxylic acids to acid chlorides (156). With explosive nitro gen trichloride, a complex cation is formed (157). PCI
+ «
J
3PC1
+
3CH C00H
>
3CH C0C1
+
H P0 ό
O
O
+
}> Cl P-N=PC1
NCI
«J
ό
«3
PC1
(156) ό
a~
ό
(157) Ό
Major industrial uses for phosphorus trichloride include con version to phosphoryl chloride, POCI3 , thiophosphoryl chloride, PSCI3, the manufacture of organophosphite esters (4-281) and the corresponding thiophosphite esters for use in the production of insecticides. The trichloride is used to make compounds such as lauroyl chloride (158), and octyl chloride (159). The former comp ound is used for synthetic detergents, and the latter in the manuf acture of rubbers, vinyl plastics and silver polishes. PC1 3
+
3CH 3 (CH 2 ) 10 COOH
> 3CH 3 (CH 2 ) 1Q C0C1
+
H3P03
(158)
PC1 3
+
3CH 3 (CH 2 ) 7 CH 2 0H
^ 3CH 3 (CH 9 ) ? C0C1
+
H3P03
(159)
The tribromide, PBr 3 , is most conveniently made by reaction between liquid bromine and a solution of white phosphorus in PBr 3 . In most of its reactions the tribromide resembles the trichloride although the former have been much less studied and in some cases the products seem to be more complex. The tri iodide, PI 3, is best prepared by reacting iodine with white phosphorus in a specially purified CS 2 solution. It may also be made by heating the chloride with gaseous HI (160). Exchange for lighter halogen may be effected in some reactions (161). PCI
+
3HI
>
PI
4P I
+
3SnCl
>
4PC1
+ 3
+
3HC1 3SnI
(160) (161)
4
Mixed trihalides are formed from reorganisation which occurs if pure trihalides are mixed (162)(163). At 300-400°C the equilibPC1 3
+
PBr 3
^
PCI Br
PC1 3
+
PF 3
^
PF
2C1
+ +
PClBr PFC1
2
(162) (163)
2.7
99
rium (162) lies well to the RHS. Chloroand bromo fluorophosphines were originally obtained by incomplete fluorination of the approp riate PX3 compounds, using SbF3 or other fluorinating agents. When fluorine halides are involved, reorganisation is slower than with non-fluorine systems, and pure specimens are generally easier to isolate. Difluoroiodophosphine is made from dimethylamino fluorophosphine (5-76 ) . Bromochlorofluorophosphine, PBrCIF, is formed in the equilib rium (164), but a more satisfactory preparation is by equation (165) Cleavage of the P — N bond to give mixed halides also occurs with dialkylamino difluorophosphines (5-75)(5-76) PFBr
+
Me NPFC1
PFC1 +
^
^
2PFClBr
^
HBr
PFClBr
(164) +
Me NH
(165)
Tetrahalides are known with the formula P2X4 where X =F,C1,I. The tetrabromide has been obtained stabilised as a metal complex (10-122). Tetrafluorodiphosphine, P 2 F 4 , is a colourless gas which can be made by reaction (166). If this gas is heated to 900°C, sub stantial quantities of a solid yellow decomposition product are formed, together with a small amount of a colourless liquid, P(PF 3 ) mp = 68° C. Tetraiododiphosphine, P 2 I 4 , which forms dark red crystals, is the most stable tetrahalide. It can be prepared by reacting iod ine with dry phosphine (167) or by direct union of the elements in CS 2 solution. 2PF2I
+
2Hg
>
P2F4
+
Hg2I2
(166)
8PH 3
+
5I2
>
P2I4
+
PH 4 I
(167)
On heating, the solid tetraiodide decomposes into tri iodide and red phosphorus. Carbon disulphide solutions undergo atmospheric oxidation on standing, to give the insoluble amorphous compound ( P 3 I 2 0 6 ) n .Hydrolysis products from P 2 I 4 include hydroiodic acid, hypophosphorous, phosphorous and phosphoric acids and phosphine. Under alkaline conditions hypophosphate is also formed. The P 2 I 5 cation (168) is present in the solid complex P2 I5+ A1I 4 which can be obtained from a CS 2 solution of A1I 3 and P I 3 . \
2-2B1
F O P - 1 (— P v ^ -1C* 5 8 7
^\
*
T ^ \ 2-21 ^ - P Ρ^2·14Θ
9
2 · 2 2
H w \ ( i . 4j ^ p - L - p
1 · 5 9
(168) 1-UiyF
\±/1
H—Poe F
A 1
2·κοΙ
I
-
All.
4
I N 2.22 +
^P
ι' 1
I/
P—I
\ΐ
2 · m
All
4
A
The tetrahalides all contain a P — P bond (168). The gaseous fluoride and the solid state iodide and its solution contain molec ules in the trans C2h configuration. Difluorophosphine, PHF2 , can be made in 55 % yield by heating
2.7
100
iododifluorophosphine with hydrogen iodide and mercury (169). It is a colourless gas stable at low pressure. In the liquid and solid some association occurs through hydrogen bonding (Chapter 12.1). Phosphinodifluorophosphine, F 2 P.PH 2 , can be made by heating phosphine and iododifluorophosphine (170). The latter reacts with cuprous oxide to form oxo-bis difluorophosphine (171), a compound which slowly decomposes to PF 3 and (P0 2 F) n . HI
PF 2 I
PH.
-> F 2 P — P H 2
Cu 0
-> F P-0-PFn ^ 2 2
2PF I
+
2Hg
H
PF 2 I
V2
(169)
+
HI
(170)
+
2CuI
(171)
-> PHFr
Pentahalides The phosphorus pentahalides can be made by addition of halogen to the trihalides or to white phosphorus. These molecules may exist as covalent trigonal molecules or in ionised form (1-10). The pentafluoride is the most stable; it is dissociated a few percent at 100°C, and completely only at 300°C. The pentachloride is consider ably dissociated at 200°C , the pentabromide totally at 35°C, and the pentaiodide does not appear to exist. These compounds hydrolyse in two stages (172)(173). PX_
H
2°
-*· POX.
2HX
(172)
POX^
3H 2 0
* V°4
3HX
(173)
Electron diffraction measurements have established the trigon al bipyramidal configuration in the vapour phases of PF5 and PC15 (174). Both these molecules have zero dipole moments, and axial bonds which are slightly weaker than the equatorial ones. This is indicated by the slightly longer bond lengths and lower stretching frequencies of the axial compared to the equatorial bonds in each halide. 1 . 5 77J F-r-PC
(174) 2.020 |
1 . 534 |
^x
Cl
F
Nuclear magnetic resonance studies have suggested the equiv alence of all five bonds in PF5 , but this has been explained on the basis of a rapid exchange of the non-equivalent F atoms (175) Such a process, which does not involve bond breaking, is known as Berry Pseudorotation (Chapters 1 & 12). It involves the simultaneous exchange of the axial F* atoms with two of the equatorial F atoms, by merely changing the bond angles Only a relatively small angular F
F ' / > F
F
(a)
\> F
-
"
(b)
F/ I^F*
(175)
-f
(C)
101
2.7
deformation of 15° is necessary to convert the initial trigonal bipyramid (175a) to an intermediate tetragonal pyramid (175b) using P — F r as a pivot, and thence a similar degree of angular adjustment to produce the alternative trigonal bipyramid (175c). A similar ex change may subsequently take place using an equatorial F* atom as pivot. If the fluorine atoms exchange at a faster rate than the frequency difference between the chemical NMR shifts from the two kinds of nuclei F and F* , all the P — F bonds would appear equival ent from the NMR data. Below -85°C, however, the rate of exchange is sufficiently slowed down for the two species F and F* to give rise to a doublet 1 9 F NMR peak. The rate of exchange in P C 1 5 is much slower : 73° C 27° C PFr 4.3 xlO 8.3 xlO 8 /sec /sec 5 3 2.3 xlO 2 /sec PCI 7.4 xi0~ /sec 5 Mass spectra of the pentachloride vapour indicate it contains a small percentage of dimer molecules (176a) - species which may also be present in some solvents at low temperatures. C1
\|:/C1\i:/C1 ci^jN^l^ci CI
ci + ci a< N u
ci^^ci ci^l>ci
CI (a)
(176)
CI (b)
Solid phosphorus pentachloride, recrystallised from nitroben zene, has been shown by X-ray diffraction to consist of an assembly of tetrahedral tetrachlorophosphonium P C 1 4 + and octahedral P C 1 6 " ions (176b). The same ions are present in methyl cyanide solution and in other solvents of high dielectric constant. Solutions in benzene or carbon disulphide contain covalent trigonal bipyramidal P C 1 5 molecules. At low concentrations in some solvents there may be slight dissociation to PCl^"1" C l ~ . Condensation of the pentachloride vapour on to a cold finger at about 90°K, produces a solid form containing covalent molecules similar to those in the vapour (174). On warming to normal temperatures these revert to the ionised form (176b). The structure of solid P F 5 i s not known, but the crystalline pentabromide consists of a n assembly of P B r 4 + and Br~ ions. In meth yl cyanide solution the compound is an electrolytic conductor (like the pentachloride) with ion species P B r 4 and P B r 6 , the anion being on the limit of stability and stabilised by solvation. The tetrahed ral P B r u ion, on the other hand, is relatively stable as indicated by the production of P B r 4 PF6 on fluoridation of P B r 5 in non-ionic
Figure
2.15
Crystal Structure of PBr Br
Sooc§oo
οφοαΙ
102
2.7
solvents. The greater stability of the cation compared to the anion is also indicated by the preferential attack on the latter during fluorination of the pentachloride (below). Under normal conditions, PF 5 is a colourless gas which fumes on contact with moist air and immediately hydrolyses with water (177). The pentafluoride may be prepared via the fluorochloride, by heating PF 3 with chlorine in the presence of calcium fluoride at ticHJ LI U l i W ± LI1 PF
+
5
H
H
+
5PF3 NaPF
Π
2°
^
^ΓΊ
V S
2
5PF2
+
PF
3
3P
4°10
2MoF bCa
PF
...T>
°PT?
*"■»·
Λ T-TT"
"ϊ*
ΑΤ> Τ?
^CJT?
4S10
3PC1
Dor „
4 5ASF
+
5
"ΓΓ5
"
ArN0PFr 2 c P
5
>rr 5
*2
2HF
V °T s 3\>F c5
ΓΊ
---*!> PT?
6
SPnF
+
+
3
!C12
^. **
6
; .
rtbr5 1 ΡΠΤΪ P 0 F
."^-
_
~>
3PF
2PC10 o
(178)
NaF
(179)
+
2MoF_ 5
(180)
+
5Ca(P03)2
(181)
+
5
+
+
P F
5 3
(177)
+
+
5
N
2
(182)
15S
(183)
5AsCl
(184)
Alternatively the pentafluoride may be made by heating alkali or alkaline earth hexafluorophosphates (179), by reacting the trifluoride with molybdenum hexafluoride (180), or by heating phosphorus pentoxide with calcium fluoride (181) Organic diazonium salts such as p-chlorophenyldiazonium hexachlorophosphate are commercially available materials which are stable when dry, but decompose at com paratively low temperatures according to (182). Other methods of preparation include the action of sulphur tetrafluoride on P U S 1 0 (183), and the direct fluorination, in solution, of the pentachlor ide with arsenic trifluoride (184). Fluorination of the crystalline pentachloride results, init ially, in preferential attack on the octahedral anion, which con tains the more loosely bound halogen (185). Further fluorination leads to the pentafluoride (186), but the tetrafluorophosphonium cation is formed in an unstable complex with SbF5 (187). It is less stable than NFM . PCI
+
PCI ~ 4 6 3PC1„ + PF ~ 4 6
+
2AsF 0 3 4AsF 0 3
+
> PCI
PF
4 >
6PF ^ 5
+
6 +
2AsCl 4AsCl
(185) (186)
3SbF^ > PF + , b F (187) 3 16 5 5 4 The pentachloride can be made by bromine displacement (188) or by simple addition (189).
PF
+
2PBr
+ o
5C1
> Δ
2PC1
+
C
5
5l3r
(188) 2
103
2.7 PCI.
S
PCI
2C12
(189)
2PSC1,.
Hydrogen and certain metals reduce the pentachloride to tri chloride (190). Phosphoryl chloride (phosphorus oxychloride), P0C1 3 can be obtained from the pentahalide by several reactions (227)(231), and thiophosphoryl chloride, PSCls, from PnS10 (79). Phosph orus pentachloride reacts with ammonium chloride to give phosphonitrilic chloride (5-163). If the pentachloride is reacted with excess liquid ammonia and the ammonium chloride removed by sublimat ion, the products include P(NH2)i|Cl (5-211). Reactions with various other nitrogen compounds are known (Chapter 5 ) .
PCI 3 ?
>
C1
2 4
~
Cl P=NPh
\
H
\
2
;
/
* I <* /
\
\
\
C13P=N-N=PC13 ^
P0C1
A
S
/
\
P C I 4 ici;
\
?C1
/
4
PF
6
^3* Ph3?-0-PClg *ζ
Ph P0
PCI,
P S
4 10
RP0C1 2 *f
>
PSC1
3
KPF„
-' I '
id„4 BC14 '
P0C1„
C13P=NP0C12
Figure 2.16 Reactions of Phosphorus Pentachloride Like the trichloride, the pentachloride is used in general organic chemistry for several purposes. These include the conversion of carboxylic acids to the corresponding chlorides (191), although in the case of oxalic acid, carbon monoxide is obtained (231). PCI, PC1 C 5 PC1C
H, +
-»
PC1
3
2HC1
+
CHCOOH o
-> CH C0C1
S0„
- ^ S0C1 2
+
(190)
HC1
+
P0C1-
P0C1
(191) (192)
2.7
104
Phosphorus pentachloride is used commercially to make thionyl chloride (192). Sulphonic acids are converted to sulphonyl chlorides (193), and the Beckmann rearrangement of oximes can be effected (194). In tetrachloroethane , the pentachloride reacts with hydrazine mainly according to (195), but if the solvent is P 0 C 1 3 , the product is a linear phosphazene (5-270). +
R.SO OH
PC1 C 5
R— C— R N.OH
PCI
+
2PCl f f 5
RSO Cl
P0C1,
HC1
(194)
R— C—NH—R
H2N-NH2
2PC1,
(193)
(195)
4HC1
Phosphonic and phosphinic acids and their esters are convert ed to the corresponding chlorides, and with phenol or catechol sub stituted phosphoranes are obtained (Chapter 4.14).Some of the react ions of the pentachloride are summarised in Fig 2.16. Phosphorus pentafluoride reacts with ammonia, amines or dialkylamino phosphines to give substituted phosphoranes (Chapter 5.3) The latter can also be obtained from trialkyl phosphites or alkoxylithium derivatives (Chapter 4 . 1 4 ) . Mixed Pentahalides Cl F~Ps
I
-f^F
Cl
C2v
Cl
C2v
t
Cl
C3v
-Cl 'Cl
C1
F
D3h
C3v
F
UF
C1
—PCT,
I^F
(196)
-f
F
F Cl-
F—P—
F
F
ci—PC Cl
Cl
I^F
Cl-
Cl
Cl F Cl
Cl
Cl
C2v
F-P-C1
|^C1 D3h
F
C2v
Like the other pentahalides, the mixed pentahalides can be based on either a covalent trigonal bipyramid, or exist in ionised form. Isomeric arrangements are also possible (196), but the stab ilities of these may be influenced by apicophilicity or pseudorotation, and one particular form is usually preferred (Chapter 1 2 . 3 ) . Electron diffraction and spectroscopic studies are generally in accord with trigonal bipyramidal configurations in which the most electronegative halogens occupy the axial (apical) positions. From (196), the correct isomers are thus C2v for P F U C 1 (u = 0.78D) and PF3CI2 ( y= 0.68D); D3h for P C 1 3 F 2 (y = 0D) and C3v for P C 1 U F (μ = 0.21D). In common with P F 5 and P C 1 5 , the most symmetrical mixed halide, PCI3F2 has a zero dipole moment. The NMR spectra of these halides indicate that they contain F
105
2.7
atoms in only one kind of environment. A pseudorotation process is therefore necessary to secure equivalence between axial and equat orial F atoms in the case of PF 3 C1 2 and PF 4 C1 (197). At low temp eratures however, these pseudorotations disappear, and the NMR spec tra indicate non-equivalent F atoms in PC1FU below -138°C and in PF 3 C1 2 below -80°C. F
F·
I ^*F '
^
Cl-j
C1
1^F
~f
<197)
Ff
F
Mixed fluorohalides can be obtained by addition of halogen to PF 3 (198). They are thermally unstable colourless gases which tend to dissociate, the bromine compound being the least stable (199) Phosphorus tetrafluorochloride can be made in 52 % yield by a reac tion between PF5 and BCI3 at low pressure (200). PF 3
+
>
Cl 2
PF
3C12
(198)
2PF Br > 3PF + 2PBr (199) 3 2 5 5 3PF^ + BC1 > 3PF Cl + BF 0 (200) 5 3 4 3 Covalent PF 3 C1 2 is isomeric with the ionic compound PC1 4 PF 6 obtained in reaction (201). In the presence of moisture, the cov alent form converts to the latter, but on heating in vacuo at 100 C the change can be reversed. In boiling carbon tetrachloride the ion ic compound decomposes into covalent PF5 , PF 3 C1 2 and PC1U PC1 5 F . If PC1 4 PF6 is sublimed in vacuo and the vapour cooled to -60°C, covalent liquid PFCI4 is obtained (202). At room temperature, this liquid changes slowly to the ionic solid form PC1 4 F . The latter exists in ionising solvents but in covalent form in non-polar liq uids. The ionic form is hydrolysed by dilute caustic potash (203). 2PF 3 C1 2 2PC1
+
>
P C 1 4 + PF 6 "
(201)
PF ~ > PFC1 + PF (202) 4 b 4 5 3PC1. + PF ~ + 7K0H > KPF^ + K ΗΡ0 Λ 4KC1 + 3 H O (203) 4 b 6 2 4 2 Unstable PCI3 F2 can be made by low temperature chlorination of PC1F2 . On heating, the covalent form transforms to PC14 PF U Cl 2 A crystalline compound of composition PCI4. 6 7 B r 0 . 33 can be obtained from bromineand phosphorus trichloride and this consists of (PC1 U ) 8 (PClö )i+ (Br ) 4 ions. Pentahalides containing three or more differ ent halogen atoms have received comparatively little study, but many isomeric arrangements would be possible (Chapter 12.2). Hydrotetrafluorophosphorane (phosphorus tetrafluorohydride) HPF 4 , mp = -lOCPC, and dihydrotrifluorophosphorane, H 2 PF 3 , mp= -52°C can be made by reacting hydrogen fluoride under anhydrous conditions with phosphorous and hypophosphorous acids respectively (204)(205)
106
2.7
H PO
+
4HF
HPO
+
3HF
_pjO°C
_-r.Y_^_^ C
->
PHF
+
3H 0
(204)
PH Q F
+
2H 0
(205)
Trihydrodifluorophosphorane can be obtained by reacting diphosphine with excess hydrogen fluoride, but the compound appears to be unstable and dissociates approximately as in (206). 2H 3 PF 2
> H 2 PF 3
+
PH 3
+
HF
(206)
According to NMR data, the axial positions are filled by F atoms in all three structures (207), and where applicable, inter change of axial and equatorial F atoms takes place. Although gaseous PHF4 and PH 2 F 3 are monomeric, there is spectroscopic evidence that in the condensed phases these compounds are associated through weak P — H — F bonding. F
»-< I
F
F
>\-*
F
H
l>\-»
H
I
I
F
F F (207) Ammonium pentafluoride reacts with PH 2 F 3 to give the PH 2 F 2 cation which appears to be more stable than the PF 4 cation or the nitrogen analogue NH 2 F 2 (208). PH
2F3
+
AsF
5
PH
^
2F2+
AsF
6_
(208)
Polyhalides A number of phosphorus polyhalides are known. In the system phosphorus tribromide-bromine at least five compounds exist : PBr3 PBr 5 , PBr 7 , PBr g , and PBr 1 7 . The crystalline heptabromide contains tetrahedral PBr 4 + and linear Br-Br-Br ions. Phosphorus pentachloride dissolves in molten ICI2 to give a strongly conducting solution containing the ions PC1 4 IC1 2 ~. Crystalline PC16I is built from ions of this kind, which also persist in methyl cyanide. The chloroiodide may be made by direct addition (209) or from the trichloride and iodine chloride (210). PCI
+
I
>
PC1 3
+
3IC1
^
PCI PC1
4
+
+
IC1 ~ IC1
2~
+ +
PC1 0 X
2
(209) (210)
Other well-defined halides have compositions such as PCl 5 BrI, PBr 5 ICl, PC1 5 IC1, PCl 3 Br 4 , and PBr6I . These all consist of tetra hedral PX 4 cations and the appropriate halide anion, both in the solid state and in the conducting solutions they form with ionising solvents. In CC1 4 solution e.g. as in acetonitrile, ionic species are obtained (212), PC1_I 6
>
PCI
5
+
IC1
(211)
107
2.7 PCI I
PC1
>
4+
+
IC1
(212)
2
Hexahalophosphates (Halophosphorides) Many salts containing the octahedral anions PC1 6 and PF 6 have been studied by spectroscopy and X-ray diffraction (213). The P — C l bond in the octahedral ion has a greater length and lower stretching frequency than in the tetrahedral PC1 U cation (Chapter 1.6). Somewhat weaker bonds in the anion are also indicated by the C1
F
I/* p_F
F
Cl — P
ri
'/
"
Cl
(213)
F preferential attack which occurs in some reactions (185). Reaction (203) suggests the PF 6 ~ anion is more stable than PC1 4 + . The order of stability of the halophosphoride anions appears to be the oppos ite to that of the halophosphonium cations : PF
4
+
PF ~ 6
PC1
< >
4+
PCI ~ 6
PBr
< >
4+
PBr ~ 6
The PF 6 ~ ion (which is isoelectronic with stable SF6 ) , can be produced by fluorination of pentahalides to give simple hexafluorophosphate salts (hexafluorophosphorides) (214) - these decom pose with heat to give PF5 (179). Reaction (215) can also be used, and this gives a mixed octahedral anion PC1 4 F 2 ~ (the PF 4 C1 2 ~ anion can be made by heating covalent PC1 3 F 2 above). PC1 C + o 2PC1 F
NaCl +
2CsF
+
6HF
> NaPF„ 6 ^ CsPF
+
6HC1
(214)
+
CsPCl F
(215)
A good method (used commercially) of preparing the acid is to mix phosphorus pentoxide with anhydrous hydrofluoric acid (216). This reaction proceeds with considerable evolution of heat and giv es white crystals of the hexahydrate, HPF 6 .6H 2 0, mp = 31.5°C, which according to NMR evidence should be formulated as H 3 0 + PF 6 ~HF.4H 2 0. The anhydrous acid can be prepared by reacting PF5 and HF under anhydrous conditions in liquid sulphur dioxide, but on standing at room temperature, the product will decompose again (217). Similar equilibria exist with PF5 and metal fluorides. The aqueous acid exists in equilibrium with difluorophosphoric acid (216)(244) and typical samples may be expected to contain some of the latter. +
24HF
PFr 5
+
HF
NH4PF6
+
P
4°10
NaOH
4HPF^ 6 -> ->-
+
10H O 2
HPF
6 NaPF^ 6
(216) (217)
+
HO 2
+
NH o 3
(218)
Soluble hexafluorophosphate salts can be made by evaporation
108
2.7
of the ammonium salt with the appropriate base (218). The ammonium salt can be obtained from a reaction between HF and (PNC1 2 ) 3 (5-168). Simple alkali metal salts of type MPFÖ are very soluble in water (103 g NaPF6 dissolve in 100 cc H 2 0 ) , and they crystallise with a rocksalt type packing of the M + and P F Ö ~ ions. Solutions of alkali and alkaline earth salts are stable except at very high concentrat ions when hydrolysis to tetrahedral fluorophosphates occurs (below). The nitronium salt, N 0 2 + PC1 6 ~, a white solid stable up to 170°C in a dry atmosphere, is available commercially as a research chemical. It can be made by decomposing the addition complex of nitrosyl fluo ride and phosphorus pentafluoride (221)(below). Addition Complexes Both trivalent and pentavalent phosphorus halides will form addition complexes with metals or metal salts. While the trivalent complexes contain metal-phosphorus bonds (Chapter 10), the penta valent complexes involve rearrangement to produce ionised assemblies of tetrahedral P X 4 + cations and various complex anions. The crystalline addition complex formed between phosphorus trichloride and tetraethylammonium chloride, is quite unusual and is correctly formulated as Eti*N+ PC1 4 ~. Unlike the tetrahedral P C 1 4 + cation, the PCl^" anion adopts a distorted trigonal bipyram idal configuration in which one equatorial arm is believed to be occupied by a lone pair of electrons (219). In this case there is a resemblance to the stereochemistry of arsenic and antimony (Chap ter 1 . 4 ) . Cl -/2 · 8 5 17 1/ 2 · 05
2 · 1 2Λ
2 · 05
Cl Complexes are formed with boron trihydrides and trihalides : F 3 P.BH 3 , Cl 3 P.BBr 3 , Br 3 P.BBr 3 . The colourless gas F 3 P.BH 3 is some , but F 2 HP.BH 3 is even what more stable than its isomer H 3 P.BF 3 more stable. Also known are F 3 P.NMe 3 and Cl 3 P.NMe 3 (Chapter 9 ) . Phosphorus pentafluoride forms many crystalline 1:1 addition complexes of the type PF5X where x may be SF 4 , NO, N0 2 F, C10 2 F, C5H5N, C 6 H 5 CH0, Me 2 0, Me 3 N etc. Some of these contain hexafluorophosphate ions (220)(221), while complexes such as Me 2 N.PF 5 are pro bably based on hexacoordinated phosphorus (Chapter 5.11). SF..PF,, 4 5 NO F.PF
V ^
SF* PF~ 3 6 NO + PF ~
(220) (221)
Phosphorus pentachloride forms numerous addition complexes with metal salts, which may be categorised as : PCI .MC1 5 5 2PC1 .MCl^ 5 4
where M = Sb,Nb,Ta,Mo,W,V. "
"
Ti,Sn,Te,Hf,Zr,V,Pt > > > > > >
109
2.7 pci
.Mm 5 4 PC1C.2MC1, 5 4 PCI ,MC1 0 5 o PCI.MC1 5 2
where
M = Ti,Zr,V,Sn,Se,Te
where
M = Ti
where
M =
where
M = Zn,Hg
B,Al,Ga,Ti,Cr,Fe,Au
These complexes contain tetrachlorophosphonium cations with various complex anions. The compound P C 1 5 . B C 1 3 e.g. is correctly formulated as P C 1 4 + B C 1 4 " . When heated to 340°C it dissociates into C l 2 , P C 1 3 and B C 1 3 . The ionised formulae are indicated by crystal structure data, and by the insolubility of these compounds in non-polar solv ents, but their ability to form conducting solutions in ionizing media. Their NMR and infra red spectra indicate P C 1 4+ ions. Crystal structure data indicate that P C 1 5 . N b C l 5 should be formulated as P C 1 4 + N b C l 6 ~ . Compound P C 1 5 . T i C l 4 has structure(222a) while PCI5.TeCl 4 exists as (222b) in the solid state. Compound P C l 5 . F e C l 3 should be formulated as PCI* , while P C 1 5 . 2 T i C l 4 FeCL contains anions as in (222c). Compounds such as 2PCI5.T1CI4 may exist as 2 P C 1 4 + T i C l 6 ~ or as (222d).
2PC1, 4
C K^iT ^ k l 1T/i 0 1 " i c1i ^ ci1 Lci nci x cC ii C
Cl Cl l/Cl l/Cl -Te — C l — T e — Cly \ / \ Cl Cl Cl Cl
nPCl ((a)
(b) (222)
Cl
C 1
PCI* 4
Cl^ / \ /Cl Cl— Ti —Cl—Ti—Cl Cl^ ΧΓ/ Cl (c)
PCI,
Cl"
Cl
ci^l^ci C l ^ C l
(d)
Oxyhali d e s 1.449
Fvi ° 1 1 . 4 3 6 F - ^ r P == 0
Cl.
F
Cl 2.002
1.524
Cl—P =
0
Br^^
1.44
Br—P = Br* 2. .06
0 (223)
With the exception of the iodide, all the oxyhalides, P 0 X 3 have been well characterised ( 2 2 3 ) , as well as mixed species such as P 0 F 2 C 1 , P 0 F C 1 2 , P0BrCl 2 etc. The phosphorus oxyhalides (phosphoryl halides) are all toxic and reactive. They may all be formed by oxid ation of the appropriate trihalide and all are hydrolysed by water. Phosphoryl fluoride, P 0 F 3 , is a very poisonous gas, very eas ily hydrolysed, which attacks silica-containing glass to form S1F4. It can be prepared by fluorination of the corresponding chlorine compound with salts such as zinc,lead or silver fluoride (224). It may also be made by using potassium fluoride dissolved in liquid sulphur dioxide (225). In the gaseous phase, the oxidation of P F 3 by pure oxygen is explosive. Phosphoryl fluoride is obtainable by hydrolysis of P F 5 or hexafluorophosphates (177)(243).
2.7
110 2P0C1,
3ZnF„
-> 2P0F.
POC1.
3KS0 F
-> POF
(224)
3ZnClr +
3KC1
+
(225)
3S0
Phosphorus oxychloride (phosphoryl chloride), P0Cl3,is a col ourless fuming liquid which can be made by the careful oxidation of the trichloride with air, oxygen or oxidising agents such as potassium chlorate (226) and others. It can be obtained by reacting the pentachloride with boric acid (227), with alcohols (228), oxid es of nitrogen (229), sulphur (230) or phosphorus pentoxide (53). The reaction with oxalic acid (231) is a convenient laboratory meth od. Direct oxidation of the chloride is the usual commercial method of production of phosphoryl chloride. Other commercial processes involve the reaction between phosgene and iron phosphate (232) or oxidation of PC1 5 with P 4 0i 0 (53). The simple reactions (55) (133) have not yet been made commercially viable. The largest technical use for phosphoryl chloride is in phosphorylation reactions (Chapters 4 & 6 ) . The manufacture of phosphate esters (Chapter 6) is a particularly important commercial use. PCI.
3KC10.
-> P0C1, ■> 3P0Clr
3KC1
(226)
PCI,. 5
+
2H
PCI
+
ROH
-> p 0Cl
+
RC1
+
N
-> P0C1
+
2N0 Cl
(229)
+
S0
-> P0C1.
+
S0C1
(230)
PC1 C 5
+
(COOH)
-> P0C1
FeP0„ 4
+
3C0Cl o 2
-> P0Clr
PCI PCI
5 5 5
3 B °3
2°5 2
C a 3 ( P 0 4 ) 2 + 6C0 + 6C1,
350°C
>2P0C1
2 3
+
+
+
6HC1
(227)
HC1
(228)
2HC1
+
CO
3C0
+
FeCl
3CaCl
+ 6C0
+
CO
(231) (232) (233)
Phosphoryl bromide, P0Br 3 , is best prepared by gently heating an intimate solid mixture of pentabromide and pentoxide (234). Mixed oxyhalides are prepared by various methods. Fluorine-contain ing compounds may be obtained by treating P0C1 3 or P0Br 3 with SbF 3 , and chlorobromides by the action of HBr on P0C1 3 at 500°C. 6PBr_
P
4°10
-> lOPOBr
(234)
If the oxychloride is mixed with oxybromide a scrambling occurs and in a few days the equilibrium mixture contains four species : P0C1 3 , P0Cl 2 Br, P0ClBr2 and P0Br 3 . Phosphoryl chloride is a non-protonic solvent, but its proper ties show a remarkable resemblance to those of water. The low elec trical conductivity of both solvents indicates only slight dissoc iation (235)(236). The extensive system of hydrogen bonds character istic of water, is of course absent in phosphoryl chloride.
2.7
111
2P0C1
^
-^
2H 0
^
—=**
Li
P0C1
2+
+
H 0+
%J
P0C1
+
4
0H~
(235)
(236)
The phosphoryl group readily accepts protons and participates in hydrogen bond formation, both in solution and solid states. This has important consequences in determining the structures (partic ularly crystal) of many phosphoryl compounds (Chapter 12.1). The phosphoryl halides, POX3, typify phosphoryl compounds whose properties are dominated by the polar and very reactive phos phoryl bond (Chapter 1.3).They have been much studied by electron diffraction and various spectroscopic techniques. Symmetrical struc tures with C3v symmetry have been established in the vapour, liquid solution and solid states. Because of the highly polar nature of the P Ä 0 linkage, the phosphoryl halides have considerably greater dipole moments than the corresponding trihalides. The phosphoryl bond is strong and rather short, and its high polarity is indicated by the great intensity of the characteristic v (P=0) infra red stretching absorption (Chapter 12.7). When metal salts are dissolved in the phosphoryl halides, add ition complexes are easily formed, and these can usually be isolat ed in the solid state. Since lone pair electrons are not available for coordination purposes as in the trihalides, complexes such as P0Cl3.SbCl5 were originally assigned an ionic formula P0C1 2 + SbCl 6 A number of crystal structure analyses of compounds of this type have, however, firmly established that coordination occurs through the phosphoryl oxygen atom. In solution, interaction through the oxygen atom is indicated by a lowering of the v (P=0) stretching frequency. Typical crystalline complexes such as BCl3.P0Cl 3 , SnCl 4 .2P0Cl 3 and TiCl 4 .P0Cl 3 have structural formulae (237). PCI Cl C 1
0
« \ l / ^ 1 /C1 .Ti Ti <*'k - N ^ c i
C1„P / 3
0 —Sn —Cl C1
3P
i>-PC13
(237)
cix ci Cl— P— 0— B — Cl Cl^ Cl A major use for phosphoryl chloride is in the synthesis of phosphate esters (Chapter 6 ) . It will also react with Grignard re agents to give phosphine oxides (4-104), and with secondary amines to give amino-substituted phosphine oxides or phosphonic dichlorides (5-43 )(5-52 ) . Phosphoryl chloride is reduced by carbon to the tri chloride (148), and with liquid ammonia, phosphoryl triamide is obtained (136). Pyrophosphoryl fluoride, P2^3^4 ,(238a) is a colourless liquid mp = 0.1°C, which can be made by the action of an electric discharge at -75°C on a gaseous mixture of P0F 3 and oxygen. The solid product
2.7
112
F
F
I
I
F
0=P—0—P=0 F
F
I
I
—P—0—P—0—P—0—
II
F
F
I
II
/
II
0
Λ
II
0
(238)
0
(a) (b) on vacuum fractionation yields P2O3F4 and a highly polymerised compound of formula (P0 2 F) n . The latter has a chain structure (238b) Pyrophosphoryl chloride, P 2 0 3 C1 4 , is a colourless oily liquid mp = -16.5 C. It can be made by passing chlorine into a suspension of phosphorus pentoxide in phosphorus trichloride and carbon tetrachloride, when PC1 5 is formed, which reacts as in (239). Studies with radioactive labelled 3 2 P atoms indicate the P — 0 — P linkages in the P40i0niolecule are utilised in the new molecule which has a structure analogous to that of (238a). P + 4PC1 4P0C1 4°10 5 ^ 2P2°3C14 + < 239 > 3 Pyrophosphoryl chloride is hydrolysed by water and it reacts readily with ammonia to give the tetramide (H 2 N) 2 P(0)0P(0)(NH 2 ) 2 and other products (Chapter 5 ) . Polymeric (P0 2 Cl) n can be made by the oxidation of PC1 3 with N 2 0. The mixed halide FC1P(0)0P(0)C1F can be obtained from P0C1 2 F and P 4 0 1 0 . The trivalent analogue of pyrophosphoryl flyuoride is stable enough to be isolated and can be made by reaction (240) as well as (171). The compound is split by HBr according to (241) to form difluorophosphine oxide as well as difluorobromophosphine . In liq uid form the oxide decomposes according to (242). The high boiling point of F 2 PH0 (67.7° C) compared to that of F3 P0 (-39.8° C) indicat es considerable hydrogen bonding in the former compound. CBu Sn^ O
V-Du any
w
+
T
F P.—O—PF
2PF Rr
41 r
l
v
>-„--P'^F
+
V
F 2 PH0
+
PF Br
> " F
+
H(K >Ά
s
DI
HRT*
r
„ F^H " F'r*0
v^
7
F
2Bu SnBr (240)
F^N)
(241) (242)
Halogeno Oxyacids Phosphorofluoridic acid (monofluorophosphoric acid) and phosphorodifluoridic acid (difluorophosphoric acid) are produced in the intermediate stages of hydrolysis of phosphoryl fluoride, and can be isolated under suitable conditions (243). /F 0«P^-F \p
HO ^-» -HF
Λ 0=Ρς-0Η ^F
H
<
?° HF
T
^0H Ο^Ρς-ΟΗ X F
?° / m ^=±^Γ 0=-Ρ<-0Η N HF 0H H
(243)
Although the reaction between hydrogen fluoride and phosphor ic acid is reversible, substantial conversion to phosphorofluoridic
2.7
113
acid can be obtained by heating phosphorus pentoxide with 69 % hydrofluoric acid (244). Sodium phosphorofluoridate is formed in 80 % yield if Graham's salt is heated with sodium fluoride for a brief period (sodium trimetaphosphate may also be used)(245). P 0 4 10 Na
4HF
4H 2 P0 3 F
2H 2 0
3NaF
3 P 3°9
(244) (245)
3Na 2 P0 3 F
Another way of making the acid is to heat pyrophosphoric and hydro fluoric acids under anhydrous conditions (246). On the other hand, if the calcium salt is heated to 550°C, condensation occurs with the elimination of HF (247). The fusion of a mixture of ammonium fluoride and phosphorus pentoxide leads to both mono- and di-fluoro salts (248). The monofluoro salt can be separated by ethanol extrac tion and precipitated as the silver salt. Aqueous solutions of KP0 2 F 2 are easily hydrolysed with caustic potash to form the monofluoro salt and potassium fluoride (249). (H0)2P(0)0P(0)(0H)2
HF
2CaP0 F.2H 0 P 0 4 10 KPO F 2 2
+
FP(0)(OH),
-> (H0)3P0 Ca
2 P 2°7
+
6NH F 4
->
2NH PO F
2K0H
->
K PO F £
2HF +
+
+
3H 2 0
2(NH ) PO F
KF
+
HO
ά
2ι
(246) (247) (248) (249)
Several phosphorofluoridates have solubilities and crystal structures very similar to those of the corresponding sulphates, although unlike the latter, they are hydrolytically unstable.Some isostructural pairs of salts are : K
2P°3F CuPO F.,5H 2 0
K
2S°4 CuSO,. 5 H 4 • 2° N i S O , , ,7H 2 0 4
NiPO F.,7H 2 0
Ovl . 51
0~P
F
12?
1.58
S^ O 1 14
0<:
562
F
Γ
X"^
1.436
(250)
Phosphorodifluoridates are obtained by heating hexafluorophosphates with metaphosphates (251) or boric oxide (252). The acid, mp = -96 C, bp = 116PC, can be made by a direct reaction between monofluoridic acid and phosphoryl fluoride under anhydrous condit ions (253). It is a clear mobile liquid which decomposes at lOCPC, 2/n(NaP0o) 3 n
NaPF^ 3KPF 6 H 9 P0 o F Δ
6
+
+
3NaP0 2 F 2
2B203 P0F o o
100°C
->
3KP0 2 F 2
^^
2HP0 2 F 2
(251) +
4BF 3
(252) (253)
114
2.7
fumes in the atmosphere, and hydrolyses slowly in water to form phosphoromonofluoridic acid. It is a strong monobasic acid when prepared fresh and is somewhat less stable thermally, than the monofluoridic acid. With phosphorus pentabromide, difluorophosphoryl bromide is obtained (254) HPO F + PBr ^ POF Br + POBr + HBr (254) 2 2 5 2 3 Phosphoromonofluoridic acid, HP0 3 F, is a colourless oil some what like concentrated sulphuric acid in appearance. It is an acid of moderate strength (Table 3.1), which is only slowly hydrolysed in mildly alkaline or neutral solution. In concentrated acid or alkaline solution this hydrolysis is more rapid. The anhydrous acid does not attack glass but with alkaline borohydrides, borane is obtained (Chapter 9 ) . NaPOF
+
2AgN0
>
Ag PO F
+
2NaN0
(255)
H PO F
+
SnF
>
SnPO F
+
2HF
(256)
Phosphoromonofluoridates in solution will give a precipitate with silver nitrate (255). The sodium and stannous salts are used in toothpaste formulations to inhibit dental caries. They act by converting the tooth hydroxyapatite into the somewhat harder and more acid resistant fluoroapatite (Chapter 3 ) . For the passivation of metal surfaces, potassium monofluorophosphate is superior to chromic acid and much less toxic. Although the chlorophosphoric (phosphorochloridic) acids are, in principle, related as in (243), they are more difficult to iso late than their fluoro analogues. Phosphorodichloridic acid can be made by the hydrolysis of pyrophosphoryl chloride at -60°C (257). It is a clear liquid, stable in the absence of air, and has a melt ing point of -18°C. Cl P(0)0P(0)C1 Δ
+ Δ
HO
>
2H0P0C1
£
(257) Δ
The dichlorophosphates of Be,Al,Ga,In and Fe are precipitated when the respective anydrous metal chlorides are dissolved in an excess of P0C13 and a stream of chlorine dioxide is bubbled into the solution. With SnCl^ obtained.
Cl
Cl
^Cl Cl Ρχ
ClvJ .0 0^1/Cl J3.T X ci-^l x o ^ X < T I ^ci
*f°
ci-Ni *™ 3
(258)
2.7 TABLE
115 2-12
Physical Data for Phosphorus Halldes bp °C
PF3 PCI 3 PBr, PI 3 PF 2 C1 PFC12 PF2Br PFBr 2 PF2I PHF 2 P2FU P 2 Iu P0F 3 POCI3
-151 - 93.6 - 40.5
-164.8 13.8 - 16.1 78.4 26.7 - 64.6 - 6.2
- 47.3 -144 -133.8 -115 - 93.8 -124.0 - 86.5 124.5 - 39.7
d
d - 39.1 105.1
bp °C
mp °C
-101 75.2 173.3
61
1.2
P0Br3 P0F2C1 P0F2Br P0FC12 P0FBr 2 P0Cl2Br P0ClBr 2
PF5
191.7
3.1
31.6 52.9 110.1
52.3 49
-84.8
mp °C PC1 4 PF 6 - 96.4 PBr 4 Br - 84.8 PBr u F - 80.1 PBr*PF6 -117.2 PSF 3 10(39mm) PSCI3 31(12rrm) PSBr 3 - 91.6 PSF 2 C1 160 PSFC1 2 -132 PSF 2 Br -124 PSFBr 2 - 61 PSCl 2 Br PSClBr 2 - 30.5
PC15 d PC1F U -43.4 PC12F3 2.5 50.4 PC13F2 PC1UF 105.9 P C 1 U P C 1 5 .F d 110
5.5
bp °C
rrp °C
d 135
d d
84 87
d 135 - 52.3 -148.8 - 40.8 125
212 6.3
64.7 35.5 125.3
80 95
38
-155.2 - 96.0 -136.9 - 75.2 - 30(60mm! - 6(60mm'
Crystalline salts of difluorodiphosphoric acid (an unstable hydrolysis product from pyrophosphoryl fluoride (238)) have been prepared (259) and the dimensions of the anion measured (260). anh P 0 4KF (259) 4 10 ■>2K2P2°5F2
(260)
2.7
116
Further Reading
(1) (2) (3) (4)
-
Chapter 2
F. Krafft, "Phosphorus from Elemental Light to Chemical Element", Angew. Chem., 3, 660 (1969). D.R. Peck, "The Physical and Chemical Properties of Phosphorus", in Mellor's Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol VIII, Supp III, Longmans, London 1971. R.E. Threlfall, "A Hundred Years of Phosphorus Making" Albright & Wilson, London 1951. R.B. Burt & J.C. Barber, "The Production of Elemental Phosphorus by the Electric Furnace Method" Chera Eng Rept No 3, TVA Wilson Dam. Alabama. 1952.
(5)
H.S. Bryant, N.G. Halloway, A.D. Silber, "Phosphorus Plant Design- New Trends" Ind.Eng.Chera. 62, 8 (1970). (6) A.D.F. Toy, "Phosphorus Compounds" in Comprehensive Inorganic Chemistry, Vol 2, Pergammon, 1973. (7) A.F. Childs in "Modern Inorganic Chemical Industry" Ed R. Thompson, Chem.So Spec. Pub 31, 1977.
(8) (9)
D.E.C. Corbridge, "The Structural Chemistry of Phosphorus Compounds", Topics in Phosphorus Chemistry, 3, 57 (1966). B. Aronsson, T. Lundstrom, S. Rundqvist, "Borides, Silicides and Phosph ides", Methuen, London, 1965.
(10) H.F. Franzen, "Structure and Bonding in Metal-Rich Pnictides, Chalcides and Halides", Prog.Solid.St.Chem., 12, 1 (1978). (11) S. Rundqvist, "Binary Transition Metal Phosphides", Arkiv. Kerai., 20, 67 (1962). (12) A. Wilson, "The Metal Phosphides" in (2). (13) H.G. Von Schnering, "Homoatomic Bonding of Main Group Elements", Angew.Chem; 20, 33 (1981). (14) V.l. Kosyakov, I.G. Vasileva, "Phosphorus Rings, Clusters, Chains and Layers" Russ. Chem. Revs., 48, 153 (1979). (15) J.E. Such, "Lower and Higher Phosphorus Oxides" in (2). (16) H. Hoffman, M. Becke-Goehring, "Phosphorus Sulphides", Topics Phos.Chem., 8, 193 (1976). (17) A.F. Childs "Phosphorus Sulphides and Oxysulphides" in (2). (18) C.A. Finch, S. Ramachandran, "Matchmaking, Science, Technology & Manufacture" Horwood, Chichester, 1983.
(19> (20) (21) (22) (23) (24)
N.L. Paddock "Recent Chemistry of Phosphine" Chem.and Ind. 900 (1955). E. Fluck, " Chemistry of Phosphine" Topics in Current Chem. 35, 3 (1973). E.J. Lowe "Phosphorus Hydrides and Phosphonium Compounds" in (2). E. Borisov, E.E. Nifantev, "Phosphorus Nitrides", Russ.Chem.Revs.,46,842 (1977). K. Utvary, "Phosphorus-Nitrogen Compounds", Method.Chim., 73, 447 (1978). M.L. Nielsen, "Phosphorus Nitrogen Chemistry", Chapter 5 in Developments in Inorganic Nitrogen Chemistry, C.B. Colburn, Elsevier, Amsterdam, 1966. (25) E. Fluck, "Phosphorus- Nitrogen Chemistry" Topics Phos. Chem.,4,291 (1967).
2.7
117
(26) D.S. Payne, "Chemistry of the Phosphorus Halides" Topics Phos.Chem. 4, 85 (1967). (27) R.H. Tomlinson, "Halides of Phosphorus" in (2). (28) R. Schmutzler, 0 Steler, "Halides of Phosphorus" MTP Science Revs., Vol 2 Butterworth. 1972. (29) S.V. Fridland, B.D. Chernokel'ski , "Structure & Reactivity of Phosphorus Pentachloride", Russ.Chem.Revs., 47, 742 (1978). (30) K. Dehnicke, A.F. Shikada., "Derivatives of Oxohalogeno Phosphoric Acids" Structure & Bonding, 28, SI Springer-Verlag, 1976. (31) R. Schmutzler, "Fluorides of Phosphorus", Adv.Fluorine Chem. 5, 1 (1965). (32) M. Webster, "Addition Compounds of Group V Pentahalides" Chem Revs., 66, 87 (1966). (33) N.M. Karayannis, C M . Mikulski and L.L. Pytlewski, "Phosphoryl & Thiophosphoryl Coordination Complexes", Inorg.Chim.Acta.Revs., 5, 69 (1971). (34) M.W.G. De Bolster, "Phosphoryl Coordination Chemistry 1975-81" Topics.Phos. Chem., 11, 69 (1983).
2
PHOSPHIDES AND SIMPLE COMPOUNDS
2.1
THE
ELEMENT
About 60 years ago three major allotropic modifications of elemental phosphorus had been recognised : white (or yellow when impure ) , red and black. These allotropes, which can be interconverted, represent successively more dense and chemically less reactive forms. In addition, gaseous and liquid states, and many other solid forms of the element are now known, although many of these can be regarded as sub-varieties of the three main allotropes. White phosphorus corresponds to the original pyrophoric form of the element prepared by Bfiand in 1669. The red allotrope was obtained in 1847 by Von Sdh&ofctQJi, and the black variety by
Bsiidgman in 1914.
White Phosphorus White phosphorus, the longest known form, is the most volatile and reactive form of the solid, and is closely related to the liquid and vapour phases of the element into which it readily transforms. White phosphorus is soft and waxy and readily soluble in many org anic solvents such as carbon disulphide and benzene. Solubilities (25°C) per 100g solvent are ; 1.27g in CC1 4 , 1.39g in Et 2 0 , 3.7g in C 6 H 6 , 0.30g in acetone and > 1000g in CS 2 . Its molecular weight in solvents corresponds to P 4 , as it is in the liquid and vapour states (see below). The density is 1.83g/cc at 20°C, mp = 44.1°C, bp = 280°C and vapour pressure = 0.173 mm The P^molecule forms a tetrahedron (see below). White phosphorus was first obtained industrially by carbon reduction of phosphoric acid, the latter having been produced by the action of sulphuric acid on bones (Chapter 3 ) . Since about 1890, white phosphorus has been obtained commercially, in about 90 % yield by heating a mixture of phosphate rock, silica (sand) and coke in an electric furnace (Fig 2.1). The mechanism of this reduction is Feed chutes
Steel casing
\
,
Gaseous P. + CO
Carbon electrodes
Carbon lining
Figure 2.1.
Phosphorus Electric Furnace (simplified).
2.1
46
quite complex, but the overall reaction is generally represented by equation (1). In spite of the heat of combustion of the coke, the reaction is strongly endothermic and requires an electrical imput of about 12,000 kW hours per ton of phosphorus produced. An 'acid displacement' mechanism is considered the most likely course of reaction (2)(3). 2CaQ(P0 )
+
6SiO
2Ca 3 (P0 4 ) 2
+
6Si0 2
P
4°10
+
+
IOC
> 6CaSiOQ > >
10C
6CaSi0 3 P
4
+
+ +
10C
P
10CO + 4
P
(1) (2)
°10
(3)
°
The phosphorus vapour is taken from the top of the furnace, condensed and collected under watert The yellow product is liable to contain As and Sb in quantities up to ^ lOOppm, as well as some Si,C,Fe and F. Decolorisation and partial purification can be effec ted by filtering the liquid element through active carbon while steam distillation will reduce the arsenic content to (\, 2ppm. The fluorine from the fluorapatite is evolved mainly as gas eous silicon tetrafluoride which is removed by scrubbers according to equation (4). If treated with caustic potash, the fluorine is recoverable as a concentrated solution of potassium fluoride (5). SSiF^ 4 H0SiF^ Δ
b
+ +
2H O Δ 6K0H
>
2H SiF„ 2 6
+
^
6KF
Si0o
+
Si0 o 2 2
(4)
+
4H O
(5)
2
M o l t e n c a l c i u m s i l i c a t e s l a g i s t a p p e d o f f from t i m e t o t i m e d u r i n g c o n t i n u o u s f u r n a c e o p e r a t i o n . Some p h o s p h o r u s combines w i t h i r o n i m p u r i t i e s t o form ' f e r r o p h o s p h o r u s ' , a p r o d u c t which c a n a l s o be t a p p e d o f f and which h a s m e t a l l u r g i c a l a p p l i c a t i o n s . The c a l c i u m s i l i c a t e s l a g c a n b e u s e d a s a low g r a d e r o a d m e t a l and a s a c o n c r e t e a g g r e g a t e . Owing t o a s m a l l d e g r e e o f n a t u r a l r a d i o a c t i v i t y , h o w e v e r , i t i s n o t u s e d i n d o m e s t i c b u i l d i n g . F o r e v e r y t o n of p h o s p h o r u s e x t r a c t e d , r o u g h l y 4 t o n s of s l a g and 0 . 3 t o n s of f e r r o p h o s p h o r u s a r e o b t a i n e d . Major o r e components a r e t y p i c a l l y : wt
P
F l o r i d a USA
2°5 34.4
K h o u r i b g a Morocco
37.2
CaO
Fe
49.3
2°3 1.2
54.2
0.1
A1
2°3 0.95 0.39
Si0 2
C0
4.21
3.8
0.97
2.64
4.2
The a n a l y s i s o f a t y p i c a l p h o s p h o r u s f u r n a c e s l a g i s a s f o l l o w s
(wt%) :
CaO
48.5
K20
1.1
F
2.8
F e ^
0.2
SiO_
40.3
S0„
0.6
P
0.5
MnO
0.2
4.9
MgO
0.5
Nao0
0.4
2
Al 0 o
F
2 3.12
«J
* E a r l y f a c t o r y workers with the element s u f f e r e d from a dangerous and f a t a l c o n d i t i o n known as n e c r o s i s of the jaw - "phossy jaw". The d i s e a s e i s now v i r t u a l l y unknown as a r e s u l t of g e n e r a l i n d u s t r i a l p r e c a u t i o n s , and t h e use of white phosphorus in match compositions was banned many y e a r s ago.
47
2.1 Phosphate rock is a potentially valuable source of fluorine and there is
currently much interest in the economic recovery of the elementfrom both the electric furnace process and the wet phosphoric acid process (Chapter 2 ) . It is estimated that the quantity of fluorine present in the annual total of mined phosphate rock exceeds that presently mined as fluorospar, CaF Most apatite contains 0.01^O.03 % uranium, but its direct recovery is not economic. Extraction from wet process phosphoric acid is more feasible (Chapter 3.1) Q
Since the world output of phosphate rock exceeds 10
tons p.a., a concentration of 4 only 0.01 % uranium represents a potential supply of over 10 tons of the heavy element. Most of the uranium in the earths crust is beleived to be associated with Apatite. Some Apatites contain relatively high concentrations (~0.01 %) of chromium and vanadium.
Elemental phosphorus can be obtained by the reduction of other minerals such as the aluminous phosphates, but, owing to tech nical difficulties, these have not yet assumed any importance in commercial processes. Aluminium phosphate can be reduced by heating with coke and lime at 1600°C (6). The byproduct calcium aluminate has a potential use a hydraulic cement, but unless the raw materials have a very low iron content, phosphorus is lost as ferrophosphorus. The process is not economic for high yields of both phosphorus and calcium aluminate. If apatite is treated with lead chloride solution it forms pyromorphite (Chapter 3 ) . This pyromorphite (or lead orthophosphate) can then be reduced by hydrogen at the comparatively low temperature of 500°C (7). 4A1P0
+
2CaO
+
2C
>·
P
+
2CaO.Al 0
+ 2C0
(6)
2Pb ^(POJ Cl + 50 H > 3Ρ„ + 20Pb + 4HC1 + 48H 0 (7) 10 4 o 2. Δ 4 2 Some electric furnace phosphorus is converted directly to phosphorus chemicals, but more than 80 % of production is 'burnt' to the pentoxide which in turn is converted to phosphoric acid (Chapter 3 ) . Ultrapure white phosphorus can be obtained by thermal decomposition of a suitable metal phosphide (Section 2.2). White phosphorus oxidises spontaneously in air, often bursting into flame.* It will burn in both oxygen and carbon dioxide to give the pentoxide (8)(9). In a restricted supply of oxygen, lower oxides are produced. Dense white smoke can be obtained in reaction (8) and the element has application as a smoke generator. Another military P4
+
P4
+ *
5 02 10C0 2
■
>
P4010
>
P4010
(8) +
10C0
The pyrophoric properties of white phosphorus were utilised in the earliest form of matches, which consisted of strips of paper tipped with the element and sealed in glass tubes. When broken, the paper would catch fire.
(9)
2.1
48
application is as a self-igniting agent in incendiary shells and tracer bullets. World War II 'Molotov cocktails' were bottles con taining white phosphorus dissolved in benzene or gasoline. Under special conditions, a cold greenish phosphorescent glow is associated with this form of the element, and it is due to a slow oxidation of the vapour emitted. This glow has been the sub ject of numerous investigations and is still not fully understood. Various chain reactions have been proposed, and spectroscopic anal ysis indicates that various molecular species are involved (Chapter 12). It has been established that the oxidation is a gas-phase re action which can take place with very low concentrations of P 4 vapour. At room temperatures the glow has a maximum intensity when the partial oxygen pressure is about 300 mm. The glow becomes weak er and finally vanishes when the partial pressure is either incre ased or decreased. The limits are influenced by vessel shape, impur ities, traces of water etc. A little ozone is also formed. The element will combine vigorously with halogens, with sul phur and many metals. It is a reducing agent and with concentrated alkalies, phosphine and hydrogen are produced (10)(11). P
+
3K0H
+
3H 0
>
PH
+
3KH PO
(10)
P
+
4K0H
+
4H 0
>· 2H
+
4KH PO
(11)
White phosphorus will precipitate copper and lead from aqueous solutions of their salts. Lumps of white phosphorus, if placed in copper sulphate solution, will rapidly become coated with black copper phosphide, which is in turn reduced to metallic copper. Sulphur chloride is reduced to sulphur (12), and thionyl chloride and potassium iodate are also reduced (13)(14). P.
+
6S 0 C1 0
>
4PC1 Q
P4
+
12KI0 3
>
4K
P4
+
8S0C1 2
>
4PC1
3P04 3
+
12S
+
6I
+
4S0
(12) +
2
+
2
10
°2
(13)
2S
2C12
(14)
White phosphorus is virtually insoluble in water (<0.0005 %) and it can be stored, fused and transported quite safely under an aqueous layer. Water dissolves in white phosphorus, however, to the extent of 0.1 mg / g of Pu at about 30°C. If it is exposed to both air and water, under conditions in which it will not inflame, a complex mixture of oxyacids is slowly produced. At elevated temper atures and pressures, water vapour will react with white phosphorus to form various products. Above 200°C in a sealed tube, phosphine and phosphorus acid are the main products (15), but above 1000°C steam will oxidise phosphorus vapour to give the pentoxide (16). P4
+
6H 2 0
>
2H
3P03
P4
+
10H 2 0
>
P4010
+
2PH
+
3
10H 2
(15)
(16)
49
2.1
White phosphorus is a strong poison and as little as 50 mg can be fatal to humans. At normal temperatures white phosphorus is cubic and has a density of 1.82 g/cc (α-form), and appears as glistening polyhedra of various kinds if grown by slow sublimation in sealed tubes. At -77°C the cubic form transforms to a hexagonal (3) form with a den sity of 1.88 g/cc. The transition point is raised to +64 C under a pressure of 11,600 atmospheres. Both solid forms of white phosphorus contain tetrahedral P4 molecules similar to those which exist in the liquid and vapour states (Fig 2.2). Electron diffraction measurements on the vapour indicate P — P = 2.21 A and interbond angles of 60°. In the cubic a form free rotation of these tetrahedra probably occurs, but in the low temperature form this freedom may be lost. White phosphorus transforms to red under the action of heat, light or X-radiation. Black Phosphorus Black phosphorus is thermodynamically the most stable form of the element and exists in three known crystalline modifications as well as in an amorphous form. Unlike white phosphorus, the black forms are highly polymeric, insoluble, practically non-inflammable and have comparatively low vapour pressures. The crystalline black varieties represent the densest and chemically the least reactive of all the forms of the element. Orthorhombic black phosphorus was originally produced by the action of high pressures on the white or red forms. It was later made by the action of heat on white mixed with mercury and in the presence of a seed crystal of black. This form of the element has a continuous double layer structure in which each P atom forms three bonds of length 2.23 A, pyramidally disposed at mutual angles of 100° (Fig 2.2). It is a semiconductor and exhibits a flakiness similar to mica and graphite which also have layer structures. Under very high pressures, orthorhombic black phosphorus under goes further (reversible) transitions to produce more dense rhombo-
white P„
rhombohedral black P n
orthorhombic black P
cubic black P n
2.1
50
hedral and cubic forms. In the rhombohedral form the simple hexag onal layers are not as folded as in the orthorhombic form, and in the cubic form each atom has an octahedral environment (Fig 2.2). Amorphous black phosphorus is made from white by the applicat ion of somewhat lower temperatures and pressures than are needed to make the crystalline varieties, and it represents a transition structure. Vitreous grey phosphorus, distinct from amorphous black, can be obtained as an intermediate product when making amorphous black. This form is dark grey, amorphous, hard and brittle, and exhibits conchoidal fracture. It can ignite on impact and is an electrical insulator. Red Phosphorus Red phosphorus is a term used to describe a variety of differ ent forms, some crystalline and all of which are more or less red in colour. They show a range of densities from 2.0 to 2.4 g/cc, and melting points in the range 585-610°C. The stabilities and reactiv ities of these red forms lie between those of the white and black forms, although they resemble the latter more closely. The vapour pressure of the red is much less than that of the white (Table 2.1). Samples of red phosphorus usually vapourise at about 450°C under atmospheric pressure. TABLE 2-1 Comparison of white and Red Phosphorus White Crystalline, waxy, translucent mp = 41.1°C Vapour pressure - high Density 1.83 g7cc Soluble in organic solvents Highly toxic Heat of sublimation 13.4 k cal/mol Chemiluminescent Spontaneous ignition near room temp Characteristic smell Spontaneously ignites in chlorine Produces phosphine v/ith alkali aqueous Contains discrete Fu molecules
Red Amorphous or crystalline, opaque mp = 585-6lO°C Vapour pressure - very lov/ Density 2.0-2.4 g/cc Insoluble in organic solvents Non-toxic (or almost so) Heat of sublimation 30.0 k cal/mol(am) Non-chemiluminescent Ignites only above 260°C. No smell Heat necessary for ignition in chlorine No reaction with aqueous alkali Highly polymeric P
Red phosphorus i s very i n s o l u b l e . It behaves as a high polymer and, unlike the white form, i t does not inflame, although i t can be more e a s i l y i g n i t e d than black. Red phosphorus i s almost non-toxic and for t h i s reason i t has replaced the white v a r i e t y in match compositions. Finely divided red P can explode when i g n i t e d in a i r . *
The cubic form i s derived from the rhombohedral form by reducing the interbond angles in the l a t t e r from 100 t o 90°, and bringing the layers c l o s e r t o g e t h e r .
2.1
51
Red can be made by heating white at 260°C, amorphous black at 125°C, or crystalline black at 550°C (Fig 2.3). Red phosphorus, like the white form, will combine directly with oxygen,sulphur, halogens and metals, although the reactions are generally less vig orous. There is no reaction with aqueous alkali , however. Red phos phorus undergoes some useful reactions with carbon compounds and is used in general chemical manufacture. Safety matches were first introduced by LuLyidU>t/l0m in 1855. The match heads contain an oxidising agent, sulphur, ground glass and glue, while the striking surface contains the red phosphorus.* Toy pistol caps contain potassium chlorate and red phosphorus in separate compartments, which,when struck together will explode. On exposure to damp air, red phosphorus will undergo a very slow oxidation to orthophosphoric acid. The oxidation of red phos phorus using concentrated nitric or sulphuric acids can be repres ented approximately by equations (17)(18). P
+
5HN0
>
HΗ ο Ρ0„ P
4P
+
8H SO
>
4Η
3 °4
0 4Hο3ΡP0 „4
++
5N0 o 5N0
++
HH o 0
2°
(17)
2H 200 7S0 ++ 2H ++ Ss ++ 7S0
(18)
Commercial red phosphorus, which is largely amorphous, is made by heating white in closed vessels at about 280-350°C for 48 hours. The product is wet milled, boiled with alkali to remove traces of white, then filtered and washed. The commercial red form exhibits conchoidal fracture and can show a range of colours from pale yellowish-red to dark violet-red. The colours obtained by heat ing pure white at various temperatures in the range 300-610°C under laboratory conditions can vary from dark red to light orange-red. Such differences can arise from particle size variations, the paler coloured forms generally being more reactive because of smaller particle size. A very fine pale red form can be obtained by expos ing a carbon disulphide solution of white to UV or visible light. On the other hand, comparatively large violet particles can some times be obtained by systematic levigation of commercial red with water. Violet phosphorus or W/JUtoh^ & phosphorus, made by recrystallisation of the element from molten lead, and Sch&nck1A scarlet phosphorus, made by heating a solution of white in phosphorus tribromide, are known to be chemically impure. The many colour varia tions reported for red phosphorus can often be associated with either particle size or surface impurity effects. *
Typical formulae are : match head KCIO 3 K
Cr
2 2°7 Mn0 2 S Fe
2°3
match bo x (wt%)
(wt %) ground glass
8.5
red P
49.5
3.5
kieselghur
1.0
Sb
27.6
4.7
ZnO
0.5
Fe
3.5
glue
5.0
3.8
water
37.0
S
2 3
2°3 gum
water
1.2 20.9
2.1
52
X-ray diffraction, optical microscopy and DTA have neverthe less established the existence of several different crystalline 'red' varieties of the pure element, in addition to the amorphous form. These crystalline varieties are generally made by heating the amorphous form to various temperatures below the melting point. Hittorf's violet phosphorus is a complex three-dimensional polymer in which each P atom has a pyramidal arrangement of three bonds linking it to neighbouring P atoms (Fig 2.2). It seems likely that all forms of red phosphorus (like black) are built from pyram idally linked atoms, the different crystalline varieties represent ing different kinds of ordered framework, or differing degrees of polymerisation.
"V W ΐ ί P
I
P
X
P I
I
P
^P I
νΛ W
Ί P
P
I
X
P I
l
^
P
\
I
ΑΡ
V,p \
v ^
/ N—p'
(19)
A,-
^* P --P'
\
It is likely that in most samples polymer growth is terminated by occluded 'impurity' atoms such as halogen, oxygen or hydroxyl. Amorphous samples probably consist of entirely random networks of atoms (19). Brown phosphorus has been obtained by condensing phosphorus vapour containing P 2 molecules on to a surface at liquid nitrogen temperatures. Above 0°C it changes to a mixture of red and white. Phosphorus Vapour When heated under normal pressures, all allotropic forms of phosphorus will produce a vapour consisting of tetrahedral P 4 mol ecules (Fig 2.2). The interbond angles of 60° represent a highly strained arrangement for which hybridised pd 2 orbitale have been proposed. The tetrahedral P 4 type structure is very unusual but not quite unique in chemistry. Elements of the same group adopt it, i.e. As 4 , Sb 4 and perhaps Bi 4 (see below). The existence of isoelectronic S i 4 4 , and Ge 4 4 ~, Sn u 4 ~and Pb 4 4 ~ anions has recently been established. At temperatures above 800°C dissociation of the tetraatomic vapour occurs. This is appreciable at 900°C, and at 1700°C roughly equal numbers of P 4 and P 2 molecules are in equilibrium. Emission spectra indicate that the P 2 molecule contains a triple bond with P = = P = 1.875 A. The equilibrium ^ ^ Λ_ _M „ , H p ^ ^ 2P - 54.5 k cals /mol lies to the left hand side except for high temperatures or low pre ssures. Further dissociation by the reaction P
^
± 2P
-
116 k cals/mol P
is very small at 1700°C, but it has been calculated to reach about 8 % at 3000°C. If any solid form of phosphorus is vapourised and then condensed at low pressure, a red variety is obtained. At low
53
2.1
pressure red phosphorus depolymerises directly to P2 molecules which recombine to form Pn molecules of white phosphorus. Under non-equilibrium conditions, the red form on vapourisation may yield other species such as P 3 , P 6 , P 8 etc. When red phosphorus is slow ly precipitated from solutions of white in CS 2 or PBr 3 , it probab ly involves photochemical dissociation into P 2 molecules which then polymerise to form the red variety. Liquid Phosphorus It is probable that all forms of phosphorus can be melted to form what is the same colourless liquid. Liquid phosphorus can be readily supercooled to a state from which the rate of crystallisat ion of the white form is extremely rapid. The Raman spectra of the solid and solution states of white phosphorus resemble that of the liquid, indicating that the latter also contains tetrahedral P4 molecules.
T n c l i nci r ed (14)
[ Rhombohedral black
Cubic r e d -
U) Cubic black
Figure 2.3
Relationships Between Forms of Elemental Phosphorus.
(1) High vapour pressure at room temperatures,(2) heat at 540°C,(3) heat at 550CC,(4) heat at 600°C, (5) heat at 125°C, (6) heat at 400°C, (7) heat at 550"C, (8) heat at 300°C at 8000 atm, (9) heat at 380°C with Hg or above 250°C at 12 kb, (10) heat at 400°C with Hg for days, (11) heat at 200°C at 12000 atm, (12) heat at 200*C at 15000 atm, (13) heat at 200°C at 12000 atm (14) reversi ble transition 50-100 kb, (15) reversible transition 110 kb, (16) recrystallize from molten Pb, (17) heat a Pßr^ solution, (18) reversible transition at 900°C, (19) reversible transition at 1700"C, (20) reversible transition at low pressure (21) reversible transition at 44.1ÖC (but can supercool), (22) reversible trans ition at -77°C or +64f,C under 1200 atm, (23) sublime under vacuum, (24)heat at 220°C at 12 kb, (25) irradiate with UV at -190°C, (26) condensation of P2 vapour at -196°C, (27) heat above -100°C, (28) heat at low pressure, (29) boils at 280 °C, (30)heat at 300°C or expose to light or X-rays, (31) melt about 600°C.
2.1
54
Phosphorus Ions Alciiough there remains some doubt about the existence of monatomic P3"" anions, the existence of a large number of polyphosphorus anions has been confirmed in recent years. These anions are found in metal phosphides and involve catenated atoms which form chains, rings, cages etc(Section 2.2). Phosphorus cations have been observed as unstable species in the mass spectrometer (Chapter 12). Comparisons of Pnictide Elements Nitrogen is unique amongst the pnictide elements because it exists solely as N 2 molecules in the gaseous, liquid and solid states. In all these forms the element is colourless, odorless, non-inflammable and non-toxic. Nitrogen, Ν = Ξ Ν , forms the most inert diatomic molecule known. The remaining pnictide elements, because of their comparative reluctance to form multiple bonds, all prefer to catenate and consequently exist in forms more highly pol ymerised than diatomic nitrogen. Elevated temperatures are required to break them down into diatomic species. Arsenic and antimony exist in yellow crystalline forms which contain tetrahedral As 4 or Sbu units similar to the Pu units of white phosphorus. These tetrahedral molecules, which pack together into cubic lattices in the solid state, persist in CS 2 solution and if the elements are melted or vapourised. On heating to higher temp eratures, the tetraatomic vapours dissociate to diatomic species and eventually to single atoms. This dissociation takes place more readily with increasing pnictide atomic weight (arsenic is complete ly dissociated to A s 2 at 1700°C). Arsenic exists in a black and a grey form, and antimony and bismuth both exist in 'metallic' forms, all of which are highly polymerised. Like the red and black forms of phosphorus, they show insolubility, greater densities, higher melting points and lower reactivities than their tetraatomic polymorphs. The black form of arsenic has the same crystal structure as orthorhombic black phosphorus. The grey form of arsenic and the grey (metallic) forms of antimony and bismuth all have the same layer type structure which is similar to that of rhombohedral black phosphorus. Although the latter appears only under pressure, the structure becomes more stable as the atomic weight increases. The rhombohedral forms of As, Sb, and Bi are the most stable forms of these elements and they can all be obtained by condensation of their tetraatomic vapours. The tetraatomic forms or the vapours from any allotropes of P, As or Sb are all extremely toxic either by inhalation or ingestion. The red, black or metallic forms, unless vapourised are, on the other hand,much less toxic.
t
heat
P
4
As,
Sb 4 -^
Asr
Sb n -^
(Bi4) decreasing thermal stability Bi n increasing thermal stability
2.1
55
The differences between the interatomic and the inter sheet distances in the rhombohedral layer structures of the pnictide elements become progressively less as the atomic weight increases: 3 closest in same layer
P 2.13
As 2.51
Sb 2.91
Bi 3.10 A
3 closest in parallel layer
2.83
3.15
3.36
3.47
This c o n s t i t u t e s an approach towards a s y m m e t r i c a l o c t a h e d r a l e n v i r onment f o r e a c h p n i c t i d e atom and i s i n a c c o r d w i t h i n c r e a s i n g met a l l i c c h a r a c t e r . This m e t a l l i c c h a r a c t e r i s s u g g e s t e d by t h e chang i n g appearance and i n c r e a s i n g v a l u e s of d e n s i t y , c o e f f i c i e n t s of e x p a n s i o n , e l e c t r i c a l and thermal c o n d u c t i v i t y which are o b s e r v e d on moving from phosphorus t o bismuth ( T a b l e 2 . 2 ) . TABLE 2-2 Physical P r o p e r t i e s of
Pnictide
Elements P
As
yellowish red
Colour
various
Crystal system Density
(g/cc)
Sb
s t e e l grey
s i l v e r white
rhombohedral
rhombohedral
rhombohedral
2.31
5.73
6.67 631
273
1380
1560
88
40
13
102
383
204
Melting point
(° C)
589 (^3 atm)
8l8 (36 atm)
Boiling point
(° C)
416 (sub)
633 (sub)
Latent heat of fusion (cals/g)
Bi reddish white
122
Latent heat of vapourisation
9.80
If data were available, a more precise comparison could be made with the isostructural black rhombohedral form of P. There is, however, a remarkable lack of fundamental physical data on even the red form.
Although the highly polymerised pnictide structures represent the least reactive forms of these elements, red P and the metallic forms of As, Sb and Bi can all be made to burn in air and they will combine directly with oxygen, sulphur and the halogens. They are all attacked by concentrated nitric and sulphuric acids and by hyd rogen chloride in the presence of oxygen. The elements are attack ed either very slowly by moist air at room temperature or more rapidly by steam at higher temperatures (Table 2.3). TABLE
2-3
Chemical Properties of Pnictide Elements P Combustion in air
P
Ignition in C0 2
P^O
Steam at high temps
H°10 Q
P^O +H~
As
Sb
As^Og (+A S l | 0 1 0 )
sb^o6
B1
ksH06
S
Bi
As
C+AS^Q)
i | O 6 + H 2 ( + A s l4°10 )
Cone HN0 o
H3POi|
H^AsO^
Hot cone H2SOi|
H^PO^
<*iPe
V6
Bi
H°6
H°6
Sbjj06+H2
Bi^0 6 +H 2
Sb
Bi(N0 3 ) 3
H°6
Sb^SO^),
Bi 2 (SO l 4 ) 3
2.2
56
Interpnictide Compounds The pnictide vapours have similar tetrahedral structure and the vapours from mixed liquid phases of phosphorus and arsenic have been shown spectroscopically to contain stable interpnictide molec ules P 3 As, P 2 As 2 , and PAs 3 , all based on tetrahedral units. Mass spectra of the vapours from various As/Sb/Bi mixtures at 300-750°C and 10~"5 atm also indicate that many stable interpnictide species exist e.g. BiSb 3 , Bi 2 Sb 2 , Bi 3 Sb, Sb 3 As, SbAs 3 etc. Orthorhombic black phosphorus will take up arsenic in solid solution and it forms a definite compound AsP. The latter can also be made by reaction (20). Arsenic phosphide resembles orthorhombic black phosphorus in physical properties and very probably has a similar puckered layer structure. AsH 3
+
PC1 3
2Quc
>
AsP
+
3HC1
(20)
Continuous series of solid solutions are formed between the common highly polymerised forms of adjacent pnictide elements i.e. P/As, As/Sb and Sb/Bi. Solid solution formation between remaining combinations i.e. P/Sb, P/Bi and As/Bi appears to be very limited and lie below 1 % of one element in the other. Nitrogen does not dissolve to any appreciable extent in either Sb or Bi, although the mononitrides SbN and BiN have been reported to exist. Nitrogen does, however,form several phosphorus nitrides (Section 2.6), and arsenic analogues probably also exist.
2,2
METALLIC
PHOSPHIDES
Almost all metals form phosphides and over 150 different binary compounds are known (Table 2.4). In addition there are many mixed-metal phosphides, phosphide sulphides and phosphide selenides. Many of the binary phosphides lie within the composition range M 3 P to MP 3 with some metals such as nickel forming several phases : Ni 3 P, Ni 5 P 2 , N i 1 2 P 5 , Ni 2 P, NiP, NiP 2 while others such as aluminium form only one : AIP. It is only since about 1960 that most of the metal phosphides have been properly characterised and obtained in a sufficiently high state of purity to enable reliable measurements of their prop erties to be made. These compounds show a wide range of crystal structures and differ greatly from one another in their physical and chemical properties. Bond type is often uncertain. The majority of metal phosphides have a metal arsenide analogue which they usually resemble closely in properties and structure (Table 2.5). Metal phosphides, arsenides and nitrides not infrequently exhibit properties similar to those of metal carbides, silicides and germanides.
2.2 TABLE
57 2-4
Conposition of Well Characterised Binary Phosphides
LUP
Be 3 P 2
B
IAPC
BeP 2
BP
UP?
AIP
Na^P
GaP
Na2P5
InP
Na3Pn Ti P
V P 3
Cr P
Mn P
FeP
Co 2 P
Ni P
CaP
Ti 5 P 3
VP
CrP
Mn 2 P
Fe 2 P
CoP
Ni 5 P 2
CaP 3
T
VP2
CrP^
MnP
FeP
CoP
Ni12P5
MnP^
FeP 2
Ni 2 P
FeP^
Ni 5 P 4
¥5
Ca 3 P 2
KP15 K
4P6
ScP
V3
K3P7
TiP
^4
Cr
KP
TiP 2
V3 V7
CrP 2
Rb 2 P 5
SrP
^4Ρ6
Sr3Pl4
RbP ?
SrP 3
YP
12P7
V12P7
NiP 2
V2P
NiP
Nb 3 P
Mo~P
Ru 2 P
Rh 2 P
Pd
ZrP 2
Nb5P3
M 0l| P 3
RuP
Rh n P 3
Pd 7 P 3
NbgP 5
MoP
RuP 2
RhP 2
PdP 2
NbP
MoP 2
Re
6P17
RuP^
RhP 3
PdP 3
Nb 2 P 5
MoP^
Re
6P13
Ba 3 P 2
LaP
Hf 2 P
Ta 2 P
Cs
BaP 3
LaP^
Hf 3 P 2
HfP
3Pl4
TlPc
BaP 1 Q
LaP
CeP
PrP
LaP 7
CeP 2
PrP 2
UP
Re 2 P
OsP 2
Ir 2 P
pt 5 P 2
TaP
3P WP
R^H
OsP^
IrP 2
PtP 2
TaP 2
WP2
ReP
Th3Pl4
U P 3 4
ih2pn
UP2
NdP
Νρ 3 Ρ 4
3P
ReP W
HfP 2
ThP
3
ZrP
CsP ?
Ba
3
NiP
NbP 2
3 P ll
13P2
PuP
IrP 3 Zn^P 2
SiP
s^?5
SiP 2
SnP
SmP
CiuP
ZnP 2
GdP
Cu 2 P
Cd^P 2
TbP
CuP 2
cd 6 P 7
r^yP
A g 3 P n CdP 2
HoP
AgP 2
CdP^
ErP
Au2P3
cd7P10
SnP
GeP GeP
3 GeP c
3
2.2
58
TABLE
2-5
Phosphides and Isostructural
Metallides
Li 3 P, Na 3 P E e 3 P 2 , Mg 3 P 2 Zn 3 P 2 , Cd 3 P 2 Cu 3 P, Cr 3 P,Mn 3 P, Fe 3 P, Ni 3 P Γ4ο3Ρ, W 3 P Ti 3 P, V 3 P , Fe 0 37B0 63, Nb 3 P, Zr 3 P, Ta 3 P Mn 5 PB 2 , Fe 5 PB 2 , Co 5 PB 2 Pd 3 P Mn 2 P, Fe 2 P, Ni 2 P Co 2 P, Ru 2 P, Re 2 P, ZrFeP, TiFeP Ta 2 P, Hf 2 P, V 3 PC, V 3 PN, Cr 3 PC, Cr 3 PN V 2 PC Li 9 TiP u Ba 3 P 2 Hf 3 P 2 U2N2P Rh u P 3 Ti5P3, V5P2N
Li 3 As, Na 3 As, K 3 As, IrSi 3 Mg 3 As 2 Zn 3 As 2 , Cd3As 2 Cu 3 As Pd 3 As aV 3 S Ti 3 Si, Zr 3 Si, Nb 3 Si, Ta 3 Si, Zr 3 As
BP, AIP, GaP, InP MgGeP 2 , CuSi 2 P 3 , CuGe 2 P 3 Zn 3 PI 3 ZnSiP 2 , ZnGeP 2 , CdGeP2 B12BP2 Rh 2 P, Ir 2 P LiMgP, LiZnP, LiyVP, Li 5 SiP 3 , Li 5 GeP 3 , Li 5 TiP 3 Li 3 AlP 2 ScP, YP, ZrP, LaP, CeP, PrP TbP, ErP, DyP, HoP, NdP, SmP, ThP, UP, PuP
8-MbP, ß-TaP TiP, ZrP, HfP
MoP VP
CrP, MnP, FeP, CoP, RuP, WP Th 3 P t ,, u 3 p u , Np 3 P L», Ti u P 3 GeP, SnP CuPS,, CuPSe , AgPS3, AsPSe Re 3 P t l SiP 2
UP2
IvbP2., WP 2 FeP 2 ., RuP 2 , 0sP 2 PtP 2i , NiP 2 , SiP 2 CoP 3 ,, NiP 3 , RhP 3 , PdP 3 , IrP NiP 2 ., RhP 2 , PdP 2 , IrP 2 , ZnP 2 VP 2 , NbP 2 , 'TaP 2 , WP 2 TiP 2 .• ZrP 2 , HfP 2
Cr 5 B 3 , Mo 5 SiB 2 , Nb 5 Si 3 , Ta 5 Si 3 , V 5 SiB 2 , Fe 5 SiB 2 , Mn 5 SiB 2 Pd 3 As, Pd 3 Si, Pd 3 B, Ni 3 B, Co 3 B, Fe 3 C, Mn 3 C Pt 2 Si, Pd 2 Si, Ni 6 Si 2 B, ß-Co 2 As, Pd 2 As 6-Rh2As, Rh 2 Si, Co 2 Si, Ru 2 Si, Ir 2 Si, 6-Ni2Si, ZrAs 2 Ti 2 S, Zr 2 S, Ti 2 Se, Zr2Se V 3 AsN, Cr 3 AsC, V 3 AsC, Re 3 B V 2 AsC Li 9 TiAsu, Li 3 Bi Ce 2 S 3 Cr 3 C 2 Ce 2 0 2 S Ni,B 3 Mn 5 Si 3 , Sc 5 Si 3 , Y 5 S i 3 , V 5 S i 3 , Mo 5 Si 3 , Zn 5 Si 3 , Nb 5 Si 3 Ta 5 Si 3 , Ti 5 As 3 , Cr 5 Si 3 , Hf 5 Si 3 , Fe 5 Si 3 AlAs, GaAs, InAs, BAs, SiC, ZnS, Si, C ZnSnAs2 Zn 3 AsI 3 ZnSiAs 2 , ZnGeAs 2 , CdGeAs 2 , CdSnAs 2 Bi 2 AsP 2 , Bi 2 Si 3 , B i 2 B C 2 , B i 2 C 3 , B i 2 S 3 Mg 2 Si, Be 2 B, Be 2 C LiMgAs, LiZnAs, Li 7 VAs u , LiMgN, LiZnN Li 5 SiAs 3 , Li 5 GeAs 3 , Li 5 TiAs 3 , Li 2 AlAs 2 YAs, LaAs, CeAs, PrAs, NdAs, SmAs, VAs, PuAs, PaC, PuN LaS, CdS, PrS, ErS, ,NdS, YbS, ThS, US, PuC, HfC, NbC, UC, SiC, HfB, ZrB, PuB, CrC, TiC, ZrC, VC, TaC, LaN, CeN, ThC, UB NbAs, TaAs MoC, TiAs, ZrAs, NbN, HfAs WC NiAs, TiAs CrAs, MnAs, FeAs, CoAs, PdSi, PtSi, VAs, HfSi, IrSi Th 3 As u , U 3 A s 4 , U 3 B i u , U 3 Sb u , C e 3 S u , P u 3 S 4 , N d 3 S 4 , Sm 3 S u G d 3 S u , La u Ge 3 , Ce u Bi 3 , GeAs, SiAs CuBS, CuBSe, AgBSe Fe 3 Se u , Cr 3 S4, Cr 2 NiS u , Ce 3 Se u SiAs 2 , GeAs 2 UAs 2 ZnSi 2 FeAs 2 , NiAs2, FeS 2 , CrFeAs u , LaC 2 FeS 2 , MnS2i PdAs 2 , CaC 2 , PtAs 2 , PdSb 2 , PtSb 2 , PtBi 2 CoAs 3 , IrAs 3 CoAs2, RhAs 2 , IrAs 2 , FeAsS ZnAs 2 VAs 2 , MoAs 2 , W A s 2 , TaSb 2 , NbSb 2 , NbAs 2 , TaAs 2 ZrAs 2 , HfAs 2 , ThS 2 , U S 2
59
2.2
Metal phosphides can, in general, be made by direct union of the elements in vacuo or protective atmosphere under conditions which prevent undue loss of phosphorus, as e.g. (21)(22). 3Li
Li
Ge
GeP
(21)
3P
(22)
In special cases they may be made by reaction of phosphine with an oxide (23), or a metal chloride (24), or by reduction under the correct conditions, of a phosphate with carbon (25) or hydrogen (26). 2PH
Ga
PH.
3ZnCl
->
2°3 2
Ca
3(P°4)2
8C
Fe
2 P 2°7
7H„
2GaP
^ -»
3 2 Ca
3P2
-> 2FeP
3H 2 0
(23)
3HC1
(24)
8C0
(25)
7H 2 0
(26)
Other methods include the reaction of calcium phosphide with an appropriate metal powder at about 1200°C (M = Ti,V,Mn,Co,Cr,Nb, Ta,Mo, or W) (27) or with a metal chloride (28). The high temperat ure electrolysis of a metal oxide-alkali phosphate melt can be used as well as simply heating a higher phosphide (29). Ca
3P2
2Ta
-> 2TaP
3Ca
(27)
Ca
3P2
2CrCl,
->2CrP
3CaCl,
(28)
4RhP.
->
(29)
4RhPr
Metal phosphides, except those with a high phosphorus content, usually have melting points above 1000°C (Table 2.6). TABLE 2 -6 Melting Points of Metal Phosphides (°C) Fe3P Cu3P Pd3P Ni3P Mn3P
1166 1023 10^7 970 1229
Fe2P Co2P Ni2P Mn2P Ti2P
1365 1386 1110 1327 1920
Rh2P Ca3P2 MnP CrP VP
1500 1600 1193 1800 1315
TIP AIP GaP NbP ReP
1100 1800 1522 1729 1204
The chemical purity of phosphides is important since impur ities can have drastic effects on their properties, particularly electrical. Many phosphides are high melting point materials which are produced by sintering processes. Specimens can consequently exhibit large variations of porosity - a factor which also affects their physical properties. Densities in particular are liable to be somewhat less than the theoretically possible values. However, the densest of all phosphorus compounds are found amongst the metal phosphides (Table 2.7). These high densities arise from the
2.2
60 TABLE 2 -7 Densities of _Metal Phosphides g/cc g/cc Fe Ni 8.90 7.87 FeP 6.92 N1 P 7-90 3 Fe2P Ni2P 6.90 6.30 FeP NIP 6.07 5.85 FeP2 NiP2 ^.58 5.11
Mo Mo^P MoP MoP2 MoP^
g/cc 10.22 9.07 7.20 5.35 3.88
g/cc Ir 22.4 Ir2P 15.6 IrP2 9.33 Pd 12.02 Pd?P3 9.53
presence of the heavy metal atoms and the efficiency with which they pack with the phosphorus atoms in the crystal lattice. A simple but highly symmetrical scheme of coordination of both the metal atoms and the phosphorus atoms is found in many phosphide structures (see below). Phosphides, in general, tend to have lower melting points and lower stabilities, and to be less hard than silicides, borides and other metallides. Metal-rich Phosphides
MxPy
(x>y)
The metal-rich transition metal phosphides (< 60 % P) are dark coloured and insoluble in water, they have high chemical and thermal stability, they are dense, hard and brittle and have high thermal and electrical conductivities. These properties they have in common with transition metal borides and silicides (and in some cases carbides and nitrides) to which they are often structurally related. With few exceptions, the transition metal phosphides borides and silicides are not attacked by dilute acids and bases and may remain unaffected by hot concentrated mineral acids.
(b)
(c) Figure 2.4
C r y s t a l S t r u c t u r e s of P h o s p h i d e s .
(b) Fe 2 P f r a g m e n t , ( c )
Rh P u n i t c e l l ,
(a) Fe~P f r a g m e n t ,
(d) ZrP u n i t c e l l ,
(unfilled circles = P).
A common s t r u c t u r a l f e a t u r e of almost a l l t r a n s i t i o n metal phosphides of the m e t a l - r i c h category i s the n i n e - f o l d ( t e t r a k a i d e c a h e d r a l ) arrangement of metal atoms around the phosphorus atom
2.2
61
(Fig 2.4a). Such tetrakaidecahedra contain metal-phosphorus distan ces expected for covalent bonds, and are arranged in various patter ns in which the metal-metal distances are greater than in the corre sponding pure metals. There are no covalent P — P linkages. In some metal-rich phosphides, one or more of the equatorial arms of the tetrakaidecahedron may be missing, resulting in only 8,7 or 6-fold coordination of the P atom by metal atoms. The 6-fold trigonal prismatic coordination (Fig 2.4b) occurs in some monophosphides (see below). Non-stoichiometry or 'extended homogeniety ranges' are found in most of the transition metal phosphides with formulae M 3 P and M 2 P . Palladium phosphide, Pd 3 P, e.g. can have any composition in the range P d 3 P 0 # 7 5 - Pd3 P 1 > 0 0 and cobalt phosphide in the range Co 2 P - C o 1 # 7 5 P . These phenomena are believed to be associated with vacant lattice sites where P or metal atoms are missing. Both boron & silicon have appreciable solid solution in some metal phosphides, where random substitution for P atoms occurs. Close structural relationships exist between some borides, suicides and phosphides. Phosphorus will dissolve to the extent of about one or two percent in some metals such as iron and copper, without detectable compound formation. This has little effect on mechanical properties and produces only slight changes of unit cell dimensions from the original metal. Small amounts of phosphorus may, under some circum stances, appear as metal phosphides which form separate phases at the grain boundaries of the metal. In this case the consequent mod ifications of mechanical properties of the metal may then be consid erable . The strength and corrosion resistance of iron is increased by the presence of small quantities of phosphorus. Grades of wrought iron and cast iron frequently contain 0.1 -0.2 % P, some of which is present as phosphide. Phosphorus is nearly always present as an unavoidable impurity in commercial steels. A maximum of 0.05 % is usually allowed, since it otherwise reduces ductility, although machinability is improved. The metal phosphide which is produced in largest quantity commercially, is ferrophosphorus, which contains Fe 3 P and Fe 2 P. This material is formed as a byproduct in the production of element al phosphorus by the electric furnace method (see above). Until recently, ferrophosphorus was added to steel furnace melts, where, together with the phosphorus already present in the iron ore, it reactedwith the basic furnace lining to form basic slags. The latter is a useful fertilizer. Phosphor-bronze contains copper, tin and about 0.1-0.5 % P, the latter element being added for de-oxidising purposes. The phos phorus is incorporated with the copper before the addition of the tin, in order to prevent the formation of tin dioxide which is more difficult to reduce than oxides of copper (the oxygen is removed from the melts as volatile phosphorus pentoxide). Phosphorus remain ing in excess of that needed for de-oxidation, increases the ten sile strength, hardness and corrosion resistance. Phosphor-bronze finds use in springs, electrical contacts and non-corrodible fixing
2.2
62
lugs in building construction. De-oxidised and 'phosphorised' copper contains 0.01 -0.05 % P and has improved hot and cold working characteristics although its electrical conductivity is less than that of the purer metal. It is used for gas and water pipes. The addition of larger quantit ies of phosphorus to copper results in the separation of Cu 3 P as a separate phase. The master alloy known as 'phosphor copper'contains 10-15 % phosphorus, much of it as Cu 3 P. This is used for de-oxidat ion and for adding phosphorus to alloy melts. Copper phosphide, CU3P, is hard, dense, brittle and electric ally conducting. In contrast to most M 3 P and M 2 P transition metal phosphides, Rh 2 P and Ir2P are very hard and inert, and the latter has been used for fountain pen tips. Schreibesite, (Fe,Ni)3P, has been detected in meteorites and lunar samples and it constitutes a rare example of a naturally occurring reduced phosphate mineral. Phosphides of this type may be present in the earth's core. Vitreous Metals So-called 'vitreous metals', 'amorphous alloys' or 'metallic glasses' can be obtained by extremely rapid chilling of molten mixtures of one or more metals with boron, carbon or phosphorus. Most compositions contain phosphorus e.g. : Fe
80 P 13 C 7
Fe
80 P 14 B 6
Fe
Ni
79 P 21
Fe
70 C r iO P 13 C 7
Fe
80
Pt
60 N i 15 P 25
M
°60 R e i6 P 10 B 10
Fe
32Ni36Cri4Pl2B6
40
N1
P
40P14B6
16C3B
Disordered alloys of this kind have high strength and corros ion resistance, extreme hardness and wear resistance/and compare favourably with conventional crystalline metal alloys. Some amorph ous alloys have unique magnetic properties with low hysteresis losses, low permeability etc . Related to these alloys are metal/ P/S and metal/P/Se glasses which can be used as electrical conduct ors or as optical fibres (Section 2.4). Monophosphides MP The monophosphides, MP, where M = Ti,V,Cr,Mn,Fe,Co,Ru,W,Mo,Zr, Nb, or Ta, are greyish black with metallic lustre and they have very high melting points and densities. Three types of hexagonal structure which are found amongst this group of compounds are indic ated in Fig 2.5abd. Trigonal prismatic coordination of the phosphor us by the metal atoms is mostly present, and the bonding is probably part metallic and part covalent. Most of these compounds are hard, chemically inert and resistant to oxidation at high temperatures. Tantalum molybdenum and tungsten monophosphides have been used as nose-cone materials in space rockets. The monophosphides MP, where M = B,Al,Ga or In, form an important group of phosphides in which each atom is tetrahedrally coordinated by atoms of the opposite kind in a cubic zinc blendtype structure. Their structures are also similar to those of
63
2.2
Figure
2.5
Structures of Monophosphides.-
diamond, silicon and boron nitride (Fig 2.6). These monophosphides are hard high melting-point compounds which have important semi conductor properties. The isomorphous arsenides have similar semi conductor properties and the system GaP-GaAs has been much studied in this connection. The value of the energy gap can be controlled in solid solutions of the type GaAs x Pi_ x (Table 2.8). The compound GaAsP is utilised in light-emitting diodes. Difficulties in obtain ing samples with the necessary high chemical purity and freedom from lattice defects have so far limited the wide-scale commercial use of these materials.
(a) Figure 2.6
(b)
(c)
(d)
C r y s t a l S t r u c t u r e s of Diamond Type P h o s p h i d e s .
(a) AIP, (b) BN, ( c ) Z n S i P 2 ,
(d) diamond ( o r s i l i c o n ) .
Various mixed phosphides such as MgGeP2, CuSi 2 P 3 and isomorph ous compounds a r e semiconductors w i t h c r y s t a l s t r u c t u r e s r e l a t e d t o
64
2.2
TABLE
2-8
Physical Properties of Monophosphides and Isostructural Pnictides Unit cell (A)
Density
g/cc
mp (°C)
Energy gap (eV)
6.0 2.5
BP AIP GaP
4.538
2.9
2500
5.451 5.450
2.85 4.13
1497 1477
InP
5.869
1057
1.27
AlAs
5.662
4.79 3.81
1598
2.16
GaAs InAs
5.653 6.058
5.32
1298
1.35
5.66
1942
0.36
AlSb GaSb
6.135
4.22
1057
1.60
6.095
5.62
0.67
InSb
6.479
5.77
707 525
2.24
0.16
those of the simple monophosphides in Fig 2.6. Small quantities of P atoms ( ^0.001 %) when substituted for Si atoms in the silicon lattice enhance its semiconductor properties . There are two well-characterised boron phosphides, BP and B 1 3 P 2 . The monophosphide exists as an amorphous brown powder or as red brown cubic crystals with the zinc blende structure. The second phosphide has a structure based on the complex icosahedral frame work of elemental boron and is believed to be isostructural with the compound B 1 2 C 3 (Fig 2.7).
Figure
2.7
Structure of B
P .
Open circles represent end views of P-B-P chains which link together B cages.
The monophosphide is made by heating boron with red phosphorus at about 900°C in sealed tubes. It is stable at ordinary temperature and up to 2500°C under pressure. Heating in vacuo induces decompo sition to the icosahedral boride (30). Thermal decomposition of some boron trihalide addition compounds will yield the monophosphide (31) and displacement from another metal phosphide may also be used (32) (33). B
red
H 3 P.BC1 3 BC1 3
*
->
BP
->» BP AIP -
-> BP , -
">
B
13 P 2
3HC1 AlClr — 3
white
(30) (31) (32)
The B/P ratio is slightly variable and the formula is sometimes quoted as BgP
65
2.2 Zn P
+
^
2B
2BP
+
3Zn
(33)
The crystalline monophosphide is inert, it is harder than most metal borides and is as hard as silicon carbide and nearly as hard as boron nitride. It resists oxidation up to 800°C and is not dissolved by boiling mineral acids or cold concentrated alkali. Boiling with the latter produces phosphine and with steam above 400°C, some phosphine and boric acid are formed. Boron phosphide reacts on heating with halogens to form addition complexes. When heated to high temperature in an atmosphere of ammonia, cubic boron nitride and phosphine are formed (34). BP
+
NH
2A1P
+
3H
2
> S0
4
>
BN 2PH
+ 3
+
PH A1
(34)
2(S°4)3
(35)
Aluminium phosphide, AIP, can be made by reacting Zn 3 P 2 or red P with excess aluminium above 900°C. Whereas boron phosphide is not attacked by water, aluminium phosphide is slowly hydrolysed in moist air, but more violently by water or dilute acid (35). Alumin ium phosphide tablets are used for grain fumigation since they will slowly release phosphine under storage conditions. Decomposition by water is a property of ionic phosphides (see below), but highly purified AIP is reported to be water stable. The monophosphides of the elements Ca,Sr,Y,Sc, and the lanthanides La to Th all crystallise with ionic-type structures, thus suggesting they may contain assemblies of anions and cations. Uranium phosphide, UP, mp = 2610°C, has a rocksalt-type struc ture (Fig 2.4d). It slowly dissolves in dilute acids evolving phos phine, but the sintered variety is inert to boiling water. It does not react with U 0 2 below 2500 C and is of interest as a nuclear power material. The monophosphides SiP, GeP and SnP are inert high temperat ure materials which can be made by direct combination of the elem ents. They exist in a variety of crystalline forms containing Si—Si and S i — P type bonds. Ionic Phosphides The phosphides of the alkali and alkaline earth metals prob ably all contain ionic bonds. Those with formulae M 3 P (M = Li to Rb) and M 3 P 2 (M = Be to Ba) liberate phosphine on contact with water and may all contain the simple phosphide ion P 3 ~ (Fig 2.5c). Metal phosphides such as LaP, CeP,Th 3 P 4 Zn 3 P 2 , LiMgP, Li 3 AlP 2 and others (see above) also liberate phosphine on contact with water or dilute acid. In general they exhibit salt-like character and ionic-type crystal structures such as rocksalt (LaP) or fluorite (LiMgP) (Fig 2.4c,d). P
~P—P"
~P—P—P~
~P—P—P—P~
¥t
P—P4-P+-P—P
(36)
The existence of polymerised phosphide ions (36) is now firmly established. Both the monophosphides CaP and SrP consist of an
2.2
66 assembly of cations and P 2 4 anions. On contact with water they liberate diphosphine (Section 2.5), which, like the anion P 2 4 T contains a covalent P — P linkage (37). 4H 2 0
2CaP
Ca 3 (PH 2 ) 2 .5NH 3
■>
H2P-PH2
CaP
">
(37)
2Ca(0H), Ca
(38)
3P2
Pure calcium monophosphide is best prepared by passing phos phine into calcium dispersed in liquid ammonia. The addition comp ound which is formed can then be heated at 150°C to give the monophosphide which above 600°C converts to a lower phosphide with loss of phosphorus (38). The lower phosphide liberates phosphine on con tact with water (39). It is made commercially by heating quicklime in phosphorus vapour, and together with Mg 3 P 2 , it finds applicat ion in sea flares. Sea flares are spontaneously inflammable due to the presence of small quantities of diphosphine which arise from impurity CaP. Reaction (37) can be compared with the action of water on calcium carbide which liberates acetylene, HC==CH . 2Ca
6H 2 0
3P2
-^ 2PH.
(39)
3Ca(0H),
The triphosphide, P 3 and pentaphosphide P5 ions have been identified in black crystalline LaP2 and the tetraphosphide P 4 6 ion is present in CeP 2 . Infinite polyphosphide P n n ~ chains exist in crystalline NaP and KP. All these compounds hydrolyse in water to PHq P n H n etc. give mixtures of various polyphosphines
Figure
2.8
Phosphide Anions
. (a) Pr
(b) P
11
(c) P
16
The interesting cage anion P 7 (Fig 2.8a), is of similar shape to P/+S3 (Section 2.4 below). It occurs in red S r 3 P 1 4 , Ba 3 P 1 4 and the highly coloured alkali phosphides M 3 P 7 where M = Li to Cs. Hydrolysis of these compounds yields mainly a hydride of composition P7H3 (Section 2.5 below). The alkali phosphide Li 3 P 7 can be obtained in 95 % yield by reaction (40). The special geometry of the P 7 3 anion makes possible valence bond tautomerism (Chapter 1.4) 3P.
6LiPH
2Li
3P7
4PH
(40)
Another cage anion of unusual shape is P 11 3 ~which is found in Na3Pi:L(Fig 2.8b) On hydrolysis this yields P U H 3 . If the sodium salt of (40) is heated with tetraphenyl phosphonium chloride, the
2.2
67
main product is (Ph u P) 2 P 16 which contains the anion (Fig 2.8c). These cage anions are all built from 3-linked P— and twolinked PC atoms, the latter bearing a formal negative charge. Arsenide analogues are known. p^P-p1
1_ 1 Ρ ·^Ρ^Ρ
p>-P^p
1-
i
II
P^P^P
p^P^p I I 11P^P^P
p^P^p1
1-
1
1-
P^P^P
ll
P
-P--
1-
P
P-
|
P
~
1
P ^ P - ^P
(41)
Two other phosphides attacked by water are K U P 6 and Rb u P 6 . These compounds both contain flat hexagonal P 6 4 ~ring anions in which all the bonds have the same rather short lengths of 2.15 A. Some multibond (π) character and a uniform spread of the four formal negative charges around the ring is indicated for these anions. In terms of conventional valence bond structures, contribu ting forms may be as (41). In contrast to the phosphides already mentioned, there are a whole series of alkali and alkaline earth phosphides which contain a relatively large proportion of phosphorus and which are insoluble and not easily attacked by water. Compounds of this kind contain highly polymerised chain, sheet or three-dimensional anions in which comparatively few P atoms form only two linkages. The two-linked (b)
(c) Figure (a)
2.9 KP15
Chain Phosphide Ion Structures (b) TIP 5
(c) BaP 3
(d) RbP ?
P atoms are associated with a formal negative charge although appar ently not conferring the same ionic properties as possessed by the lower phosphorus content compounds just discussed. Examples of in soluble chain anion structures are found in LiP 5 , LiP 7 , K P 1 5 , BaP 3 LaP 7 , ZnPbP 14 (Fig 2.9). RbP^ TIP. The infinite chains in KP 1 5 resemble those in Hittorf phos phorus (Fig 2.2). Many of these compounds can be regarded as derived from a basic polymerised phosphorus network in which a few of the P atoms are linked to two rather than to three other P atoms. The formal negative charges so introduced have to be balanced by the introduction of a suitable number of cations into the structure. There are probably many more phosphides of this type which are capable of synthesis.
2.2
68 Phosphorus-rich Phosphides
MxPy
(χ < y)
Phosphorus-rich phosphides of most metals other than alkali or alkaline earths (above) contain polymerised P atoms, but cannot be satisfactorily represented by ionic formulae. In their structure each P atom is covalently linked to at least one other P atom and up to three metal atoms in a tetrahedral configuration. On heating, these compounds lose phosphorus and usually revert to monophosphide or a metal-rich phosphide. Semiconductor properties are frequently found amongst these compounds. ^Ni -Ρς—Ni
I
I
-P-
I
V I
-Ir
•SIL
/ V
(b)
(a)
I
I
I
^Pv
^Sn
I x NP/ N> \P
/P
\>
(42) (c)
Dimeric covalent P — P units are found in PtP2 FeP 2 , 0sP 2 , and RuP 2 (42a). Square four-membered rings of P atoms occur in MP 3 type phosphides where M = Co,Ni,Rh,Pd or Ir (42b). Chains of polymerised P atoms are found in PdP2 ,NiP2 ,ZnP2, TiP2 , and CdP 2 . and sheets in CuP2.CdP4 , and in SnP 3 . The last compound has a layer structure similar to that of orthorhombic black phos phorus in which every fourth P atom is replaced by an Sn atom (42c) Recently InP3 has been found to be isostructural with SnP 3 , but A1P3 remains an unknown compound.
Figure
2.10
^r
Structural
Units found in Re_P,_. 6 13 (Open circles P atoms)
The crystalline compound Re 6 P 1 3 is remarkable in that it contains P 2 dimers, P4 chains and P6 rings (Fig 2.10).The ternary phosphide Cu 4 SnP 1 0 , prepared by direct synthesis from the elements, contains the highly symmetrical adamantane-type P 10 cage unit (43).
.A/
v
'_/
V-
P'
(43)
2.3
69
A number of phosphide-sulphides and phosphide-selenides are known e.g. CuPS, CuPSe, AgPS , PdPS, PdPSe, NbPS and TaPS. The compounds MPS (M = Rh,Co,Ni,Ir) are semiconductors. Compounds MPS 3 contain the P2S£~anion and are also of interest for their electric al properties (Chapter 7 ) . Metal phosphide-nitrides can be obtained by the reaction of P 3 N 5 (Section 2.6) with lithium (43A) and magnesium (43B) nitrides. The compounds obtained are formally salts of phosphenimidic nitride HN=P.^N, and phosphenodiimidic amide (HN) 2 P—NH 2 respectively. o P N + Li N 800_C ^ 3LiPN (43A)
+
P3N5
2M
g3N2 '
3M
I^IS—^
e 2 PN 3
(43B)
The lithium compound contains PN 4 tetrahedra which link to gether by sharing all four of their corners to build up a contin uous 3-dimensional cristobalite type structure (Chapter 3.2), which contains the lithium (cations) in the cavities.
2,3
OXIDES 0
ϋ^\ / 0 - -P.
rC/Xf
r^>
i i^r P^T jkzr u^r 0
(a) (b) (c) (d) (e) Five oxides of phosphorus form a series of molecules which are based on a tetrahedral arrangement of atoms (44). The two end members of the series correspond to the well-known trioxide, P 4 0 6 (P 2 0 3 ), and the pentoxide P u 0 1 0 ( P 2 0 5 ) . Their symmetrical dimeric oxide structures have been confirmed by vapour density, electron diffraction and x-ray diffraction studies. The structures may be regarded as derived from a Pz* tetrahedron by adding oxygen atoms to the centres of the edges, and then to each of the corners as well. Phosphorus tetroxide, Ρ20** , consists of various proportions of mol ecules with structures (44b-d), which contain P atoms in two diff erent valency states. In all these structures the terminal P — 0 bonds have multiple character and are considerably shorter than the remaining P — 0 — ( P ) bonds in these molecules. These oxide struct ures have many analogues in chemistry, as e.g. in the phosphorus sulphides (below). (Table 2.10). Phosphorus Pentoxide Phosphorus pentoxide, Ρ 4 0 1 0 , is obtained when phosphorus is burnt in an excess of dry air or oxygen. On the commercial scale,
2.3
70
phosphorus vapour is burnt in a specially designed burner, in a current of air which has been dried by refrigeration or other methods. The phosphoric oxide vapor so formed passes to a cooling chamber where it condenses to the familiar white powder. It may be purified from possible traces of the lower oxides by sublimation in an oxygen atmosphere. Phosphorus pentoxide is an exceedingly hygroscopic white pow der, density 2.30 g/cc, which sublimes at 359°C, but if heated rap idly melts at about 423°C. It combines avidly with water, forming orthophosphoric acid (45). The oxide finds much use as a desiccant, P 0 4 10
6H
2°
(45)
4H 3 P0 4
but has the great disadvantage that it 'skins' with a mixture of metaphosphoric acids, formed as intermediate products in reaction (45). This effect may be alleviated to some extent by spreading the oxide over a large surface such as glass wool. Industrial uses are many and varied. Worthy of note are de hydration in methyl methacrylate manufacture, the preparation of triethyl phosphate (6-18) and phosphorus oxychloride (53)(54), and for raising the softening point of asphalt by inducing cross-linking. -10° C + 4HP0. (46) P 4HN0 4°10 ■> 2 N 2°5
2H2S04
+
P
4HC1CK 4
+
P
+
P
C0NH2 CONH
4°10 4°10 4°10
cnr
^COOH
-> ">
2S0 o 2C1
2°7
CN
(47)
4HPC)
(48)
4HP0.
(49)
4HPC)
(50)
CN
c=oo 4 10
+
4HP0^
^c=co
Phosphorus pentoxide will dehydrate acids, forming metaphos phoric acid and the acid anhydride (46)-(48). It will also remove water from organic compounds as for example amides and carboxylic acids (49)(50). Under controlled conditions of hydrolysis, the cage-like mol ecule of Ρ 4 0 1 0 will disintegrate by rupture of successive P — 0 — P linkages to form a variety of products. The main process entails the rupture of two P — 0 — P linkages to give cyclic tetrametaphosphoric acid as indicated in (51). Ice-cold alkaline hydrolysis can be used to prepare the sodium salt of this acid in nearly 100 % yield. The next stage of hydrolysis results in breakage of the ring to give linear tetraphosphoric acid, which may then hydrolyse further by either of the two routes indicated. In a parallel process, a small proportion of cage molecules hydrolyse by breaking a ring bond first, giving rise to trimetaphosphoric acid and isotetraphosphoric acid. In all cases, however, the final hydrolysis product is orthophosphoric acid.
2.3
71
/1\
H20
^
i
•s^ Λ
/
0=F^-OH 0
\
^O-f-O ÖH
I
°vj _^——f~* 0 H
N
\H20
OH
+
0
OH
I
I
OH
OH
OH /2H.O
1
1
OH
OH OH
OH
OH
1
I OH
I
OH OH
OH
|
HO—P=0 — ^
HO—P=0 11 OH
1
1 OH
I
HO—P=0
OH
OH
1
°T° OH
OH
0
II
+
\>H
I 0— P — 0— P = 0 II I
OH -
+
j /
N 7°
1
0—P —0— P = 0
o
Ηθ" > 0 ^
OH
1
I 0 = P —OH I on
I 0=P—
0=P—0— P=0
x
o'
I ■ 0 — P — 0 — PI = r O 0=P— OH OH OH
^
0 — P— 0—P=rO
HOv
I
OH 1
•OH ' 0H 07-
4
HO-P=0
\
0~P—OH
"\
OH
0=P-OH
0
(51)
\
!
(T 0 H X 0
^
main < \ P " . ° tl
^0 OH 0 0=P-0H χ ^ \
... OH
2
Ό
HO-P=0 0 = PP-OH - O H 0 HO-1
YQ P==o K> —
> OR
,0
I
4 HO— P = 0
OH
I
OH
Phosphorus pentoxide reacts with dry or wet ammonia to produce a variety of both linear and cyclic amido derivatives and ammonium salts. In addition, bridge oxygen atoms may also be replaced, giving P — N H — P linkages. The course of low temperature ammonolysis is probably similar to that of hydrolysis (52). With sodamide, mono and diamido salts are obtained directly (Chapter 5 ) . Ethyl alcohol reacts with the pentoxide to give a mixture of ethyl phosphoric acids (Chapter 6 ) . Phosphoryl halides are obtained from phosphorus pentahalides
o
2NH
3
y
ό
\£
0 ^
H
2
N
X
p
,PC
ONH
NH 2
.
o
P H 4 N0
2
V
v
t--
0NH,
^°
NH 4 0
^•p
H
^*Q
H2N
0
,NH4
N
Λ
->
ΌΝΗ. / 2NH 3
V
2NH„
I
% / 0 ^
N
NH 2
H 0 N — P — 0 — P —NH 2
I
I
ONH,
ONH.
(52)
2.3
72
(53) or hydrogen halides (54), but with hydrogen fluoride the pro duct is hexafluorophosphoric acid (216) or phosphorofluoridic acid. (245). P
+
4°10
P 0
+
6PC1
5
^
10P0C1 3
>
3HC1
P0C1
(53) +
3
3HP0
<54>
3
At 500°C phosphoryl chloride can be obtained by heating the pentoxide with rocksalt (55), and if calcium fluoride is added, mixed phosphoryl halides are produced together with calcium metaphosphate (56). With calcium fluoride alone, phosphorus pentafluoride is ontained (181). Metaphosphates are also produced by heating with sodium carbonate (57). P 0 4P 0 T;
3P
+ JL U
4°10
6NaCl
+ 6NaCl + 3CaF +
6Na
> 2P0C1 £
2P0F Cl + 2P0FC1 ci
2C°3
^
+ £
2Na PO
(55)
+ 3/n{Ca(P0 ) } ό
4(NaP0 3 ) 3
+
£
n
6C0 2
(56) (57)
At high temperatures phosphorus pentoxide attacks fused silica ware and many silica-containing ceramic materials. Above 400°C it is reduced by carbon (58). The reduction of Ρ 4 0 10 (obtained from the dehydration of phosphoric acid (45)) by carbon;formed the basis of the earliest commercial preparation of elemental phosphorus (Sect ion 2.1) . P 0
+
IOC
^ 10CO
+
P
(58)
Phosphorus pentoxide exists in at least four polymorphic forms as well as a glass. The common variety of laboratory and commerce already discussed, consists of cage molecules (44a) arranged in a hexagonal (rhombohedral) crystal lattice with only weak van der waal-type forces between the cages. This is sometimes known as the 'Η' form or 'Form I' of phosphorus pentoxide. There are also two orthorhombic forms which contain highly polymerised arrangements ( P 2 0 5 ) n . The 0' form or 'Form II' has a sheet structure built from rings of six P 0 4 tetrahedra (59b), and the 0 form (Form III) is built from puckered rings of ten P0 4 tetrahedra which are linked laterally to form a three-dimensional structure. Another solid form I
I
1
?
9
9
^p^
Λ^
/Pv
/°\
I
I
0=P — 0 — P = 0
0
0
.
ό
/k
0
X
(b) 0
<59>
2.3
73
of unknown structure exists at high pressures. The vapour is stable up to at least 1400°C, but the existence of dimeric species such as (59a) may be possible. The t h r e e forms which a r e s t a b l e at ordinary p r e s s u r e s have i n t e r e s t i n g d i f f e r e n c e s in p r o p e r t i e s which can be r e l a t e d t o t h e i r c r y s t a l s t r u c t u r e s . The highly polymerised 0 and 0 f forms have higher melting p o i n t s and higher d e n s i t i e s than t h e H form. On h e a t i n g in a closed system, the H form transforms f i r s t i n t o t h e 0 form which then changes i n t o the more s t a b l e sheet 0 ' form. All t h r e e forms can be vapourised to produce the same cage molecules as e x i s t i n t h e H form, but t h e r e are d i f f e r e n c e s in the l i q u i d s produced by m e l t i n g . The H form f i r s t melts a t 420°C t o produce a m e t a s t a b l e l i q u i d with a high vapour p r e s s u r e and c o n s i s t i n g of d i s c r e t e P u 0 1 0 u n i t s . This l i q u i d then r a p i d l y polymerises t o form a g l a s s c o n t a i n i n g some c r y s t a l s of the 0 form. The 0 and 0 ' forms melt at 562°C and 580°C r e s p e c t i v e l y , to give viscous l i q u i d s with much lower vapour p r e s s u r e s than t h e l i q u i d H form. The l i q u i d orthorhombic forms presumably contain r e l a t i v e l y large fragments of the o r i g i n a l polymers and a r e consequently not e a s i l y v o l a t a l i s e d to P u 0 1 0 m o l e c u l e s . Both orthorhombic forms a r e considerably l e s s d e l i q u e s c e n t than the rhombohedral form. The H form r e a c t s v i o l e n t l y with water evolving much h e a t , whereas the 0 ' form r e a c t s much more slowly, evolving l e s s heat and forming a s t i f f gel which slowly d i s a p p e a r s in s o l u t i o n . This gel probably contains fragments of the sheet s t r u c t u r e in various s i z e s and s t a t e s of h y d r a t i o n , the f u r t h e r a c t i o n of the water then causing more breakdown and eventual s o l u t i o n . The 0 form d i s s o l v e s in water very slowly, even at 90°C, and t h i s i s because of t h e g r e a t e r d i f f i c u l t y of p e n e t r a t i o n of t h e t h r e e dimensional s t r u c t u r e by t h e water molecules. The h y d r o l y s i s products from t h e orthorhombic forms a r e i n i t i a l l y much higher molecular s p e c i e s than those obtained from t h e common H form. Unlike the l a t t e r however, t h e i r path of eventual breakdown t o phosph o r i c acid i s not properly known.
Phosphorus T r i o x i d e P h o s p h o r u s t r i o x i d e , P 4 0 6 , c a n be made by b u r n i n g p h o s p h o r u s i n a r e s t r i c t e d s u p p l y of o x y g e n . I t i s a c o l o u r l e s s c r y s t a l l i n e m a t e r i a l w i t h a m e l t i n g p o i n t of 2 3 . 8 ° C and a b o i l i n g p o i n t of 1 7 5 . 4 ° C and i t c a n be c r y s t a l l i s e d from c a r b o n d i s u l p h i d e s o l u t i o n . The s o l i d i s b u i l t from t e t r a h e d r a l u n i t s ( 4 4 e ) w h i c h a r e s i m i l a r t o t h o s e e s t a b l i s h e d i n t h e v a p o u r by e l e c t r o n d i f f r a c t i o n and vapour d e n s i t y measurements. T h i s o x i d e h a s an u n p l e a s a n t s m e l l and i s v e r y p o i s o n o u s . I t o x i d i s e s r a p i d l y i n a i r t o t h e p e n t o x i d e and t a k e s f i r e i f h e a t e d . The v a p o u r i s c o n s i d e r a b l y l e s s s t a b l e t h a n t h a t of t h e p e n t o x i d e , and i f t h e s o l i d i s h e a t e d s t r o n g l y i n t h e a b s e n c e of a i r , i t d e composes t o t h e t e t r o x i d e and r e d p h o s p h o r u s ( 6 0 ) . 200° C 2P40g > 3P204 + 2P (60) P
4°6
+
6H
2°
>4H3P°3
( 6 1 )
W h i t e p h o s p h o r u s w i l l d i s s o l v e i n P 4 0 6 t o t h e e x t e n t of 1.7 g p e r 100 g of P 4 0 6 . Each m o l e c u l e r e t a i n s i t s i d e n t i t y b u t t h e p h o s p h o r u s c a n b e removed by c o n v e r s i o n t o t h e r e d form by UV l i g h t . I t c a n t h e n be s e p a r a t e d from t h e o x i d e by s o l u t i o n of t h e l a t t e r i n
2.3
74
CS 2 . With an excess of cold water the hydrolysis product is phosph orous acid (61). With hot water the process is more complex, and the products include phosphoric acid, phosphine and phosphorus. The cold water hydrolysis may proceed in a manner analogous to that of the pentoxide (62).
°"V° P-0
\
0'
0
/
/ 0
-» P-OH 2H 0 \ ^
0
^ 0 (62)
HO-P /
OHO P^
Phosphorus trioxide reacts violently with chlorine or bromine to produce the corresponding phosphoryl halides (63). With hydrogen chloride, phosphorous acid is obtained (150). In carbon disulphide under pressure the di-iodide is formed (64). Addition of sulphur readily takes place to give an oxysulphide, P u 0 6 S 4 (65), and with ammonia, phosphonic diamide is obtained (5-23). Phosphorus trioxide forms various addition complexes using its lone-pair electrons to complete a tetrahedral configuration. With diborane it forms P 4 0 6 nBH 3 , and with nickel carbonyl it forms Ρ^Οβ nNi(C0) 4 , where n = 1-4 (Chapter 10). Phosphorus is obtained on heating phosphorus trioxide with arsenic or antimony in a sealed tube (66). 4 6 5P„0^ 4 6 4 6 4 6
Br
2
8I
2
4S 4Sb
->
P0Br„
■> ^
4P I 2 4
■> ■*
P
4°6 S 4
Sb
4°6
(63) +
3P 0 i n 4 10
(64) (65)
(66)
Phosphorus Tetroxide Phosphorus tetroxide, f P 2 0 4 f , which can be made by thermal decomposition of the trioxide as in (60), forms white crystals which sublime at about 180°C. At about 350°C oxidation to the pen toxide takes place in air. The oxide dissolves in water with cons iderable evolution of heat to give a mixture of phosphorous and phosphoric acids. There are two crystalline forms of this oxide. The rhombohedral α-form contains P 4 0 8 and P 4 0 9 molecules in varying proportions to give an average composition in the range P u 0 8 m λ -P.0 portions which cover a composition range P 4 0 7 7 -Ρ 4 0 8 . 0 · T n e u n i t cell dimensions of these forms remain almost constant, but their crystal densities vary with composition. Completely pure samples of the three types of molecule (44b-d) are difficult to obtain, but
75
2.3 there is evidence that Pu0 8 exists in the amorphous state.
Miscellaneous Oxides Molecules or radicals such as PO, P 0 2 , P0 3 2 ~have been obser ved spectroscopically . The diatomic molecule PO has an interatomic distance of 1.447 A corresponding to a multiple bond. Early workers described the existence of several solid yellow or orange sub-oxides to which various empirical formulae such as P 2 0 and P u 0 were assigned. A stable brown solid, insoluble in wat er, with empirical formula PO can be made by the electrolysis of anhydrous phosphoryl chloride at 0°C (67)(68). P0C1 3nP0Cl
> +
>
P0C1
2
(PO)
+
C1
+
(67)
~
2nP0Cl
+
+
2nd"
(68)
These amorphous unreactive and ill-characterised solids prob ably belong to a class of polymeric phosphorus networks to which various terminal groups such as H or OH may be attached. A contin uous range of composition between P u 0 6 and P 4 may be possible, with colours ranging from white through yellow, orange and brown to red. A violet solid of composition P 2 0 6 > phosphorus peroxide, is formed by condensing a mixture of oxygen and pentoxide vapour at low pressure under an electric discharge. The peroxide reacts with water to form peroxydiphosphoric acid (Chapter 3 ) . Pnictide Oxides The oxides of nitrogen bear little resemblance to those of the other pnictides, but arsenic, antimony and bismuth form oxides with the same empirical formulae as those of phosphorus : P
2°3
P
2°4
P
2°5
AS
2°3
AS
2°4
AS
2°5
Sb
2°3
Sb
2°4
Sb
2°5
Bi
2°3
(Bi
2 0 5>
The vapours of P 4 0 6 , A s 4 0 6 and Sb 4 0 6 all have the same molec ular structure (44e) and these structural units persist in the solid states where they pack into similar cubic lattices, and in solution in organic solvents. Above 800°C dissociation of A s 4 0 6 occurs and at 1800°C only As 2 0 3 molecules are present. On the other hand S b 4 0 6 is more stable and persists up to at least 1560°C. The least stable trioxide seems to be P 4 0 6 which decomposes above 210°C. The trioxides can all be obtained by reacting the elements or their sulphides in air. They show increasing thermal stability and basic character, but reluctance to oxidise to the pentavalent state on progressing from P to Bi. Although less soluble in water than its phosphorus analogue (Table 2.9), As 4 0 6 eventually produces arsenous acid As(OH) . Unlike phosphorus acid, however, the latter compound
2.3
76 TABLE
2- 9
Properties of Pnlctide Trloxides
P
4°6
mp
bp
(°C)
(°C)
23.8
175.4
solubility
density
g/100g H 2 0
g/cc
vs, d
2.13
acidic
2.04
3.7
weakly acidic
As406
218
-
S
\°6
655
1425
0.002
5.67
airphoteric
Bi20^
817
1900
vss
8.9
weakly basic
does not exist in tetrahedral form with an A s — H linkage (Chapter 3) In addition to the cubic forms based on discrete AsuOe and S b 4 0 6 molecules, there are alternative crystalline forms of ( A s 2 0 3 ) n and ( S b 2 0 3 ) n which are highly polymerised structures. In all of these, the pnictide atoms form pyramidal configurations of three bonds to 0 atoms, which are linked to give two-dimensional sheet structures as in ( 6 9 ) .
?1
,
|
J
i1
As
f
J1
1
As ^As^
I
^
1
1
X
As^
Is 1
^
^As
1
1 01 I
As
^As
(69)
0
1
^As
X 0 0^
M.
^As
1 The double oxide As 2 0 3 .P1 2 0 5 , sometimes described as arsenic phosphate, AsP0 4 , contains a network of ASO3 pyramida and P0 4 tetrahedra which share all their corner 0 atoms to give a continuous structure . The arrangement is similar to that adopted by ars enic tetroxide, AS2O4, which can be formulated as As 2 0 3 .As 2 0 5 . Among the pentavalent oxides, P 2 0 5 is thermally the most stable and Bi 2 0 5 the least. Whereas P^Oiois produced on heating Pu0 6 in oxygen, the arsenic analogue cannot be made by this method. On heating in air, As 4 0 10 loses oxygen. Although P u 0 l o and As u 0 10 both dissolve in water to produce similar ortho acids H 3 P0 4 and H 3 As0 4 , their solid state structures are quite different. In contrast to discrete Ρ^0 1 0 molecules,
2.4
77
c r y s t a l l i n e a r s e n i c p e n t o x i d e i s b u i l t from As0 6 o c t a h e d r a and AsOu t e t r a h e d r a which s h a r e c o r n e r s t o p r o d u c e a c o n t i n u o u s t h r e e - d i m e n s i o n a l s t r u c t u r e . O c t a h e d r a l c o o r d i n a t i o n of p h o s p h o r u s by oxygen does not o c c u r i n p h o s p h a t e s t r u c t u r e s (Chapter 3 ) . TABIE 2-10 Adamantane Type Structures A Sjj Se 6
Ρ
P Se
As^fWe)^
p
ti°6 s H
B
Pl4(NMe)6
(W?)H(Cti2)6
p
*s604
^ 1 0
P(CH) 3 (CH 2 ) 3 O s
(CH) 1) (CH ? ) 6
P
As
(CH)„S6
Ve10
Ga Se
(SiH) l 4 (SiH ? ) 6
P^NMe
In^S108-
P
H°G 4 6
4°6
As^Sg
As,4(NMe)g
Λθ
S
i| S 10
Ga
i) 10
3-
/) S 10 "
n io8
The P u 0 l o and PU 0 6 structures were the first examples found in phosphorus chemistry of the adamantane, (CH) u (CH 2 ) 6 or hexamethylene tetramine, (NH 2 ) u (CH 2 ) 6 type tetrahedral structures. Many examples of this structure type are now established (Table 2.10).
2Λ
SULPHIDES
AND
SELENIDES
The phosphorus sulphides can be prepared by heating mixtures of red phosphorus and sulphur in an inert atmosphere, or by react ing white phosphorus with sulphur in a high boiling point solvent. The phosphorus sulphides, which are formed above lOCPc, are all sol uble in carbon disulphide but are generally less stable than the oxides. They dissolve in water only with decomposition.
Pse
KΓ^ ^ v h \ r
» ! i-
p-
Liquid
\
\
\
\
Γ Γ
\
P4
^^\
af ♦ liquid
\
^/p
*
p-
/
I 1"
1 /
/
\
Mixture of < and fl h
♦ liquid
\ \
/
1
.
1 1 1 40 60 Atom-·/, of phosphorus
l_
, , 1
Figure 2.11 The System Phosphorus-Sulphur -below the reaction temperature.
2.4
78
If white phosphorus and sulphur are mixed together at temper atures below 100°C, solid solutions are formed as indicated in the phase diagram (Fig 2.11). The α-phase has the crystal structure of orthorhombic sulphur built from Ss rings with Pu molecules in solid solution. The ß -phase, on the other hand, has the structure of white phosphorus, with Se ring molecules in solid solution. The properties of elastic sulphur, Sy , can be stabilised by adding a few percent of phosphorus. A vulcanisation process occurs in which crosslinking is produced via the P atoms, and as more phosphorus is added, the product becomes more brittle and finally vitreous when crosslinking is excessive. The firmly established phosphorus sulphide molecules P4S3, P4S4 , P4S5 , P4S7 and Pi+Sxo are all based on P 4 cage structures which in some cases are related to those of the oxides (Fig 2.12). These have been confirmed by X-ray and electron diffraction, and by NMR studies. The structures of P 4 S 9 and P 4 S 1 0 are analogous to those of the oxides. The remainder are almost unique in chemistry except for the following pnictide chalcogenides which are known to form isostructural molecules : P
P
4S3
α
4 4
4Se3
P 7 3As 7 3-
4 N
P S
4 4
„ ,i
4
„4S „4 4
As3S4+ + As 3 Se 4
B
P S, i „
„4S *5
P
4S7
P
^4 S e c5
P
4Se7
P,S i A 4 10 p
„Se_ 4 10
s> i N ~ E
4 5
4 4 4
As
P
„4S e „4
AS
4S4
AS
4S5
AS
P
4S10
4S9N"
As 4 Se 3 The industrially important phosphorus sulphides are P 4 S 3 and P4S10 Phosphorus selenides have been much less studied than the sulphides. Reported compounds, additional to those listed above, include P 4 Se, P 2 Se and P 2 Se 3 . The best defined telluride is P 2 Te 3 . Phosphorus selenides form glassy phases more readily than phosphorus sulphides. During the last 100 years the existence of numerous phosphor us sulphides has been claimed, with compositions ranging from P 4 S to P4S24 . It seems likely that some of these e.g. P 4 S 2 do in fact exist, in addition to the confirmed ones mentioned above. The exist ence of P4S6 has been persistently reported, and molecules of this composition are probably present in a sulphur-deficient form of P 4 S 7 (below). At least five types of phosphorus-sulphur anions exist (Chapter 7 ) . Some properties of the four longest-established phosphorus sulphides are listed in Table 2.11.
2.4
Figure 2 . 12
79
Molecular Structures of Oxides and Sulphides of Phosphorus
2.4
80 TABLE 2-11 Properties of the Common Phosphorus Sulphides P4O7
305-310 523 2.19 0.029 decomp nearly white Monoclinic
Melting point °C 171-172 170-220 Boiling point °C 407-^08 Density g/cc 2.03 2.17 10 Solubility in CS2/100g 100 Action in air slow oxidation Colour yellow yellow Crystal system Orthorhombic Monoclinic s
s s
i00
s s
I 1 · 86
s
1103 ^v£0
P
s
'feH
2.235
1124
11.91 ^Pv^.io S 109S S 1110
"^v
'^
(70)
*^43^ (d)
(c)
(b)
(a)
P4S10
286-290 513-515 2.09 0.222 slow decomp yellow Triclinic
Phosphorus sulphide melts with low P/S ratios contain mainly the established cage molecules in Fig 2.12, but with high sulphur contents, long chain polymers are also present. An insoluble yellow powder of composition (PS) n can be made by reacting magnesium with thiophosphoryl bromide (71). 2PSBr
3Mg
2/
n
(PS) n
3MgBrr
(71)
Sulphur dichloride will combine with phosphorotetrathioic acid with the elimination of hydrogen chloride (72). Further react ion of the remaining SH groups enables three dimensional networks of general composition (PS ) x to be built up. HS^ S-^P—SH + Cl.S. HS
,SH \ S-^P~S — S — S — P ~ S + 2HC1 HS SH
(72)
Purification of P4S3 and P 4 S 1 0 can be effected by recrystallisation fromcarbon disulphide, but the less soluble P 4 S 7 is best purified by solvent extraction of the impurities. The order of thermal stabilities is : P4S3
>
P^S7
>
P4S10 >
P4S5
Tetraphosphorus trisulphide (also known as phosphorus sesquisulphide) and the heptasulphide show no appreciable decomposition up to 700°C, but the pentasulphide decomposes at its melting point to give tri and hepta sulphide (73). Similarly the disulphide, mp = 47°C, decomposes according to reversible reaction (74), which can also be used for its preparation 2P„Sc 4 5
4 7
(73)
81
2.4 3P 4 S 2
> 2P 4 S 3
<
2P 4
+
(74)
Tetraphosphorus decasulphide (still known often as 'pentasulphide') decomposes a few degrees above its boiling point, but this decomposition is reversible since the compound can be purified by distillation. If this sulphide is melted and re-solidified, some decomposition occurs and the product consists of a mass of crystall ine P 4 S 1 0 embedded in a mixture of amorphous P^Sc, and sulphur. Vapour density and mass spectra indicate the vapour contains P 2 S 5 molecules which seem to be a good deal more stable than their poss ible oxy analogue (59b). Free radicals may be produced in the de composition of the decasulphide since the vapour can be condensed on a cold surface to give a green solid. In carbon disulphide solution, P 4 S 3 readily adds on sulphur and undergoes rearrangement to give a -P4S5 . In view of the low thermal stability of the latter, this is the best method for its preparation. On cooling a CS 2 solution, this compound rearranges to g i v e ß P 4 S 5 . if a CS 2 solution of P 4 S 3 is allowed to oxidise in air, an amorphous pale yellow precipitate of composition P 4 S 3 0 u is formed. The sesquisulphide, P 4 S 3 , is used together with potassium chlorate, powdered glass and glue, to make 'strike anywhere' match es, which were first introduced by SdV£n
+
18S
P
>
4 S in
+
8FeS
(75)
The heptasulphide exists in two crystalline forms a and ß . The 3 form is composed of wholly symmetrical molecules of the type indicated in Fig 2.12. The a form however, has an approximate comp osition P 4 S 6 - 5 and is believed to contain a small proportion of molecules with the composition P 4 S 6 or P4S5 corresponding to either one or two sulphur atoms missing from terminal positions in the P 4 S 7 molecule. The nonasulphide, P 4 S 9 , can be made by heating appropriate proportions of hepta and decasulphide (76). Alternatively it can be made by reacting the decasulphide with either PCI 3 or PPh 3 in a sealed tube or in C S 2 solution (77). P
4S7
+
2P
4S10
VlO
+
PPh
> >
3
3P
P
4S9
4S9
(76)
+
Ph
3PS
(77)
Like a P4S7, the nonasulphide has a variable composition and may appear with a sulphur deficiency corresponding to P4S8.5. This *
A typical formula is
: KC103 P
S
4 3
Fe
2°3
20 %
ground glass ZnO
9
glue
11 Vi'ater
14 % 7 10
2.4
82
might arise from the presence of a sulphur analogue of P 4 0 8 . Two forms of the nonasulphide are known, a with a mp = 240-270°C, and 3 with mp = 250-259°C. At 255°C the nonasulphide disproportionates to P4S7 and P|+S10 . The phosphorus sulphides differ in their hydrolytic stabili ties. The sesquisulphide reacts only slowly with cold dilute HC1, whereas the heptasulphide is readily attacked by atmospheric moist ure. The order of stability is :
p4s3
>
P4S10
>
>
PUS9
PUS7
These hydrolyses are complex processes involving the initial rupture of P — S — P followed by the P — P linkages (where present), and the eventual replacement of S by 0. There have been conflicting reports about the nature of the products which depend on pH, temperature and other factors. P
4 S 10
+
16H
2°
^
4H
3P°4
+
10H
2S
(?8)
Acid hydrolysis of P 4 S 1 0 at 100°C quickly produces only orthophosphate (78). On the other hand if cautious alkaline hydrolysis is carried out, various thioated ortho ions (Chapter 7) can be found as intermediate products. These later react under oxidative condit ions to give mostly orthophosphate P 0 4 3 ~ together with some phosph orous HP0 3 2 ~and hypophosphorous H2P02~ions.In addition, traces of phosphine, PH 3 may be produced, and those sulphides containing P — P linkages can give rise to hypophosphate P20^~ions. The decasulphide is very reactive with halogens. It reacts with PCI3 in a sealed tube at 150°C to give thiophosphoryl chloride (79). Anhydrous HF or PF 3 under pressure will give thiophosphoryl fluoride (80). A reaction also occurs with carbon tetrachloride (Chapter 7 ) . Air combustion of P4S 1 0 produces Pi,0lo and S0 2 . P S^ 4 10 P4S
+
>
6PC1
10PSC1
5 +
12HF
(79) o
>
4PSF 3
+
6H S
(80)
The decasulphide undergoes nucleophilic attack by sodium fluoride in acetonitrile, to give ions (81), which can be isolated as their n-propyl ammonium salts. By reacting the decasulphide with F
S" >>
S^ yS~ F— P— S—P— F
S^ ΐ—Ρ—S—S—P—
SF
(81)
sodium azide in acetonitrile and treating the product with n-propyl ammonium bromide, a salt containing the (N 3 ) 2 PS 2 ~ion can be isolated. Reaction of this latter salt with more decasulphide will give (Pr^N) (P 4 S g N) which contains an anion which is the structural analogue of the decasulphide, except that one terminal S atom is replaced by N. Potassium phosphorothioate is produced in a reaction with KC1 or K S (82), and with KCNS a large cage molecule is formed (7-162).
2.4 P
83 6K 2 S
4 S 10
4K 3 PS 4
(82)
Grignard reagents will react with P 4 S 3 , P4S7 or P4S 1 0 to give products in which one, two or three alkyl groups are attached to the P atom (83)(84). With alcohols, phenols and thiols, dithio est ers are the predominant products (85) (Chapter 7 ) . Primary amines 12RMgX
VlO PS
+
9RMgX
+
-> 4R 3 PS 3H 0
6MgS
GMgX^
-^3R (H)P + R P + 3MgX 2
3
2
P„S^ 4 10
+
8R0H
->
4(R0) P(S)SH
PS 4 10
+
8RNHr
->
4(RNH) P(S)SH
P4S10
+
12RNH2
->
4(RNH) PS
+
+2MgS +3MgOHX (84)
2H S +
(83)
2H 0 S
6H 2 S
(85) (86) (87)
react with P 4 S 1 0 to yield either thiophosphoric (phosphorothioic) diamides or the triamides, depending on the conditions (86)(87). Tetraphosphorus decasulphide is used in organic chemistry to convert OH, C = 0 , COOH or C0NH 2 groups into their sulphur analogues. This sulphide is an important intermediate in the manufacture of insecticides (Chapter 6) and a large industrial use is in the manu facture of zinc dialkyl or diaryl dithiophosphates for oil additives (Chapter 7 ) . Some of the reactions of P 4 S 1 0 are summarised in Fig 2.13.
PSCl
^VP12S12N14>
RNH ^
Na3PS202
Figure 2.13
+
Na3PS30
Reactions of P.S -. ^
.
·* (RO) PSSH 2 (RNH) 2 PSSH
2.4
84
The phosphorus sulphides react readily with ammonia. At -33°C in liquid ammonia, the trisulphide and pentasulphide form solid compounds with formulae P 4 S 3 .4NH 3 and P 4 S 5 .6NH 3 respectively. These compounds are in fact ammonium salts and should be formulated as (NH 4 ) 2 P 4 S 3 (NH 2 ) 2 and (NH 4 ) 5 P 4 S 5 (NH 2 ) 3 . The hepta and deca sulph ides react with ammonia to give a variety of ammonium salts and and amino-substituted ions including (NH 4 ) 3 PS U and (NH^) 2 (PS 3 NH 2 ). At -78°C liquid ammonia reacts with both Pi+Sg and P 4 S 1 0 to give cyclophosphorothioate anions (7-129). On heating with ammonium chloride, the decasulphide forms polymeric (PSN) n . The sesquisulphide reacts with iodine to form P 4 S 3 I 2 , a cage-like molecule which converts to an isomer on heating to 125°C. These isomers react with (Me3Sn)2S to give the a and 3 forms of P n S 4 (88).
P
4S3
I fI \/PI
\l/ PI I
IP
f I
(Me 3 Sn) 2 S
(88)
2Me SnI
IV? 1
The reaction between P4S3 and iodine can be made to go further (89), and zinc phosphide can be obtained with zinc (90). 7P
P
4S3
4S3
241,
16PI
3P
9Zn
3ZnS
2Zn
(89)
4S7
(90)
3P2
Phosphorus - Sulphur Glasses Various phosphorus sulphide glasses have been prepared. When P/S = 1, nearly 60 % of the total phosphorus is present as P 4 S 3 according to NMR data. Solid solutions and glasses are formed in the system P U S 3 P 4 Se 3 . Mass spectra and NMR studies indicate all species (91) can be present.
£N
1
V
/
i
S
Se S
Se
ρ 1
P
\p'—
P
S Se
/ ρI
φ
/
Se 1
P
N \
Se Se (91)
/
'
Suitable thermal treatment of P, S^ As 4 S 3 mixtures leads to _ x Xi,J Asx~3 compositions of the type P u4-x xSc »where ^ ^ ~x - -1-3. ". Mass spectra and NMR studies indicate the occurrence of six different structures in which the P atoms occupy either apical or basal cage positions
85
2.5
S
I
S
/
S !
S^
S
S
As-^As
1 /
·
Sx 1
S
S
/
1
1
/
P s / vs \ s
'
(92)
As-·!—·— As
(92). Products in the system P/S/Ge have applications as semicond uctors or in optical fibre technology, and glasses in the system P/S/Li/I are electrical conductors. The heptasulphide P 4 S 7 , reacts with bromine in CS 2 solution to give two compounds, P 2 S 5 Br 4 , mp = 90°C, and P 2 S 6 Br 2 mp = 118°C. The latter has the ring structure (93).
y^
/p\
(93)
Four oxysulphides are known : P406S4
P4S604
P4S304
P6010S5
The first of these, mp = 110°C, can be made by heating the trioxide with sulphur (65) or a sulphide-oxide mixture (94). The structure 6P
+
4°10
4P
4S10
10P
>
4°6S4
(94)
of this oxysulphide in the vapour and solid states is similar to that of P 4 0 1 0 , except that the terminal 0 atoms in the latter are replaced by S. Controlled hydrolysis of this compound will yield initially the species (95). S
s—P—o—P5-O_
">P—0-,"PVS-
(95)
The structure of the cage compound P 4 S 6 0 4 , mp = 290-295°C, is similar to that of P^S^Og with the S and 0 atoms reversed (Fig 2.12) The compound can be made by reaction (96). There is spectroscopic evidence that PuS 6 0 3 (70d) can be obtained by heating P S and P u 0 1 0 together to high temperatures. 4P0C1 3
+
6(Me 3 Si) 2 S
2.5
HYDRIDES
>>
P
4°4 S 6
+
12Me
3
SiC1
< 96 >
There are six possible hydrides based on a single P atom (97) H
H-(<Ü H
H
- H H
H /?X H
HA
f
' -
2.5
86
are well Phosphine, PH 3 , and the phosphonium cation PH characterised and the existence of the PH 2 anion has also been established. Pentaphosphorane, PH 5 , and the hexaphosphoride anion PH 6 , based on outer valence shells of 10 and 12 electrons respect ively, remain hypothetical compounds, although halides and many other derivatives are known. Long established and well recognised are the nitrogen analog ues, ammonia, NH 3 , the ammonium NH 4 , amide NH 2 and imide NH ions. Established hydrides of the remaining pnictide elements are the AsH2 and SbH2 anions, arsene, AsH 3 , stibine, SbH 3 , and bismuthine, BiH 3 . The arsonium cation AsH 4 + , has been detected spectroscopically at low temperatures, but it does not form simple salts stable at room temperature. The cations SbH 4 + and BiH u + do not appear to exist and penta or hexacoordinated hydrides of these elements are also unknown. Unlike nitrogen, which forms only NH3 and N 2 H 4 , phosphorus forms a very large number of hydrides based on more than one P atom. While the simpler compounds are gases or liquids, others are highly polymeric, amorphous, insoluble and highly coloured solids Phosphine, PH3 , diphosphine, P 2 H 4 , and triphosphine, P 3 H , are the first three members of the series of composition P n H n + 2 . Other well characterised phosphorus hydrides (phosphanes) include cyclopentaphosphine, P 5 H 5 , representing the series P n H n , and the cage hydride P 7 H 3 , which is a member of a whole series of compounds that are now known to exist (98). Condensed phosphorus H /?\ > _ p
ά
H
H
P
P
ά
/
"~P\
ι
H p'
- v\ H—P
H
U
/
H ^P
\\
H
~HP
HHE>
-PH
,ΛΩΧ
/P~H
P
1 H γ—H hydride anions such as P 7 H 2 , P 7 H , P 5 H 2 and P 9 H 2 are also known. Various arsenic and antimony hydrides with empirical composit ions As 2 H 4 , As 3 H 5 , As 2 H, As 2 H 2 , As 2 H 5 , Sb 2 H 2 , Sb 2 H 4 , Sb 2 H 5 etc, are have been made. Although somewhat less precisely characterised, they are believed to belong to series analogous to those formed by the phosphides (see below). No phosphorus analogues of multiply bonded nitrogen species such as diazene, HN=NH, tetrazene, H 2 N-N=N-NH 2 , and hydrazoic acid HN=N-N have been isolated. As far as the hydrides are concerned, those of phosphorus and the heavier pnictides are , as yet of little more than academic significance. They stand in complete contrast to NH3 and NH4 which are of overwhelming industrial, biochemical and environmental impor tance. Phosphine is virtually absent from biological processes (Chapter 11). Simple XH3 Hydrides The gaseous XH3 molecules (X = N,P,As,Sb,Bi) all have symmet rical pyramidal (C3v) configurations which have been established by
2.5
87
/ ζ \ 1 . 14 3 7
\ \ 1 .015
4 bp(°C)
Η^,-VVH
H
-73
-88
χ Ά δ Ό -519
K2y-n
^ ^ v1 . 7 0 7
/Bi\
H^AJ
-62
+22
-18
(99) v(X-H)
cm"1
3337
y (D)
(NH )
2327
1.45
(PH )
2122
(AsH
)
0.55
numerous infrared, microwave, electron diffraction and nuclear mag netic resonance studies (99). In pyramidal XH 3 pnictide molecules, the central X atom osc illates from one side of the plane of H atoms to the other. The inv ersion frequency is about 10 3 -10 u times less in PH 3 than it is with NH 3 , and this frequency decreases progressively with increase of molecular weight of X. The calculated inversion times are : -11
NH
2.5
xlO
PH
1.1
xi0~7
AsH
1.4
sees
years.
Phosphine has a smaller dipole moment than ammonia (99).This arises from the increased electron drift twards the H atoms and the smaller polarity of the bond in the case of PH 3 . Bond stretching frequencies become lower with increasing pnictide weight (99). The shapes of these molecules and much of their chemistry can be interpreted in terms of sp 3 hybridisation, and d orbitals are not likely to be much involved. The progressive reduction of interbond angle with increasing molecular weight can be associated with a change from nearly pure sp 3 hybridisation in NH 3 , to nearly pure p bonding in SbH 3 as the angle approaches 90°. Mainly p orbitals are involved with PH 3 but some s character predominates in the lone pair electrons. Owing to the smaller electronegativity differences involved (xp^ x H = 0, x^ ^ XJJ =0.9) , hydrogen bonding between XH 3 molecules other than NH 3 is likely to be very weak. The available evidence indicates that intermolecular association in liquid PH 3 and the heavier hydrides is indeed very slight or non-existant. The lack of internal association is indicated by the anomalous position of NH 3 with regard to its melting and boiling points (Chapter 12.1). Phosphine PHg Phosphine, PH 3 , bp = -88°C, mp = -133.8°C, is the best known hydride of phosphorus. At ordinary temperatures it is a colourless and very poisonous gas which has a characteristically unpleasant garlic like odour. (Appendix III).
^ i SΊ \t "
H'
η
|
n
h = 0.764 A ° 50 ' β = 57° 30«
α = 93
C3V
P—H = 1.42 A
2.5
88 PH
+
3
2
°2
^
H
3P°4
(100)
Pure phosphine ignites in air at about 150°C and burns to produce phosphoric acid (100). When impure, the gas is spontaneous ly inflammable at room temperature, and this is usually attributed to traces of diphosphine, P2H4, or possibly P^ . A slow oxidation of phosphine can occur by a branched chain reaction, and, like the slow oxidation of white phosphorus, there appear to be critical pressure limits for the reaction. Phosphine solidifies at -133.8°C (triple point) under its own vapour pressure of 27.3 mm Hg, and there are at least four differ ent crystalline forms existing at lower temperatures. Association in liquid phosphine is negligible compared to that in liquid ammon ia where there is extensive hydrogen bonding (above). The gas is only very slightly soluble in water, to give a neutral solution (26 ccs of gas dissolve in 100 ccs at 20°C). It is somewhat more soluble in organic solvents such as cyclohexane and carbon disulphide. An aqueous solution of phosphine gradually decomposes forming phosphorus, hydrogen and a yellow solid of approximate composition P2H.
Phosphine is both a weak acid and a weak base (101)(102). It is however more strongly acidic but much more weakly basic than NH3 . PH 3
+
H20
PH
+
H20
^
PH 2 " > PH4+
%
+
H30+
(101)
+
OH"
(102)
+
3
Protonation to form PHj,. (i.e 3p ^ sp ) involves greater hybridis ation changes than in the case of the change NH 3 — * N H 4 + , and the base strength of PH 3 is accordingly much lower than that of NH 3 . Phosphine acts as a weak donor towards protons and Lewis acids. The dissociation of phosphine is negligible unless heated to several hundred degrees. It is thermally more stable than AsH3 but less so than NH 3 . Photodissociation also occurs (Chapter 12.5 ) . At 0°C and atmospheric pressure, activated charcoal (1 cc ) absorbs considerably less PH 3 (69ccs) than NH 3 (170 ccs). Phosphine can be made by the action of water or dilute acids on certain metal phosphides (35)(39)(103). The hydrolysis of white phosphorus is a viable commercial method (104)and the gas may also be made by the action of caustic potash on phosphonium iodide (105) or by heating dry phosphorous acid ( 106). Phosphine and nitrogen are obtained by the interaction of phosphorus vapour and ammonia at red heat (107) and the gas is liberated in many other reactions of phosphorus compounds. Mg P + 6H + 3 2 2° " * 2PH3 3Mg(0H) 2 (103) *
The faint flickering light sometimes seen in marshes, *will-of-the-wisp' ,
has been attributed to the spontaneous ignition of impure phosphine, formed by the bio reduction of phosphate esters.
89
2.5 P
4
PH I 4 4H
3K0H
+
+
3H20_
KOH
+
*N-
s
3P03
2P
->
2NH 3
PH
3
+
V
3H 3 P0 4
Sfc
2PH 3
7
+
3KH PC) 2 4
y
+ KI
PH 0 3 +
HO 2
PH3
+
(105) (106)
N2
+
(104)
(107)
Phosphine is a strong reducing agent and it will e.g reduce many metal salts to free metal and pentahalides to trihalides (108). 3PC1
+
PH
>
4PC1
+
3HC1
(108)
When heated with sulphur, hydrogen sulphide and a mixture of phosphorus sulphides are produced. Direct union of phosphine with a hydrogen halide produces a phosphonium halide (109). PH
+
HI
>
PH I
(109)
Phosphine dissolves in liquid ammonia to give a salt-like compound NHz*+ PH 2 , and it also reacts with lithium aluminium hydride to give a salt of the PH 2 anion, which is soluble only in ammonia (110). This compound is analogous to the amide LiAl(NH 2 ) u . Phosphine is liberated on contact with water (111). LiAlH
+
LiAKPH )
+
4PH
>
LiAl(PH )
4H 0
^
LiAl(OH)
+ +
4H 4PH
(HO) (111)
At low temperatures phosphine reacts with perchloric acid to produce explosive crystals of phosphonium perchlorate, PHu + ClOu". Phosphine readily forms addition complexes in which a metal-phosph orus bond is present e.g. PH 3 .A1C1 3 , PH 3 .TiCl 4 , Cr(CO) 3 (PH 3 ) 3 and Co(NO)(C0)2PH3 (Chapter 10). Borane complexes are also known (Chapter 9 ) . Important reactions of phosphine are with formaldehyde to form THPC (4-348 ) and with olefins to produce trialkyl phosphines, Phosphonium Salts Phosphonium salts containing the tetrahedral PHu cation are generally less stable than the corresponding NH 4 salts and dissoc iate more easily. The chloride and bromide easily form gases at room temperature and only the iodide, PH 4 I , mp = 18.5°C, is crys talline. Phosphonium halides are produced by direct union of phos phine and hydrogen halide (109) or acid. Phosphonium salts of the PH 4 cation have few uses but their derivatives are important (Chapter 4 ) . Phosphide Anion PH 2 The phosphide anion, PH2 , is obtained from phosphine by reaction with the amide (112) or lithium aluminium hydride (110). PH
+
KNH
>
KPH
+
NH
(112)
2.5
90
The PH molecule does not exist at room temperature, but can be detected spectroscopically in reactions between hydrogen and phosphorus vapour at higher temperatures. H
^p_p-^-H (113) H ^*^H Diphosphine, P 2 H U , bp ~ 52°C, mp = -99°C, unlike hydrazine N 2 H 4 / ignites spontaneously in air and has no basic properties. It can be made by the action of water on calcium monophosphide (37). Decomposition of diphosphine yields triphosphine, P 3 H 5 together with higher members of the series, and a yellow solid of approxim ate composition P 2 H . Diphosphine
Polyphosphines During the last two decades a large number of phosphorus hyd rides (phosphanes) have been detected in the thermolysis products from diphosphine, or amongst the volatile hydrolysis products from calcium and other metal phosphides. These phosphanes have been characterised largely by mass spectra and NMR studiesand they form various series : P H ,P H ,P H P H n n+2 n n n n-2 n n-14 Members of the first series are the open chain compounds PH 3 , P 2 H 4 , P 3 H 5 etc. The second series are cyclic, one member being P 5 H 5 ( 98b) and the remaining compounds (e.g. P7H3 (98c)) are based on condensed networks of P atoms, only some of which may have H atoms directly attached. The majority of these compounds have only been prepared and studied as mixtures because, in many cases there is a very close similarity in properties, or they have a very marked tendency to disproportionate. Apart fromthe long established phosphine, PH 3 and diphosphine, P 2 H4 ; only the three phosphanes already cited (98) have so far been prepared in pure form. Diphosphine under the correct conditions of thermolysis will decompose mainly according to (114), and the desired triphosphine can be concentrated by fractionation. Triphosphine is a colourless 2P
2H4
P
>
3H5
+
PH
(114)
3
liquid, soluble in diphosphine, stable at -80°C, but turning yellow at room temperature giving eventually solid products. If liquid diphosphine and triphosphine are heated, tetraphosphine PnH 6 , and cyclopentaphosphine P 5 H 5 are among the products which can be obtain ed. Both n- (115a) and iso- (115b) forms of tetraphosphine have been characterised, moreover the n derivative exists in diastereoisomeric forms (Chapter 12). II
H
/p\
\„/ \ 1
H
H
H 1
p1
1 H
H
(a)
\P/H
I \p/P\p^H I I H
H
(115) (b)
2.6
91
The phosphorus-rich heptaphosphide, P 7 H 3 (98c) is obtained by methanolysis of (Me 3 Si) 3 P 7 (9-130) or hydrolysis of Ba3Pi4 (Section 2.2), and is amongst the thermolysis products from diphosphine. This compound is an amorphous white powder which is stable up to about 300°C and is insoluble in diphosphine and most common solvents. If potassium phosphide, KPH 2 is reacted with white phosphorus in dimethylformamide solution, a deep red amorphous compound with composition KP 5 H 2 is obtained. Formula (116a) has been proposed on the basis of molecular weight and NMR data. More highly polymerised ions such as P 9 H 2 (116b) also exist.
κ+ "Xj^P—PH 2 P
K+ X J ^ P — p \ l / p — P H 2 P
(a)
P
(116)
(b)
and Lithium salts containing the heptaphosphide anions P 7 H P H 2 can be obtained by reacting the heptaphosphane with Li 3 P 7 or LiPH2 at -78°C (117) . P?H3
+
2,6
^Li2HP7
2LiPH 2
+
2PH
3
(117)
NITRIDES
A whole series of amorphous polymeric materials appear to ex ist within the composition range P 3 N 3 to P 3 N 5 . They are white, yellow or brown, chemically very inert, they have high melting points and upon heating evolve gaseous PN molecules. The structures of these compositions are believed to be based on random networks of P — P and P — N linkages (118), although multiply-bonded units such as — P = N may be involved. Only P3N5 has been obtained cryst alline and this presumably has a regular structure of some kind.
I
1
I
I
-N
l
'N'PNV/IV 1
/P
\
/p
1
1
/P
\
\/
\ Ν /-Γ i Ϊ?-- Ν ^ ~ .p—ή /
},
(a)
!
,
(us)
pP—
; _N>-P-
(b)
Spectroscopic studies of gaseous PN molecules indicate multi ple bonding with P ^ N = 1.491 A . Dissociation occurs above 800 C and the bond is weaker than N = N (Table 1.20) An explosive colourless oil of composition P 3 N 2 i is an azide based on the phosphazene ring (Chapter 5 ) . Phosphorus triazide, P ( N 3 ) 3 , and the penta azide, P ( N 3 ) 5 have been prepared by reacting sodium azide with PC1 3 and PC1 5 respect-
2.6
92
ively (119)(120). Reaction of the latter with tetraphenylphosphonium chloride gives an explosive hexa-azidophosphate (hexa azidophosphoride) (121) (Chapter 5 ) . PC1 3
+
3NaN 3
P(N
>
> + 5NaN 0 5 o P(N 0 )_ + NaN + PhPCl 3 5 3 4 The penta azide decomposes to the phosphate hydrolyses according to PCI
P(N3)5
M£CN
Ph P + P(N.)_4
^
+
P(N
3)3
3NaCl
(119)
P(N0)„ + 5NaCl (120) Jo > Ph / P(NJ " + NaCl (121) 4 3 6 triazide (122) and the hexa azido (123). +
3>3
2HO
>
Ρη
Δ
o b
+
3N
„Ρ+
PO
(122)
2
o( N o>o"
4
z
+ 4HN
d z
< 123 >
o J
The earliest effective method of producing P 3 N was that due to Stock and Ho^mann who in 1903, heated P 4 S 10 with ammonia (124) 3P
4 S 10
+
80NH
>
3
4P
3N5
+
30(NH 4 ) 2 S
(124)
Amorphous material is obtained by this method, but on heating to 800°C it is converted to the crystalline variety. Another method of synthesis is by heating a diaminophosphazene (5-247). Triphosphorus pentanitride is insoluble in water and organic solvents, and it is not attacked by dilute acids or alkalis. Decomposition occurs at about 800°C according to (125). P3N5
^ ^
>
PN
£20_£
^
3PN
g
ip 4
+ +
N2
(125)
iN2
(126)
Monomeric PN is stable in the gaseous phase at 450-800 C. At higher temperatures it starts to decompose to the elements (which may then appropriately polymerise)(126), and at lower temperatures it is deposited as an amorphous polymer (PN) n . Polymeric phosphorus nitride is obtained when phosphine and nitrogen react at 80-290°C. This can be represented approximately as (127). 2PH 3
+
N2
>
2/n (PN) n
+
3H 2
(127)
No reaction occurs with P3N5 in hot water, but hydrolysis can be effected by steam at 800 C to give ammonia and phosphoric acid (128). Ammonium phosphates can be obtained if the reaction is carr ied out at 250°C under pressure. P3N5
+
12H 2 0
>*
3H
3P04
+
5NH
3
(128)
When heated in nitrogen, P 3 N 5 decomposes according to (125): in hydrogen, phosphorus and ammonia are obtained, and in air the final residue is polymeric (P 2 0 5 ) n · Triphosphorus pentanitride is decomposed on heating with concentrated nitric acid or by fusion with alkalis.
93
2.6
A series of somewhat ill-defined compounds containing Ν,Η &Ρ can be obtained from reactions between ammonia and the phosphorus halides. Some of these materials, e.g. phospham, P 2 NH and phosphor us amide imide, P(NH)NH2 are inert, insoluble and highly polymer ised (129). (129)
P — -NHNHr
L NH In
When phospham, made by reacting NH 3 with PC1 5 , is heated to 500 C, network rearrangement occurs with the elimination of H to give P 3 N 5 , which itself changes to P 3 N 3 at high temperatures (130) PN(NH)
P
P N (NH ) 3 3 2 6 2P red
->
-4
■>
3N5
4NH
l/n{(NPNH) 3 ) n 450°c
>
(130)
(PN) +
3NH
2PN(NH)
+
(131) 5H
(132)
The cyclic trimer, (PNC1 2 ) 3 (Chapter 5 ) will react with liq uid ammonia in a sealed tube to give the fully ammoniated derivat ive {PN(NH2)2}3 . The action of heat on this compound first prod uces a form of phospham in which the trimeric rings probably remain intact (131), but this eventually loses more ammonia and decomposes along the route (130). The normal form of phospham, made from PC1 5 and NH3, probably has a random network structure (133), like that obtained from reaction (132). 1
/^
NH
N H N
-<
(133)
NH
p «N^V V - k N / Polymeric phosphorus amide imide is produced directly by re acting phosphorus trichloride with ammonia in ethereal solution at -20°C (134). If PC1 3 is added to a saturated solution of NH 3 in chloroform at -78 C, phosphorus triamide is produced. Phosphorus triamide will lose ammonia, then hydrogen and nitrogen to form first the amide imide and eventually (PN) n (135). PCI. PCI,
-> HN.PNH
5NH +NH
3
>
P(NH
3NH Cl 4
-NHc
2}3
->
(134)
P(NH)NH„ (135)
-NHC P2(NH)3
->
P
4N6
*
(PN)
n
^
P
+
N
2.6
94
Numerous compounds based on N,P and H are possible in princi ple. Those containing a multiple bond and based on a single P atom include : H—P—NH iminophosphine
2
N
phosphenimidic nitride
phosphenimidous amide /
H
HN=P=N
H N-P=NH
- %
N H
2
NSP.
NH
2 phosphonitrilic amide
phosphenodiimidic amide Ηχ^ΝΗ
H
W
Η
^Η
N
^NH
NHn 2 phosphinimidic amide
phosphine imine
phosphazyne
H2NN^NH N H N H 2 phosphonimidic diamide
H
2
N
H
2
N /
\p^
N H
X N H
2
phosphorimidic triamide.
Most of these monomeric hydrides remain hypothetical compounds but H-substituted derivatives are in some cases known (2-43A)(2-43B) (see Chapter 5 ) . Amorphous polymeric materials with compositions (PON) and (PSN) can be obtained from reactions of NH 3 with P0C1 3 and PSC1 3 respectively. With more limited reactions, phosphoryl triamide (136) or thiophosphoryl triamide (137) can be made. These compounds are colourless crystalline solids which are soluble in water, but the action of heat will eventually transform them into (PON) n and (PSN) n . The oxynitride can be made crystalline by heating to 700°C under 45 kb pressure. Another route to the polymeric sulphur comp ound is to heat P 4 S 1 0 with ammonium chloride (138). P0C1
6NH
+
liq
PSC1.
6NH
P
4NH Cl 4
4S10
OP(NH 2 ) 3 SP(NH 2 ) 3
+
3NH Cl 4 3NH4C1
-> 4/n (PSN) + 6H S + 4HC1 n 2
(136) (137) (138)
On heating with dry HC1, reaction (139) takes place and with chlorine, polymeric chlorophosphazenes are obtained (140)(Chapter 5) PSN
+
4HC1
SPClr
2PSN
+
3C1„
2/n(PNCl 2 ) n
NH Cl 4 +
S2C12
(139) (140)
The insoluble compound P 2 0 3 N 3 H 5 , obtained from pyrophosphoryl chloride and liquid ammonia, probably has the structure (141) 0
0
?_0_?■f NH
NH
-NH-
(141)
2.7
95
Phosphoryl trihydrazide, Ο Ρ ( Ν Η — N H 2 > 3 , can be made by react ing hydrazine with phosphoryl chloride in anhydrous ether at -12°C (142). The compound forms colourless hygroscopic needles which are more stable than phosphoryl triamide, 0P(NH 2 >3 , in aqueous sol ution, but slowly decompose to give hydrazine N2H4 . 0PClo
+
2.7
6H N-NH
>
OP(NH—NH
)
+
3Ν H Cl
(142)
HALIDES Well-characterised phosphorus halide species include : PF 3
PC133 PCI
PBr 3
PF r
P PCU CU 5 5
PBr r
5 P
2F4
PF*
5
PP CC11
22 44
p
V4
1+
PC1," ci„ "
PBr +
4
4 PCI 4~
4 ( PBr 4 ~ )
PF ß
PCI6"
( PBr " )
6 POF PO3F"
Pig
6
POCI33
P0C1
6
POBr
( PO3CI~")
P
°2F2"
P
°2 C 1 2 _
P
2°3F4~
P
2°3 C1 4~
Trihalides The phosphorus halides are generally very reactive compounds which can cause both acute and chronic poisoning. They are strong electron donors by virtue of the lone-pair electrons on the P atom.
/ p \\i.57o F
X
F
F
U(D) 1 Ό 3
y^N^.oua X
C1^°C1 C1 0.80
^A\V2-220 Br^oiBrXBr 0.61
2 , U 6
^A\ ΙΛθ2Ι X I 0
(143)
The trivalent halides, P X 3 , are well known (143). All exist as pyramidal molecules with X/P/X angles of about 100 . Bond lengths in the trifluoride are abnormally short and this may indicate that, unlike other trihalides, some π-bonding may be present. Physical properties are listed in Table 2.12. Each trihalide may be made (although not necessarily most
2.7
96
conveniently) by direct union of the elements. They will undergo atmospheric oxidation to the oxyhalide, add sulphur to form the thiohalide, hydrolyse under acid conditions to form phosphorous acid, readliy add more halogen to form the pentahalide, and form addition complexes with various metals (Fig 2.14) The trifluoride, PF3 , is a colourless gas, odourless in tox ic concentrations, which burns in air in the presence of nitric oxide as a catalyst. It is best made by fluorination of the chlor ide with arsenic or zinc fluorides, or potassium fluoride dissolved in liquid SO2 .It may also be made by the action of HF on red phos phorus at elevated temperatures (144), or by reacting copper phos phide with lead fluoride (145). 2P red
+
6HF
2Cu P
+
3PbF
>
2PF 2PF
^
o
+ +
3
3H
6Cu
+
(144) 3Pb
(145)
The trifluoride has the largest dipole moment, corresponding to the largest phosphorus-halogen electronegativity difference in the series (143). The fluoride is slow to hydrolyse compared to the other trihalides (it can be washed with water during its preparat ion) , but the ultimate products under acid conditions are phosphor ous and hydrofluoric acids (146). Under controlled alkaline conditPF
+
>
3H 0
Η
·3 Ρ0 Τ
+
3HF
(146)
ions of hydrolysis with KOH, the product is potassium phosphite K2HPO3 , whereas if KHC03 is used the product is potassium fluorophosphite, KFHPO3, (Chapter 3 ) . Phosphorus trifluoride will react with carbon at high temperatures to produce tetrafluoroethylene, and at temperatures above 500°C it will attack silica (in glass), producing substantial quantities of S1F4 . Various metals react with PF3 at high temperatures to produce fluorides and phosphides. Phosphorus trifluoride forms a complex with arsenic pentafluoride below -78 C, but above this temperature it is fluorinated by the latter (147). PF
3'ASF5 ^ 7 ^ C
1
PF
3
+
ASF
>-7^CV
5
?F
5
+
AsF
3
(147)
The trichloride is made commercially by the direct action of dry chlorine gas on red phosphorus suspended in PC1 3 . White phosph orus will give a purer product. Commercial material is usually 99.5 % purity, with P0C13 as the most likely contaminant. Alternat ively the trichloride may be made by the reduction of oxychloride by passage over red hot coke (148), by reaction of the element with certain halides such as HgCl2 , CuCl2 or S0 2 C1 2 (149), or by the action of hydrogen chloride on phosphorus trioxide (150). P0C1
+
C
2P
+
3S0 Cl
> ^
PCI 2PC1
+ 3
+
CO 3S0
2
(148) (149)
2.7
97 6HC1
4 6
2H
-»
(150)
2PC1
3P03
The lone pair 3s electrons, the highly polar nature of the P — C l linkage and its donor or acceptor capacity, means the tri chloride will participate in many chemical reactions (Fig 2.14). Although acid hydrolysis yields phosphorous acid as in the case of the fluoride (146), controlled conditions of pH can give other products such as pyrophosphite and hypophosphate (Chapter 3)
Ni(PCV4
«
3
*
BBi
2 ^
5
f 3
λ
CH_?C1Q A l C l " ά ό 4
GT
^
ί
P0C1
er /*
/ /v
0
PSCI„
7*
**0Λ RPC1
ν
T
Cl PNP0C12
\
^
.H3P°3
* * > . <0*
H
}
(RS)3P^.
RSH
VC\r
2° - > PC1C
Cl,
(RO) 3 P «r~ * ° t t
P(NCO) PCl2Br + PBrCl2
P(CN) P C 1
P(NH2)3
5
F i g u r e 2 . 1 4 R e a c t i o n s of P h o s p h o r u s T r i c h l o r i d e
Phosphorus t r i c h l o r i d e w i l l reduce many o x i d e s , some v i o l e n t l y , i n r e a c t i o n s such as ( 1 5 1 ) - ( 1 5 3 ) , but i s i t s e l f reduced by ant imony, a r s e n i c or a r s e n e ( 1 5 4 ) ( 1 5 5 ) . PCI3
+
5PC13
+
PC1„
+
PCI3
+
PCI
+
2CrC>
- >
P0C1 3
2C1CL
->
4P0C1
sc)
->
POCI3
+
Sb
->
SbC1
+
>
AsP
AsH
+
Cr 0 2 3 +
PCI
3
3
+
0ft 2
(151) (152)
5
+
so 2 P 3HC1
(153) (154) (155)
2.7
98
Phosphorus trichloride is said to react rapidly with pure oxygen, even at low temperatures. It appears that various trace impurities can inhibit this reaction, although it is the commercial route to POCI3 . With ammonia various amides are formed (134)(135) and with nitrogen compounds the reaction products include dialkyl amino-substituted phosphonous halides, phosphazenes and cage comp ounds (Chapter 5 ) . Phosphines may be obtained by reaction with Grignard reagents (4-24), phosphites from reactions with alcohols (4-281,291-294)and phosphonic and phosphinic halides from reactions with metal alkyls (4-126)(4-137) or other organic compounds (4-129)(4-130)(4-132) (4-135). The trichloride is used in general organic chemistry to con vert carboxylic acids to acid chlorides (156). With explosive nitro gen trichloride, a complex cation is formed (157). PCI
+ «
J
3PC1
+
3CH C00H
>
3CH C0C1
+
H P0 ό
O
O
+
}> Cl P-N=PC1
NCI
«J
ό
«3
PC1
(156) ό
a~
ό
(157) Ό
Major industrial uses for phosphorus trichloride include con version to phosphoryl chloride, POCI3 , thiophosphoryl chloride, PSCI3, the manufacture of organophosphite esters (4-281) and the corresponding thiophosphite esters for use in the production of insecticides. The trichloride is used to make compounds such as lauroyl chloride (158), and octyl chloride (159). The former comp ound is used for synthetic detergents, and the latter in the manuf acture of rubbers, vinyl plastics and silver polishes. PC1 3
+
3CH 3 (CH 2 ) 10 COOH
> 3CH 3 (CH 2 ) 1Q C0C1
+
H3P03
(158)
PC1 3
+
3CH 3 (CH 2 ) 7 CH 2 0H
^ 3CH 3 (CH 9 ) ? C0C1
+
H3P03
(159)
The tribromide, PBr 3 , is most conveniently made by reaction between liquid bromine and a solution of white phosphorus in PBr 3 . In most of its reactions the tribromide resembles the trichloride although the former have been much less studied and in some cases the products seem to be more complex. The tri iodide, PI 3, is best prepared by reacting iodine with white phosphorus in a specially purified CS 2 solution. It may also be made by heating the chloride with gaseous HI (160). Exchange for lighter halogen may be effected in some reactions (161). PCI
+
3HI
>
PI
4P I
+
3SnCl
>
4PC1
+ 3
+
3HC1 3SnI
(160) (161)
4
Mixed trihalides are formed from reorganisation which occurs if pure trihalides are mixed (162)(163). At 300-400°C the equilibPC1 3
+
PBr 3
^
PCI Br
PC1 3
+
PF 3
^
PF
2C1
+ +
PClBr PFC1
2
(162) (163)
2.7
99
rium (162) lies well to the RHS. Chloroand bromo fluorophosphines were originally obtained by incomplete fluorination of the approp riate PX3 compounds, using SbF3 or other fluorinating agents. When fluorine halides are involved, reorganisation is slower than with non-fluorine systems, and pure specimens are generally easier to isolate. Difluoroiodophosphine is made from dimethylamino fluorophosphine (5-76 ) . Bromochlorofluorophosphine, PBrCIF, is formed in the equilib rium (164), but a more satisfactory preparation is by equation (165) Cleavage of the P — N bond to give mixed halides also occurs with dialkylamino difluorophosphines (5-75)(5-76) PFBr
+
Me NPFC1
PFC1 +
^
^
2PFClBr
^
HBr
PFClBr
(164) +
Me NH
(165)
Tetrahalides are known with the formula P2X4 where X =F,C1,I. The tetrabromide has been obtained stabilised as a metal complex (10-122). Tetrafluorodiphosphine, P 2 F 4 , is a colourless gas which can be made by reaction (166). If this gas is heated to 900°C, sub stantial quantities of a solid yellow decomposition product are formed, together with a small amount of a colourless liquid, P(PF 3 ) mp = 68° C. Tetraiododiphosphine, P 2 I 4 , which forms dark red crystals, is the most stable tetrahalide. It can be prepared by reacting iod ine with dry phosphine (167) or by direct union of the elements in CS 2 solution. 2PF2I
+
2Hg
>
P2F4
+
Hg2I2
(166)
8PH 3
+
5I2
>
P2I4
+
PH 4 I
(167)
On heating, the solid tetraiodide decomposes into tri iodide and red phosphorus. Carbon disulphide solutions undergo atmospheric oxidation on standing, to give the insoluble amorphous compound ( P 3 I 2 0 6 ) n .Hydrolysis products from P 2 I 4 include hydroiodic acid, hypophosphorous, phosphorous and phosphoric acids and phosphine. Under alkaline conditions hypophosphate is also formed. The P 2 I 5 cation (168) is present in the solid complex P2 I5+ A1I 4 which can be obtained from a CS 2 solution of A1I 3 and P I 3 . \
2-2B1
F O P - 1 (— P v ^ -1C* 5 8 7
^\
*
T ^ \ 2-21 ^ - P Ρ^2·14Θ
9
2 · 2 2
H w \ ( i . 4j ^ p - L - p
1 · 5 9
(168) 1-UiyF
\±/1
H—Poe F
A 1
2·κοΙ
I
-
All.
4
I N 2.22 +
^P
ι' 1
I/
P—I
\ΐ
2 · m
All
4
A
The tetrahalides all contain a P — P bond (168). The gaseous fluoride and the solid state iodide and its solution contain molec ules in the trans C2h configuration. Difluorophosphine, PHF2 , can be made in 55 % yield by heating
2.7
100
iododifluorophosphine with hydrogen iodide and mercury (169). It is a colourless gas stable at low pressure. In the liquid and solid some association occurs through hydrogen bonding (Chapter 12.1). Phosphinodifluorophosphine, F 2 P.PH 2 , can be made by heating phosphine and iododifluorophosphine (170). The latter reacts with cuprous oxide to form oxo-bis difluorophosphine (171), a compound which slowly decomposes to PF 3 and (P0 2 F) n . HI
PF 2 I
PH.
-> F 2 P — P H 2
Cu 0
-> F P-0-PFn ^ 2 2
2PF I
+
2Hg
H
PF 2 I
V2
(169)
+
HI
(170)
+
2CuI
(171)
-> PHFr
Pentahalides The phosphorus pentahalides can be made by addition of halogen to the trihalides or to white phosphorus. These molecules may exist as covalent trigonal molecules or in ionised form (1-10). The pentafluoride is the most stable; it is dissociated a few percent at 100°C, and completely only at 300°C. The pentachloride is consider ably dissociated at 200°C , the pentabromide totally at 35°C, and the pentaiodide does not appear to exist. These compounds hydrolyse in two stages (172)(173). PX_
H
2°
-*· POX.
2HX
(172)
POX^
3H 2 0
* V°4
3HX
(173)
Electron diffraction measurements have established the trigon al bipyramidal configuration in the vapour phases of PF5 and PC15 (174). Both these molecules have zero dipole moments, and axial bonds which are slightly weaker than the equatorial ones. This is indicated by the slightly longer bond lengths and lower stretching frequencies of the axial compared to the equatorial bonds in each halide. 1 . 5 77J F-r-PC
(174) 2.020 |
1 . 534 |
^x
Cl
F
Nuclear magnetic resonance studies have suggested the equiv alence of all five bonds in PF5 , but this has been explained on the basis of a rapid exchange of the non-equivalent F atoms (175) Such a process, which does not involve bond breaking, is known as Berry Pseudorotation (Chapters 1 & 12). It involves the simultaneous exchange of the axial F* atoms with two of the equatorial F atoms, by merely changing the bond angles Only a relatively small angular F
F ' / > F
F
(a)
\> F
-
"
(b)
F/ I^F*
(175)
-f
(C)
101
2.7
deformation of 15° is necessary to convert the initial trigonal bipyramid (175a) to an intermediate tetragonal pyramid (175b) using P — F r as a pivot, and thence a similar degree of angular adjustment to produce the alternative trigonal bipyramid (175c). A similar ex change may subsequently take place using an equatorial F* atom as pivot. If the fluorine atoms exchange at a faster rate than the frequency difference between the chemical NMR shifts from the two kinds of nuclei F and F* , all the P — F bonds would appear equival ent from the NMR data. Below -85°C, however, the rate of exchange is sufficiently slowed down for the two species F and F* to give rise to a doublet 1 9 F NMR peak. The rate of exchange in P C 1 5 is much slower : 73° C 27° C PFr 4.3 xlO 8.3 xlO 8 /sec /sec 5 3 2.3 xlO 2 /sec PCI 7.4 xi0~ /sec 5 Mass spectra of the pentachloride vapour indicate it contains a small percentage of dimer molecules (176a) - species which may also be present in some solvents at low temperatures. C1
\|:/C1\i:/C1 ci^jN^l^ci CI
ci + ci a< N u
ci^^ci ci^l>ci
CI (a)
(176)
CI (b)
Solid phosphorus pentachloride, recrystallised from nitroben zene, has been shown by X-ray diffraction to consist of an assembly of tetrahedral tetrachlorophosphonium P C 1 4 + and octahedral P C 1 6 " ions (176b). The same ions are present in methyl cyanide solution and in other solvents of high dielectric constant. Solutions in benzene or carbon disulphide contain covalent trigonal bipyramidal P C 1 5 molecules. At low concentrations in some solvents there may be slight dissociation to PCl^"1" C l ~ . Condensation of the pentachloride vapour on to a cold finger at about 90°K, produces a solid form containing covalent molecules similar to those in the vapour (174). On warming to normal temperatures these revert to the ionised form (176b). The structure of solid P F 5 i s not known, but the crystalline pentabromide consists of a n assembly of P B r 4 + and Br~ ions. In meth yl cyanide solution the compound is an electrolytic conductor (like the pentachloride) with ion species P B r 4 and P B r 6 , the anion being on the limit of stability and stabilised by solvation. The tetrahed ral P B r u ion, on the other hand, is relatively stable as indicated by the production of P B r 4 PF6 on fluoridation of P B r 5 in non-ionic
Figure
2.15
Crystal Structure of PBr Br
Sooc§oo
οφοαΙ
102
2.7
solvents. The greater stability of the cation compared to the anion is also indicated by the preferential attack on the latter during fluorination of the pentachloride (below). Under normal conditions, PF 5 is a colourless gas which fumes on contact with moist air and immediately hydrolyses with water (177). The pentafluoride may be prepared via the fluorochloride, by heating PF 3 with chlorine in the presence of calcium fluoride at ticHJ LI U l i W ± LI1 PF
+
5
H
H
+
5PF3 NaPF
Π
2°
^
^ΓΊ
V S
2
5PF2
+
PF
3
3P
4°10
2MoF bCa
PF
...T>
°PT?
*"■»·
Λ T-TT"
"ϊ*
ΑΤ> Τ?
^CJT?
4S10
3PC1
Dor „
4 5ASF
+
5
"ΓΓ5
"
ArN0PFr 2 c P
5
>rr 5
*2
2HF
V °T s 3\>F c5
ΓΊ
---*!> PT?
6
SPnF
+
+
3
!C12
^. **
6
; .
rtbr5 1 ΡΠΤΪ P 0 F
."^-
_
~>
3PF
2PC10 o
(178)
NaF
(179)
+
2MoF_ 5
(180)
+
5Ca(P03)2
(181)
+
5
+
+
P F
5 3
(177)
+
+
5
N
2
(182)
15S
(183)
5AsCl
(184)
Alternatively the pentafluoride may be made by heating alkali or alkaline earth hexafluorophosphates (179), by reacting the trifluoride with molybdenum hexafluoride (180), or by heating phosphorus pentoxide with calcium fluoride (181) Organic diazonium salts such as p-chlorophenyldiazonium hexachlorophosphate are commercially available materials which are stable when dry, but decompose at com paratively low temperatures according to (182). Other methods of preparation include the action of sulphur tetrafluoride on P U S 1 0 (183), and the direct fluorination, in solution, of the pentachlor ide with arsenic trifluoride (184). Fluorination of the crystalline pentachloride results, init ially, in preferential attack on the octahedral anion, which con tains the more loosely bound halogen (185). Further fluorination leads to the pentafluoride (186), but the tetrafluorophosphonium cation is formed in an unstable complex with SbF5 (187). It is less stable than NFM . PCI
+
PCI ~ 4 6 3PC1„ + PF ~ 4 6
+
2AsF 0 3 4AsF 0 3
+
> PCI
PF
4 >
6PF ^ 5
+
6 +
2AsCl 4AsCl
(185) (186)
3SbF^ > PF + , b F (187) 3 16 5 5 4 The pentachloride can be made by bromine displacement (188) or by simple addition (189).
PF
+
2PBr
+ o
5C1
> Δ
2PC1
+
C
5
5l3r
(188) 2
103
2.7 PCI.
S
PCI
2C12
(189)
2PSC1,.
Hydrogen and certain metals reduce the pentachloride to tri chloride (190). Phosphoryl chloride (phosphorus oxychloride), P0C1 3 can be obtained from the pentahalide by several reactions (227)(231), and thiophosphoryl chloride, PSCls, from PnS10 (79). Phosph orus pentachloride reacts with ammonium chloride to give phosphonitrilic chloride (5-163). If the pentachloride is reacted with excess liquid ammonia and the ammonium chloride removed by sublimat ion, the products include P(NH2)i|Cl (5-211). Reactions with various other nitrogen compounds are known (Chapter 5 ) .
PCI 3 ?
>
C1
2 4
~
Cl P=NPh
\
H
\
2
;
/
* I <* /
\
\
\
C13P=N-N=PC13 ^
P0C1
A
S
/
\
P C I 4 ici;
\
?C1
/
4
PF
6
^3* Ph3?-0-PClg *ζ
Ph P0
PCI,
P S
4 10
RP0C1 2 *f
>
PSC1
3
KPF„
-' I '
id„4 BC14 '
P0C1„
C13P=NP0C12
Figure 2.16 Reactions of Phosphorus Pentachloride Like the trichloride, the pentachloride is used in general organic chemistry for several purposes. These include the conversion of carboxylic acids to the corresponding chlorides (191), although in the case of oxalic acid, carbon monoxide is obtained (231). PCI, PC1 C 5 PC1C
H, +
-»
PC1
3
2HC1
+
CHCOOH o
-> CH C0C1
S0„
- ^ S0C1 2
+
(190)
HC1
+
P0C1-
P0C1
(191) (192)
2.7
104
Phosphorus pentachloride is used commercially to make thionyl chloride (192). Sulphonic acids are converted to sulphonyl chlorides (193), and the Beckmann rearrangement of oximes can be effected (194). In tetrachloroethane , the pentachloride reacts with hydrazine mainly according to (195), but if the solvent is P 0 C 1 3 , the product is a linear phosphazene (5-270). +
R.SO OH
PC1 C 5
R— C— R N.OH
PCI
+
2PCl f f 5
RSO Cl
P0C1,
HC1
(194)
R— C—NH—R
H2N-NH2
2PC1,
(193)
(195)
4HC1
Phosphonic and phosphinic acids and their esters are convert ed to the corresponding chlorides, and with phenol or catechol sub stituted phosphoranes are obtained (Chapter 4.14).Some of the react ions of the pentachloride are summarised in Fig 2.16. Phosphorus pentafluoride reacts with ammonia, amines or dialkylamino phosphines to give substituted phosphoranes (Chapter 5.3) The latter can also be obtained from trialkyl phosphites or alkoxylithium derivatives (Chapter 4 . 1 4 ) . Mixed Pentahalides Cl F~Ps
I
-f^F
Cl
C2v
Cl
C2v
t
Cl
C3v
-Cl 'Cl
C1
F
D3h
C3v
F
UF
C1
—PCT,
I^F
(196)
-f
F
F Cl-
F—P—
F
F
ci—PC Cl
Cl
I^F
Cl-
Cl
Cl F Cl
Cl
Cl
C2v
F-P-C1
|^C1 D3h
F
C2v
Like the other pentahalides, the mixed pentahalides can be based on either a covalent trigonal bipyramid, or exist in ionised form. Isomeric arrangements are also possible (196), but the stab ilities of these may be influenced by apicophilicity or pseudorotation, and one particular form is usually preferred (Chapter 1 2 . 3 ) . Electron diffraction and spectroscopic studies are generally in accord with trigonal bipyramidal configurations in which the most electronegative halogens occupy the axial (apical) positions. From (196), the correct isomers are thus C2v for P F U C 1 (u = 0.78D) and PF3CI2 ( y= 0.68D); D3h for P C 1 3 F 2 (y = 0D) and C3v for P C 1 U F (μ = 0.21D). In common with P F 5 and P C 1 5 , the most symmetrical mixed halide, PCI3F2 has a zero dipole moment. The NMR spectra of these halides indicate that they contain F
105
2.7
atoms in only one kind of environment. A pseudorotation process is therefore necessary to secure equivalence between axial and equat orial F atoms in the case of PF 3 C1 2 and PF 4 C1 (197). At low temp eratures however, these pseudorotations disappear, and the NMR spec tra indicate non-equivalent F atoms in PC1FU below -138°C and in PF 3 C1 2 below -80°C. F
F·
I ^*F '
^
Cl-j
C1
1^F
~f
<197)
Ff
F
Mixed fluorohalides can be obtained by addition of halogen to PF 3 (198). They are thermally unstable colourless gases which tend to dissociate, the bromine compound being the least stable (199) Phosphorus tetrafluorochloride can be made in 52 % yield by a reac tion between PF5 and BCI3 at low pressure (200). PF 3
+
>
Cl 2
PF
3C12
(198)
2PF Br > 3PF + 2PBr (199) 3 2 5 5 3PF^ + BC1 > 3PF Cl + BF 0 (200) 5 3 4 3 Covalent PF 3 C1 2 is isomeric with the ionic compound PC1 4 PF 6 obtained in reaction (201). In the presence of moisture, the cov alent form converts to the latter, but on heating in vacuo at 100 C the change can be reversed. In boiling carbon tetrachloride the ion ic compound decomposes into covalent PF5 , PF 3 C1 2 and PC1U PC1 5 F . If PC1 4 PF6 is sublimed in vacuo and the vapour cooled to -60°C, covalent liquid PFCI4 is obtained (202). At room temperature, this liquid changes slowly to the ionic solid form PC1 4 F . The latter exists in ionising solvents but in covalent form in non-polar liq uids. The ionic form is hydrolysed by dilute caustic potash (203). 2PF 3 C1 2 2PC1
+
>
P C 1 4 + PF 6 "
(201)
PF ~ > PFC1 + PF (202) 4 b 4 5 3PC1. + PF ~ + 7K0H > KPF^ + K ΗΡ0 Λ 4KC1 + 3 H O (203) 4 b 6 2 4 2 Unstable PCI3 F2 can be made by low temperature chlorination of PC1F2 . On heating, the covalent form transforms to PC14 PF U Cl 2 A crystalline compound of composition PCI4. 6 7 B r 0 . 33 can be obtained from bromineand phosphorus trichloride and this consists of (PC1 U ) 8 (PClö )i+ (Br ) 4 ions. Pentahalides containing three or more differ ent halogen atoms have received comparatively little study, but many isomeric arrangements would be possible (Chapter 12.2). Hydrotetrafluorophosphorane (phosphorus tetrafluorohydride) HPF 4 , mp = -lOCPC, and dihydrotrifluorophosphorane, H 2 PF 3 , mp= -52°C can be made by reacting hydrogen fluoride under anhydrous conditions with phosphorous and hypophosphorous acids respectively (204)(205)
106
2.7
H PO
+
4HF
HPO
+
3HF
_pjO°C
_-r.Y_^_^ C
->
PHF
+
3H 0
(204)
PH Q F
+
2H 0
(205)
Trihydrodifluorophosphorane can be obtained by reacting diphosphine with excess hydrogen fluoride, but the compound appears to be unstable and dissociates approximately as in (206). 2H 3 PF 2
> H 2 PF 3
+
PH 3
+
HF
(206)
According to NMR data, the axial positions are filled by F atoms in all three structures (207), and where applicable, inter change of axial and equatorial F atoms takes place. Although gaseous PHF4 and PH 2 F 3 are monomeric, there is spectroscopic evidence that in the condensed phases these compounds are associated through weak P — H — F bonding. F
»-< I
F
F
>\-*
F
H
l>\-»
H
I
I
F
F F (207) Ammonium pentafluoride reacts with PH 2 F 3 to give the PH 2 F 2 cation which appears to be more stable than the PF 4 cation or the nitrogen analogue NH 2 F 2 (208). PH
2F3
+
AsF
5
PH
^
2F2+
AsF
6_
(208)
Polyhalides A number of phosphorus polyhalides are known. In the system phosphorus tribromide-bromine at least five compounds exist : PBr3 PBr 5 , PBr 7 , PBr g , and PBr 1 7 . The crystalline heptabromide contains tetrahedral PBr 4 + and linear Br-Br-Br ions. Phosphorus pentachloride dissolves in molten ICI2 to give a strongly conducting solution containing the ions PC1 4 IC1 2 ~. Crystalline PC16I is built from ions of this kind, which also persist in methyl cyanide. The chloroiodide may be made by direct addition (209) or from the trichloride and iodine chloride (210). PCI
+
I
>
PC1 3
+
3IC1
^
PCI PC1
4
+
+
IC1 ~ IC1
2~
+ +
PC1 0 X
2
(209) (210)
Other well-defined halides have compositions such as PCl 5 BrI, PBr 5 ICl, PC1 5 IC1, PCl 3 Br 4 , and PBr6I . These all consist of tetra hedral PX 4 cations and the appropriate halide anion, both in the solid state and in the conducting solutions they form with ionising solvents. In CC1 4 solution e.g. as in acetonitrile, ionic species are obtained (212), PC1_I 6
>
PCI
5
+
IC1
(211)
107
2.7 PCI I
PC1
>
4+
+
IC1
(212)
2
Hexahalophosphates (Halophosphorides) Many salts containing the octahedral anions PC1 6 and PF 6 have been studied by spectroscopy and X-ray diffraction (213). The P — C l bond in the octahedral ion has a greater length and lower stretching frequency than in the tetrahedral PC1 U cation (Chapter 1.6). Somewhat weaker bonds in the anion are also indicated by the C1
F
I/* p_F
F
Cl — P
ri
'/
"
Cl
(213)
F preferential attack which occurs in some reactions (185). Reaction (203) suggests the PF 6 ~ anion is more stable than PC1 4 + . The order of stability of the halophosphoride anions appears to be the oppos ite to that of the halophosphonium cations : PF
4
+
PF ~ 6
PC1
< >
4+
PCI ~ 6
PBr
< >
4+
PBr ~ 6
The PF 6 ~ ion (which is isoelectronic with stable SF6 ) , can be produced by fluorination of pentahalides to give simple hexafluorophosphate salts (hexafluorophosphorides) (214) - these decom pose with heat to give PF5 (179). Reaction (215) can also be used, and this gives a mixed octahedral anion PC1 4 F 2 ~ (the PF 4 C1 2 ~ anion can be made by heating covalent PC1 3 F 2 above). PC1 C + o 2PC1 F
NaCl +
2CsF
+
6HF
> NaPF„ 6 ^ CsPF
+
6HC1
(214)
+
CsPCl F
(215)
A good method (used commercially) of preparing the acid is to mix phosphorus pentoxide with anhydrous hydrofluoric acid (216). This reaction proceeds with considerable evolution of heat and giv es white crystals of the hexahydrate, HPF 6 .6H 2 0, mp = 31.5°C, which according to NMR evidence should be formulated as H 3 0 + PF 6 ~HF.4H 2 0. The anhydrous acid can be prepared by reacting PF5 and HF under anhydrous conditions in liquid sulphur dioxide, but on standing at room temperature, the product will decompose again (217). Similar equilibria exist with PF5 and metal fluorides. The aqueous acid exists in equilibrium with difluorophosphoric acid (216)(244) and typical samples may be expected to contain some of the latter. +
24HF
PFr 5
+
HF
NH4PF6
+
P
4°10
NaOH
4HPF^ 6 -> ->-
+
10H O 2
HPF
6 NaPF^ 6
(216) (217)
+
HO 2
+
NH o 3
(218)
Soluble hexafluorophosphate salts can be made by evaporation
108
2.7
of the ammonium salt with the appropriate base (218). The ammonium salt can be obtained from a reaction between HF and (PNC1 2 ) 3 (5-168). Simple alkali metal salts of type MPFÖ are very soluble in water (103 g NaPF6 dissolve in 100 cc H 2 0 ) , and they crystallise with a rocksalt type packing of the M + and P F Ö ~ ions. Solutions of alkali and alkaline earth salts are stable except at very high concentrat ions when hydrolysis to tetrahedral fluorophosphates occurs (below). The nitronium salt, N 0 2 + PC1 6 ~, a white solid stable up to 170°C in a dry atmosphere, is available commercially as a research chemical. It can be made by decomposing the addition complex of nitrosyl fluo ride and phosphorus pentafluoride (221)(below). Addition Complexes Both trivalent and pentavalent phosphorus halides will form addition complexes with metals or metal salts. While the trivalent complexes contain metal-phosphorus bonds (Chapter 10), the penta valent complexes involve rearrangement to produce ionised assemblies of tetrahedral P X 4 + cations and various complex anions. The crystalline addition complex formed between phosphorus trichloride and tetraethylammonium chloride, is quite unusual and is correctly formulated as Eti*N+ PC1 4 ~. Unlike the tetrahedral P C 1 4 + cation, the PCl^" anion adopts a distorted trigonal bipyram idal configuration in which one equatorial arm is believed to be occupied by a lone pair of electrons (219). In this case there is a resemblance to the stereochemistry of arsenic and antimony (Chap ter 1 . 4 ) . Cl -/2 · 8 5 17 1/ 2 · 05
2 · 1 2Λ
2 · 05
Cl Complexes are formed with boron trihydrides and trihalides : F 3 P.BH 3 , Cl 3 P.BBr 3 , Br 3 P.BBr 3 . The colourless gas F 3 P.BH 3 is some , but F 2 HP.BH 3 is even what more stable than its isomer H 3 P.BF 3 more stable. Also known are F 3 P.NMe 3 and Cl 3 P.NMe 3 (Chapter 9 ) . Phosphorus pentafluoride forms many crystalline 1:1 addition complexes of the type PF5X where x may be SF 4 , NO, N0 2 F, C10 2 F, C5H5N, C 6 H 5 CH0, Me 2 0, Me 3 N etc. Some of these contain hexafluorophosphate ions (220)(221), while complexes such as Me 2 N.PF 5 are pro bably based on hexacoordinated phosphorus (Chapter 5.11). SF..PF,, 4 5 NO F.PF
V ^
SF* PF~ 3 6 NO + PF ~
(220) (221)
Phosphorus pentachloride forms numerous addition complexes with metal salts, which may be categorised as : PCI .MC1 5 5 2PC1 .MCl^ 5 4
where M = Sb,Nb,Ta,Mo,W,V. "
"
Ti,Sn,Te,Hf,Zr,V,Pt > > > > > >
109
2.7 pci
.Mm 5 4 PC1C.2MC1, 5 4 PCI ,MC1 0 5 o PCI.MC1 5 2
where
M = Ti,Zr,V,Sn,Se,Te
where
M = Ti
where
M =
where
M = Zn,Hg
B,Al,Ga,Ti,Cr,Fe,Au
These complexes contain tetrachlorophosphonium cations with various complex anions. The compound P C 1 5 . B C 1 3 e.g. is correctly formulated as P C 1 4 + B C 1 4 " . When heated to 340°C it dissociates into C l 2 , P C 1 3 and B C 1 3 . The ionised formulae are indicated by crystal structure data, and by the insolubility of these compounds in non-polar solv ents, but their ability to form conducting solutions in ionizing media. Their NMR and infra red spectra indicate P C 1 4+ ions. Crystal structure data indicate that P C 1 5 . N b C l 5 should be formulated as P C 1 4 + N b C l 6 ~ . Compound P C 1 5 . T i C l 4 has structure(222a) while PCI5.TeCl 4 exists as (222b) in the solid state. Compound P C l 5 . F e C l 3 should be formulated as PCI* , while P C 1 5 . 2 T i C l 4 FeCL contains anions as in (222c). Compounds such as 2PCI5.T1CI4 may exist as 2 P C 1 4 + T i C l 6 ~ or as (222d).
2PC1, 4
C K^iT ^ k l 1T/i 0 1 " i c1i ^ ci1 Lci nci x cC ii C
Cl Cl l/Cl l/Cl -Te — C l — T e — Cly \ / \ Cl Cl Cl Cl
nPCl ((a)
(b) (222)
Cl
C 1
PCI* 4
Cl^ / \ /Cl Cl— Ti —Cl—Ti—Cl Cl^ ΧΓ/ Cl (c)
PCI,
Cl"
Cl
ci^l^ci C l ^ C l
(d)
Oxyhali d e s 1.449
Fvi ° 1 1 . 4 3 6 F - ^ r P == 0
Cl.
F
Cl 2.002
1.524
Cl—P =
0
Br^^
1.44
Br—P = Br* 2. .06
0 (223)
With the exception of the iodide, all the oxyhalides, P 0 X 3 have been well characterised ( 2 2 3 ) , as well as mixed species such as P 0 F 2 C 1 , P 0 F C 1 2 , P0BrCl 2 etc. The phosphorus oxyhalides (phosphoryl halides) are all toxic and reactive. They may all be formed by oxid ation of the appropriate trihalide and all are hydrolysed by water. Phosphoryl fluoride, P 0 F 3 , is a very poisonous gas, very eas ily hydrolysed, which attacks silica-containing glass to form S1F4. It can be prepared by fluorination of the corresponding chlorine compound with salts such as zinc,lead or silver fluoride (224). It may also be made by using potassium fluoride dissolved in liquid sulphur dioxide (225). In the gaseous phase, the oxidation of P F 3 by pure oxygen is explosive. Phosphoryl fluoride is obtainable by hydrolysis of P F 5 or hexafluorophosphates (177)(243).
2.7
110 2P0C1,
3ZnF„
-> 2P0F.
POC1.
3KS0 F
-> POF
(224)
3ZnClr +
3KC1
+
(225)
3S0
Phosphorus oxychloride (phosphoryl chloride), P0Cl3,is a col ourless fuming liquid which can be made by the careful oxidation of the trichloride with air, oxygen or oxidising agents such as potassium chlorate (226) and others. It can be obtained by reacting the pentachloride with boric acid (227), with alcohols (228), oxid es of nitrogen (229), sulphur (230) or phosphorus pentoxide (53). The reaction with oxalic acid (231) is a convenient laboratory meth od. Direct oxidation of the chloride is the usual commercial method of production of phosphoryl chloride. Other commercial processes involve the reaction between phosgene and iron phosphate (232) or oxidation of PC1 5 with P 4 0i 0 (53). The simple reactions (55) (133) have not yet been made commercially viable. The largest technical use for phosphoryl chloride is in phosphorylation reactions (Chapters 4 & 6 ) . The manufacture of phosphate esters (Chapter 6) is a particularly important commercial use. PCI.
3KC10.
-> P0C1, ■> 3P0Clr
3KC1
(226)
PCI,. 5
+
2H
PCI
+
ROH
-> p 0Cl
+
RC1
+
N
-> P0C1
+
2N0 Cl
(229)
+
S0
-> P0C1.
+
S0C1
(230)
PC1 C 5
+
(COOH)
-> P0C1
FeP0„ 4
+
3C0Cl o 2
-> P0Clr
PCI PCI
5 5 5
3 B °3
2°5 2
C a 3 ( P 0 4 ) 2 + 6C0 + 6C1,
350°C
>2P0C1
2 3
+
+
+
6HC1
(227)
HC1
(228)
2HC1
+
CO
3C0
+
FeCl
3CaCl
+ 6C0
+
CO
(231) (232) (233)
Phosphoryl bromide, P0Br 3 , is best prepared by gently heating an intimate solid mixture of pentabromide and pentoxide (234). Mixed oxyhalides are prepared by various methods. Fluorine-contain ing compounds may be obtained by treating P0C1 3 or P0Br 3 with SbF 3 , and chlorobromides by the action of HBr on P0C1 3 at 500°C. 6PBr_
P
4°10
-> lOPOBr
(234)
If the oxychloride is mixed with oxybromide a scrambling occurs and in a few days the equilibrium mixture contains four species : P0C1 3 , P0Cl 2 Br, P0ClBr2 and P0Br 3 . Phosphoryl chloride is a non-protonic solvent, but its proper ties show a remarkable resemblance to those of water. The low elec trical conductivity of both solvents indicates only slight dissoc iation (235)(236). The extensive system of hydrogen bonds character istic of water, is of course absent in phosphoryl chloride.
2.7
111
2P0C1
^
-^
2H 0
^
—=**
Li
P0C1
2+
+
H 0+
%J
P0C1
+
4
0H~
(235)
(236)
The phosphoryl group readily accepts protons and participates in hydrogen bond formation, both in solution and solid states. This has important consequences in determining the structures (partic ularly crystal) of many phosphoryl compounds (Chapter 12.1). The phosphoryl halides, POX3, typify phosphoryl compounds whose properties are dominated by the polar and very reactive phos phoryl bond (Chapter 1.3).They have been much studied by electron diffraction and various spectroscopic techniques. Symmetrical struc tures with C3v symmetry have been established in the vapour, liquid solution and solid states. Because of the highly polar nature of the P Ä 0 linkage, the phosphoryl halides have considerably greater dipole moments than the corresponding trihalides. The phosphoryl bond is strong and rather short, and its high polarity is indicated by the great intensity of the characteristic v (P=0) infra red stretching absorption (Chapter 12.7). When metal salts are dissolved in the phosphoryl halides, add ition complexes are easily formed, and these can usually be isolat ed in the solid state. Since lone pair electrons are not available for coordination purposes as in the trihalides, complexes such as P0Cl3.SbCl5 were originally assigned an ionic formula P0C1 2 + SbCl 6 A number of crystal structure analyses of compounds of this type have, however, firmly established that coordination occurs through the phosphoryl oxygen atom. In solution, interaction through the oxygen atom is indicated by a lowering of the v (P=0) stretching frequency. Typical crystalline complexes such as BCl3.P0Cl 3 , SnCl 4 .2P0Cl 3 and TiCl 4 .P0Cl 3 have structural formulae (237). PCI Cl C 1
0
« \ l / ^ 1 /C1 .Ti Ti <*'k - N ^ c i
C1„P / 3
0 —Sn —Cl C1
3P
i>-PC13
(237)
cix ci Cl— P— 0— B — Cl Cl^ Cl A major use for phosphoryl chloride is in the synthesis of phosphate esters (Chapter 6 ) . It will also react with Grignard re agents to give phosphine oxides (4-104), and with secondary amines to give amino-substituted phosphine oxides or phosphonic dichlorides (5-43 )(5-52 ) . Phosphoryl chloride is reduced by carbon to the tri chloride (148), and with liquid ammonia, phosphoryl triamide is obtained (136). Pyrophosphoryl fluoride, P2^3^4 ,(238a) is a colourless liquid mp = 0.1°C, which can be made by the action of an electric discharge at -75°C on a gaseous mixture of P0F 3 and oxygen. The solid product
2.7
112
F
F
I
I
F
0=P—0—P=0 F
F
I
I
—P—0—P—0—P—0—
II
F
F
I
II
/
II
0
Λ
II
0
(238)
0
(a) (b) on vacuum fractionation yields P2O3F4 and a highly polymerised compound of formula (P0 2 F) n . The latter has a chain structure (238b) Pyrophosphoryl chloride, P 2 0 3 C1 4 , is a colourless oily liquid mp = -16.5 C. It can be made by passing chlorine into a suspension of phosphorus pentoxide in phosphorus trichloride and carbon tetrachloride, when PC1 5 is formed, which reacts as in (239). Studies with radioactive labelled 3 2 P atoms indicate the P — 0 — P linkages in the P40i0niolecule are utilised in the new molecule which has a structure analogous to that of (238a). P + 4PC1 4P0C1 4°10 5 ^ 2P2°3C14 + < 239 > 3 Pyrophosphoryl chloride is hydrolysed by water and it reacts readily with ammonia to give the tetramide (H 2 N) 2 P(0)0P(0)(NH 2 ) 2 and other products (Chapter 5 ) . Polymeric (P0 2 Cl) n can be made by the oxidation of PC1 3 with N 2 0. The mixed halide FC1P(0)0P(0)C1F can be obtained from P0C1 2 F and P 4 0 1 0 . The trivalent analogue of pyrophosphoryl flyuoride is stable enough to be isolated and can be made by reaction (240) as well as (171). The compound is split by HBr according to (241) to form difluorophosphine oxide as well as difluorobromophosphine . In liq uid form the oxide decomposes according to (242). The high boiling point of F 2 PH0 (67.7° C) compared to that of F3 P0 (-39.8° C) indicat es considerable hydrogen bonding in the former compound. CBu Sn^ O
V-Du any
w
+
T
F P.—O—PF
2PF Rr
41 r
l
v
>-„--P'^F
+
V
F 2 PH0
+
PF Br
> " F
+
H(K >Ά
s
DI
HRT*
r
„ F^H " F'r*0
v^
7
F
2Bu SnBr (240)
F^N)
(241) (242)
Halogeno Oxyacids Phosphorofluoridic acid (monofluorophosphoric acid) and phosphorodifluoridic acid (difluorophosphoric acid) are produced in the intermediate stages of hydrolysis of phosphoryl fluoride, and can be isolated under suitable conditions (243). /F 0«P^-F \p
HO ^-» -HF
Λ 0=Ρς-0Η ^F
H
<
?° HF
T
^0H Ο^Ρς-ΟΗ X F
?° / m ^=±^Γ 0=-Ρ<-0Η N HF 0H H
(243)
Although the reaction between hydrogen fluoride and phosphor ic acid is reversible, substantial conversion to phosphorofluoridic
2.7
113
acid can be obtained by heating phosphorus pentoxide with 69 % hydrofluoric acid (244). Sodium phosphorofluoridate is formed in 80 % yield if Graham's salt is heated with sodium fluoride for a brief period (sodium trimetaphosphate may also be used)(245). P 0 4 10 Na
4HF
4H 2 P0 3 F
2H 2 0
3NaF
3 P 3°9
(244) (245)
3Na 2 P0 3 F
Another way of making the acid is to heat pyrophosphoric and hydro fluoric acids under anhydrous conditions (246). On the other hand, if the calcium salt is heated to 550°C, condensation occurs with the elimination of HF (247). The fusion of a mixture of ammonium fluoride and phosphorus pentoxide leads to both mono- and di-fluoro salts (248). The monofluoro salt can be separated by ethanol extrac tion and precipitated as the silver salt. Aqueous solutions of KP0 2 F 2 are easily hydrolysed with caustic potash to form the monofluoro salt and potassium fluoride (249). (H0)2P(0)0P(0)(0H)2
HF
2CaP0 F.2H 0 P 0 4 10 KPO F 2 2
+
FP(0)(OH),
-> (H0)3P0 Ca
2 P 2°7
+
6NH F 4
->
2NH PO F
2K0H
->
K PO F £
2HF +
+
+
3H 2 0
2(NH ) PO F
KF
+
HO
ά
2ι
(246) (247) (248) (249)
Several phosphorofluoridates have solubilities and crystal structures very similar to those of the corresponding sulphates, although unlike the latter, they are hydrolytically unstable.Some isostructural pairs of salts are : K
2P°3F CuPO F.,5H 2 0
K
2S°4 CuSO,. 5 H 4 • 2° N i S O , , ,7H 2 0 4
NiPO F.,7H 2 0
Ovl . 51
0~P
F
12?
1.58
S^ O 1 14
0<:
562
F
Γ
X"^
1.436
(250)
Phosphorodifluoridates are obtained by heating hexafluorophosphates with metaphosphates (251) or boric oxide (252). The acid, mp = -96 C, bp = 116PC, can be made by a direct reaction between monofluoridic acid and phosphoryl fluoride under anhydrous condit ions (253). It is a clear mobile liquid which decomposes at lOCPC, 2/n(NaP0o) 3 n
NaPF^ 3KPF 6 H 9 P0 o F Δ
6
+
+
3NaP0 2 F 2
2B203 P0F o o
100°C
->
3KP0 2 F 2
^^
2HP0 2 F 2
(251) +
4BF 3
(252) (253)
114
2.7
fumes in the atmosphere, and hydrolyses slowly in water to form phosphoromonofluoridic acid. It is a strong monobasic acid when prepared fresh and is somewhat less stable thermally, than the monofluoridic acid. With phosphorus pentabromide, difluorophosphoryl bromide is obtained (254) HPO F + PBr ^ POF Br + POBr + HBr (254) 2 2 5 2 3 Phosphoromonofluoridic acid, HP0 3 F, is a colourless oil some what like concentrated sulphuric acid in appearance. It is an acid of moderate strength (Table 3.1), which is only slowly hydrolysed in mildly alkaline or neutral solution. In concentrated acid or alkaline solution this hydrolysis is more rapid. The anhydrous acid does not attack glass but with alkaline borohydrides, borane is obtained (Chapter 9 ) . NaPOF
+
2AgN0
>
Ag PO F
+
2NaN0
(255)
H PO F
+
SnF
>
SnPO F
+
2HF
(256)
Phosphoromonofluoridates in solution will give a precipitate with silver nitrate (255). The sodium and stannous salts are used in toothpaste formulations to inhibit dental caries. They act by converting the tooth hydroxyapatite into the somewhat harder and more acid resistant fluoroapatite (Chapter 3 ) . For the passivation of metal surfaces, potassium monofluorophosphate is superior to chromic acid and much less toxic. Although the chlorophosphoric (phosphorochloridic) acids are, in principle, related as in (243), they are more difficult to iso late than their fluoro analogues. Phosphorodichloridic acid can be made by the hydrolysis of pyrophosphoryl chloride at -60°C (257). It is a clear liquid, stable in the absence of air, and has a melt ing point of -18°C. Cl P(0)0P(0)C1 Δ
+ Δ
HO
>
2H0P0C1
£
(257) Δ
The dichlorophosphates of Be,Al,Ga,In and Fe are precipitated when the respective anydrous metal chlorides are dissolved in an excess of P0C13 and a stream of chlorine dioxide is bubbled into the solution. With SnCl^ obtained.
Cl
Cl
^Cl Cl Ρχ
ClvJ .0 0^1/Cl J3.T X ci-^l x o ^ X < T I ^ci
*f°
ci-Ni *™ 3
(258)
2.7 TABLE
115 2-12
Physical Data for Phosphorus Halldes bp °C
PF3 PCI 3 PBr, PI 3 PF 2 C1 PFC12 PF2Br PFBr 2 PF2I PHF 2 P2FU P 2 Iu P0F 3 POCI3
-151 - 93.6 - 40.5
-164.8 13.8 - 16.1 78.4 26.7 - 64.6 - 6.2
- 47.3 -144 -133.8 -115 - 93.8 -124.0 - 86.5 124.5 - 39.7
d
d - 39.1 105.1
bp °C
mp °C
-101 75.2 173.3
61
1.2
P0Br3 P0F2C1 P0F2Br P0FC12 P0FBr 2 P0Cl2Br P0ClBr 2
PF5
191.7
3.1
31.6 52.9 110.1
52.3 49
-84.8
mp °C PC1 4 PF 6 - 96.4 PBr 4 Br - 84.8 PBr u F - 80.1 PBr*PF6 -117.2 PSF 3 10(39mm) PSCI3 31(12rrm) PSBr 3 - 91.6 PSF 2 C1 160 PSFC1 2 -132 PSF 2 Br -124 PSFBr 2 - 61 PSCl 2 Br PSClBr 2 - 30.5
PC15 d PC1F U -43.4 PC12F3 2.5 50.4 PC13F2 PC1UF 105.9 P C 1 U P C 1 5 .F d 110
5.5
bp °C
rrp °C
d 135
d d
84 87
d 135 - 52.3 -148.8 - 40.8 125
212 6.3
64.7 35.5 125.3
80 95
38
-155.2 - 96.0 -136.9 - 75.2 - 30(60mm! - 6(60mm'
Crystalline salts of difluorodiphosphoric acid (an unstable hydrolysis product from pyrophosphoryl fluoride (238)) have been prepared (259) and the dimensions of the anion measured (260). anh P 0 4KF (259) 4 10 ■>2K2P2°5F2
(260)
2.7
116
Further Reading
(1) (2) (3) (4)
-
Chapter 2
F. Krafft, "Phosphorus from Elemental Light to Chemical Element", Angew. Chem., 3, 660 (1969). D.R. Peck, "The Physical and Chemical Properties of Phosphorus", in Mellor's Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol VIII, Supp III, Longmans, London 1971. R.E. Threlfall, "A Hundred Years of Phosphorus Making" Albright & Wilson, London 1951. R.B. Burt & J.C. Barber, "The Production of Elemental Phosphorus by the Electric Furnace Method" Chera Eng Rept No 3, TVA Wilson Dam. Alabama. 1952.
(5)
H.S. Bryant, N.G. Halloway, A.D. Silber, "Phosphorus Plant Design- New Trends" Ind.Eng.Chera. 62, 8 (1970). (6) A.D.F. Toy, "Phosphorus Compounds" in Comprehensive Inorganic Chemistry, Vol 2, Pergammon, 1973. (7) A.F. Childs in "Modern Inorganic Chemical Industry" Ed R. Thompson, Chem.So Spec. Pub 31, 1977.
(8) (9)
D.E.C. Corbridge, "The Structural Chemistry of Phosphorus Compounds", Topics in Phosphorus Chemistry, 3, 57 (1966). B. Aronsson, T. Lundstrom, S. Rundqvist, "Borides, Silicides and Phosph ides", Methuen, London, 1965.
(10) H.F. Franzen, "Structure and Bonding in Metal-Rich Pnictides, Chalcides and Halides", Prog.Solid.St.Chem., 12, 1 (1978). (11) S. Rundqvist, "Binary Transition Metal Phosphides", Arkiv. Kerai., 20, 67 (1962). (12) A. Wilson, "The Metal Phosphides" in (2). (13) H.G. Von Schnering, "Homoatomic Bonding of Main Group Elements", Angew.Chem; 20, 33 (1981). (14) V.l. Kosyakov, I.G. Vasileva, "Phosphorus Rings, Clusters, Chains and Layers" Russ. Chem. Revs., 48, 153 (1979). (15) J.E. Such, "Lower and Higher Phosphorus Oxides" in (2). (16) H. Hoffman, M. Becke-Goehring, "Phosphorus Sulphides", Topics Phos.Chem., 8, 193 (1976). (17) A.F. Childs "Phosphorus Sulphides and Oxysulphides" in (2). (18) C.A. Finch, S. Ramachandran, "Matchmaking, Science, Technology & Manufacture" Horwood, Chichester, 1983.
(19> (20) (21) (22) (23) (24)
N.L. Paddock "Recent Chemistry of Phosphine" Chem.and Ind. 900 (1955). E. Fluck, " Chemistry of Phosphine" Topics in Current Chem. 35, 3 (1973). E.J. Lowe "Phosphorus Hydrides and Phosphonium Compounds" in (2). E. Borisov, E.E. Nifantev, "Phosphorus Nitrides", Russ.Chem.Revs.,46,842 (1977). K. Utvary, "Phosphorus-Nitrogen Compounds", Method.Chim., 73, 447 (1978). M.L. Nielsen, "Phosphorus Nitrogen Chemistry", Chapter 5 in Developments in Inorganic Nitrogen Chemistry, C.B. Colburn, Elsevier, Amsterdam, 1966. (25) E. Fluck, "Phosphorus- Nitrogen Chemistry" Topics Phos. Chem.,4,291 (1967).
2.7
117
(26) D.S. Payne, "Chemistry of the Phosphorus Halides" Topics Phos.Chem. 4, 85 (1967). (27) R.H. Tomlinson, "Halides of Phosphorus" in (2). (28) R. Schmutzler, 0 Steler, "Halides of Phosphorus" MTP Science Revs., Vol 2 Butterworth. 1972. (29) S.V. Fridland, B.D. Chernokel'ski , "Structure & Reactivity of Phosphorus Pentachloride", Russ.Chem.Revs., 47, 742 (1978). (30) K. Dehnicke, A.F. Shikada., "Derivatives of Oxohalogeno Phosphoric Acids" Structure & Bonding, 28, SI Springer-Verlag, 1976. (31) R. Schmutzler, "Fluorides of Phosphorus", Adv.Fluorine Chem. 5, 1 (1965). (32) M. Webster, "Addition Compounds of Group V Pentahalides" Chem Revs., 66, 87 (1966). (33) N.M. Karayannis, C M . Mikulski and L.L. Pytlewski, "Phosphoryl & Thiophosphoryl Coordination Complexes", Inorg.Chim.Acta.Revs., 5, 69 (1971). (34) M.W.G. De Bolster, "Phosphoryl Coordination Chemistry 1975-81" Topics.Phos. Chem., 11, 69 (1983).