Phosphorus

12 Phosphorus 12.1 Introduction

As shown in the Panel on the next page, phos­ phorus is probably unique among the elements in being isolated first from animal (human) excreta, then from plants, and only a century later being recognized in a mineral.

Phosphorus has an extensive and varied chem­ istry which transcends the traditional bound­ aries of inorganic chemistry not only because of its propensity to form innumerable covalent "organophosphorus" compounds, but also because of the numerous and crucial roles it plays in the biochemistry of all living things. It was first isolated by the alchemist Hennig Brandt in 1669 by the unsavoury process of allowing urine to putrify for several days before boiling it down to a paste which was then reductively distilled at high temperatures; the vapours were condensed under water to give the element as a white waxy substance that glowed in the dark when exposed to a i r . Robert Boyle improved the process (1680) and in subsequent years made the oxide and phosphoric acid; he referred to the element as "aerial noctiluca", but the name phos­ phorus soon became generally accepted (Greek φως phos, light; Greek φόρος- phoros, bringing).

In much of its chemistry phosphorus stands in relation to nitrogen as sulfur does to oxygen. For example, whereas N and 0 are diatomic gases, Ρ and S have many allotropie modifications which reflect the various modes of catenation adopted. Again, the ability of Ρ and S to form multiple bonds to C, Ν and O, though it exists, is less highly developed than for Ν (p. 416), whereas the ability to form extended networks of - P - O - P - O - and - S - O - S - O bonds is greater; this is well illustrated by comparing the oxides and oxoanions of Ν 2

(1)

M . E. W E E K S , Discovery of the Elements, Journal of Chemical Education Publ., Easton, Pa., 1956; Phosphorus, pp. 1 0 9 - 3 9 . 1

473

2

474

Phosphorus

Ch. 12

§12.2.1

Abundance

and

and P. "Valency expansion" is another point of difference between the elements of the first and second periods of the periodic table for, although compounds in which Ν has a formal oxidation state of + 5 are known, no simple "single-bonded" species such as NF5 or NCl6~ have been prepared, analogous to P F and PCl6~~. This finds interpretation in the availability of 3d orbitals for bonding in Ρ (and S) but not for Ν (or O). The extremely important Wittig reaction for olefin synthesis (p. 545) is another manifestation of this property. Discussion of more extensive group trends in which Ν and Ρ are compared with the other Group 15 elements As, Sb and Bi, is deferred until the next chapter (pp. 5 5 0 - 4 ) . Because of the great importance of phosphorus and its compounds in the chemical industry, several books and reviews on their preparation and uses are a v a i l a b l e / Some of these applications reflect the fact that Ρ is a vital element for the growth and development of all plants and animals and is therefore an important constituent in many fertilizers. Phosphorus compounds are involved in energy transfer

distribution

475

processes (such as photosynthesis (p. 126), metabolism, nerve function and muscle action), in heredity (via DNA), and in the production of bones and t e e t h . ~ Topics in phosphorus chemistry are regularly reviewed. ( l l

1 4 )

(15)

5

2 - 1 0 )

12.2 The Element 12.2.1 Abundance and distribution Phosphorus is the eleventh element in order of abundance in crustal rocks of the earth and it occurs there to the extent of M 1 2 0 ppm (cf. H M 5 2 0 p p m , Mn M 060 ppm). All its known terrestrial minerals are orthophosphates though the reduced phosphide mineral schriebersite (Fe,Ni) P occurs in most iron meteorites. Some 200 crystalline phosphate minerals have been described, but by far the major amount of Ρ occurs in a single mineral family, the apatites, and these are the only ones of industrial importance, the others being rare curiosities/ Apatites (p. 523) have the idealized general formula 3Ca (P0 )2.CaX , that is C a i o ( P 0 ) X , and common members are fluorapatite Ca5(P0 ) F, chloroapatite C a ( P 0 ) C l , and hydroxy apatite Ca5(P0 ) (OH). In addition, there are vast deposits of amorphous phosphate rock, phos­ phorite, which approximates in composition to fluoroapatite. These deposits are widely 3

16)

3

2

J. EMSLEY and D. HALL, The Chemistry Harper & Row, London 1976, 534 pp.

of

Phosphorus,

3

4

5

L. D. QUIN and J . D . VERKADE (eds.), Phosphorus Chemistry: Proceedings of the 1981 International Conference, ACS Symposium Series No. 171, 1981, 640 pp. 6

H. GOLDWHITE, Introduction to Phosphorus Chemistry, Cambridge University Press, Cambridge, 1981, 113 pp. 7

E. C. ALYEA and D. W. M E E K (eds.), Catalytic Aspects of Metal Phosphine Complexes, A C S Symposium Series No. 196, 1982, 421 pp. 8

D. E. C. CORBRIDGE, Phosphorus: An Outline of its Chemistry, Biochemistry and Technology, 5th edn. Elsevier, Amsterdam, 1995, 1208 pp. 9

A. D. F. T O Y and Ε. N. W A L S H , Phosphorus Chemistry in Everyday Living, (2nd edn). Washington, A C S , 1987, 362 pp. E. N . W A L S H ,

E.J.GRIFFITH,

R. W. PARRY

2

and

L. D. QUIN (eds.), Phosphorus Chemistry: Developments in American Science, A C S Symposium Series No. 486, 1992, 288 pp.

4

6

2

4

5

A. F. CHILDS, Phosphorus, phosphoric acid and inorganic phosphates, in The Modern Inorganic Chemicals Industry, (R. THOMPSON, ed.), pp. 3 7 5 - 4 0 1 , The Chemical Society, London, 1977. Proceedings of the First International Congress on Phosphorus Compounds and their Non-fertilizer Applications, 17-21 October 1977 Rabat, Morocco, IMPHOS (Institut Mondial du Phosphat), Rabat, 1978, 767 pp.

1 0

4

4

4

3

3

3

(1U7)

J. R . VAN W A Z E R (ed.), Phosphorus and its Vol. 2, Technology, Biological Functions and Interscience, New York, 1961, 2046 pp. 1 2

F. H. PORTUGAL and J . S. C O H E N , A Century

Compounds, Applications, of DNA.

A

History of the Discovery of the Structure and Function of the Genetic Substance, M I T Press, Littleton, Mass., 1977, 384 pp. 1 3

R . L. RAWIS, Chem. pp. 2 6 - 3 9 . 1 4

J. K . BARTON, Chem. pp. 3 0 - 4 2 .

and Eng. News,

Dec. 2 1 , 1987,

and Eng. News,

Sept. 26, 1988,

1 5

Topics in Phosphorus Chemistry, Wiley, New York, Vol. 1 (1964)-Vol. 11 (1983). J. O. NRIAGU and P. B. M O O R E (eds.), Phosphate Miner­ als, Springer Verlag, Berlin, 1984, 442 pp. 1 6

1 7

W. BUCHNER, R . SCHLIEBS, G . W I N T E R and K . H. BUCHEL,

Industrial Inorganic Chemistry, (transi. D. R . TERRELL), VCH, Weinheim, 1989, Phosphorus, pp. 6 8 - 1 0 5 .

476

Phosphorus

Table 12.1

Estimated reserves of phosphate rock (in gigatonnes of contained P)

Continent Africa North America South America Europe Asia/Middle East Australasia

Ch. 12

Main areas

9

Reserves/10 tonnes Ρ

Morocco, Senegal, Tunisia, Algeria, Sahara, Egypt, Togo, Angola, South Africa USA (Florida, Georgia, Carolina, Tennessee, Idaho, Montana, Utah, Wyoming), Mexico Peru, Brazil, Chile, Columbia Western and Eastern Kola Peninsula, Kazakhstan, Siberia, Jordan, Israel, Saudi Arabia, India, Turkey Queensland, Nauru, Makatea

4.6 1.6 0.4 0.7 1.4 0.4

Total

9.1

spread throughout the world as indicated in Table 12.1 and reserves (1982 estimates) are ade­ quate for several centuries with present technol­ ogy. The phosphate content of commercial phos­ phate rock generally falls in the range (72 ± 10)% BPL [i.e. "bone phosphate of lime", C a ( P 0 ) 2 ] corresponding to (33 ± 5)% P O i or 1 2 - 1 7 % P. The USA is the principal producer, having produced one-third of the total world output in 1985, and Morocco is the largest exporter, mainly to the UK and continental Europe. World pro­ duction is a staggering 151 million tonnes of phosphate rock per annum (1985), equivalent to some 20 million tonnes of contained phosphorus (p. 480). Phosphorus also occurs in all living things and the phosphate cycle, including the massive use of phosphatic fertilizers, is of great cur­ rent i n t e r e s t / The movement of phospho­ rus through the environment differs from that of the other non-metals essential to life (H, C, Ν, Ο and S) because it has no volatile com­ pounds that can circulate via the atmosphere. Instead, it circulates via two rapid biological 3

4

4

0

1 8 _ 2 0 )

cycles on land and sea (weeks and years) super­ imposed on a much slower primary geological inorganic cycle (millions of years). In the inor­ ganic cycle, phosphates are slowly leached from the igneous or sedimentary rocks by weather­ ing, and transported by rivers to the lakes and seas where they are precipitated as insoluble metal phosphates or incorporated into the aquatic food chain. The solubility of metal phosphates clearly depends on pH, salinity, temperature, etc., but in neutral solution C a ( P 0 ) (solubil­ ity product M O m o l 1 ~ ) may first precipitate and then gradually transform into the less solu­ ble hydroxyapatite [ C a ( P 0 ) ( O H ) ] , and, finally, into the least-soluble member, fluoropatite (sol­ ubility product M 0 m o l l ~ ) . Sedimentation follows and eventually, on a geological time scale, uplift to form a new land mass. Some idea of actual concentrations of ions involved may be obtained from the fact that in sea water there is one phosphate group per million water molecules; at a salinity of 3.3%, pH 8 and 20°C, 87% of the inorganic phosphate exist as [ H P 0 ] ~ , 12% as [ P 0 ] " and 1% as [ H P 0 ] ~ . Of the [ P 0 ] " species, 99.6% is complexed with cations other than N a . The secondary biological cycles stem from the crucial roles that phosphates and particularly organophosphates play in all life processes. Thus organophosphates are incorporated into the back­ bone structures of DNA and RNA which regu­ late the reproductive processes of cells, and they 3

- 2 9

Β . H . SVENSSON and R. SΤDERLUND (eds.), Nitrogen, Phosphorus, and Sulfur-Global Biogeochemical Cycles, SCOPE Report, No. 7, Sweden 1976, 170 pp.; also SCOPE Report No. 10, Wiley, New York, 1977, 220 pp, and SCOPE Newsletter 47, Jan. 1995, pp. 1 - 4 . 1 9

E. J. GRIFFITH,

A . BEETON,

J.M.SPENCER,

D. T. MITCHELL (eds.), Environmental book, Wiley, New York, 1973, 718 pp.

Phosphorus

and

Hand­

4

2

5

5

4

_ 6 0

3

9

9

2

4

3

3

4

2

+

1 S

5

4

4

( 2 1 )

2 0

Ciba Foundation Symposium 57 (New Series), Phospho­ rus in the Environment: Its Chemistry and Biochemistry, Else­ vier, Amsterdam, 1978, 320 pp.

2 1

Ε . T . DEGENS, Topics in Current Chem. 64, 1 - 1 1 2 ( 1 9 7 6 ) .

§12.2.1

Abundance

are also involved in many metabolic and energytransfer processes either as adenosine triphos­ phate (ATP) (p. 528) or other such compounds. Another role, restricted to higher forms of life, is the structural use of calcium phosphates as bones and teeth. Tooth enamel is nearly pure hydroxyapatite and its resistance to dental caries is enhanced by replacement of O H by F~ (fluoridation) to give the tougher, less soluble [Ca5(PU4)3F]. It is also commonly believed that the main inorganic phases in bone are hydroxya­ patite and an amorphous phosphate, though many crystallographers favour an isomorphous solu­ tion of hydroxyapatite and the carbonate-apatite mineral dahlite, [ ( N a , C a ) ( P 0 , C 0 ) ( O H ) ] , as the main crystalline phase with little or no amorphous material. Young bones also contain brushite, [ C a H P 0 . 2 H 0 ] , and the hydrated octacalcium phosphate [ C a g H ( P 0 ) 6 . 5 H 0 ] which -

5

4

4

3

3

and

distribution

477

.is composed, essentially, of alternate layers of apatite and water oriented parallel to ( 0 0 1 ) . The land-based phosphate cycle is shown in The amount of phosphate in untilled Fig. 1 2 . 1 . soil is normally quite small and remains fairly stable because it is present as the insoluble and Al . To be used by salts of Ca", F e plants, the phosphate must be released as the soluble [ H P 0 ] ~ anion, in which form it can be taken up by plant roots. Although acidic soil conditions will facilitate phosphate absorption, phosphorus is the nutrient which is often in shortest supply for the growing plant. Most mined phosphate is thus destined for use in fertilizers and this accounts for up to 7 5 % of phosphate rock in technologically advanced countries and over 90% in less advanced (more (2l)

(22)

m

2

111

4

2

2

4

2

Figure 1 2 . 1

J. E M S L E Y , Chem. Br.

The land-based phosphate cycle.

13, 4 5 9 - 6 3

(1977).

478

Ch. 12

Phosphorus

agriculturally based) countries. Moderation in all things, however: excessive fertilization of natural waters due to detergents and untreated sewage in run-off water can lead to heavy overgrowth of algae and higher plants, thus starving the water of dissolved oxygen, killing fish and other aquatic life, and preventing the use of lakes for recreation, etc. This unintended overfertilization and its consequences has been termed eutrophication (Greek εύ, eu, well; τρέφειν, trephein, to nourish) and is the subject of active environmental legislation in several countries. Reclamation of eutrophied lakes can best be effected by addition of soluble A l salts to precipitate the phosphates. m

As just implied, the land-based phosphorus cycle is connected to the water-based cycle via the rivers and sewers. It has been estimated that, on a global scale, about 2 million

Figure

12.2

tonnes of phosphate are washed into the seas annually from natural processes and rather more than this amount is dumped from human activities. For example in the UK some 200000 tonnes of phosphate enters the sewers each year: 100000 tonnes from detergents (now decreasing), 75 000 from human excreta, and 25 000 tonnes from industrial processes. Details of the subsequent water-based phosphate cycle are shown schematically in Fig. 12.2. The waterbased cycle is the most rapid of the three phosphate cycles and can be completed within weeks (or even days). The first members of the food chain are the algae and experiments with radioactive P (p. 482) have shown that, within minutes of entering an aquatic environment, inorganic phosphate is absorbed by algae and bacteria (50% uptake in 1 min. 80% in 3 min). In the seas and oceans the various phosphate anions 3 2

The water-based phosphate c y c l e .

( 2 2 )

§12.2.3

Allotropes

of

form insoluble inorganic phosphates which gradually sink to the sea bed. The concentration of phosphate therefore increases with depth (down to about 1000 m, below which it remains fairly constant); by contrast the sunlight, which is necessary for the primary photosynthesis in the food chain, is greatest at the surface and rapidly diminishes with depth. It is significant that those regions of the sea where the deeper phosphate-rich waters come welling up to the surface support by far the greatest concentration of the world's fish population; such regions, which occur in the mid-Pacific, the Pacific coast of the Americas, Arabia and Antarctica, account for only 0 . 1 % of the sea's surface but support 50% of the world's fish population.

12.2.2 Production and uses of elemental phosphorus For a century after its discovery the only source of phosphorus was urine. The present process of heating phosphate rock with sand and coke was proposed by E. Aubertin and L. Boblique in 1867 and improved by J. B. Readman who intro­ duced the use of an electric furnace. The reactions occurring are still not fully understood, but the overall process can be represented by the ideal­ ized equation: 1400-

2 C a ( P 0 ) + 6 S i 0 + 10C 3

4

2

» 6CaSi0

2

3

1500

+ 10CO + P ; 4

Δ / / = -3060kJ/mol P

4

phosphorus

479

In the second possible mechanism, the rock is considered to be directly reduced by CO and the CaO so formed then reacts with the silica to form slag: 2 C a ( P 0 ) + 10CO 3

4

> 6CaO + 10CO + P

2

6CaO + 6 S i 0

2

> 6CaSi0

2

10CO + 10C

4

3

> 20CO

2

Whatever the details, the process is clearly energy intensive and, even at 90% efficiency, requires M 5 M W h per tonne of phosphorus (see Panel).

{23)

12.2.3 Allotropes of phosphorus

Phosphorus (like C and S) exists in many allotropie modifications which reflect the variety of ways of achieving catenation. At least five crystalline polymorphs are known and there are also several "amorphous" or vitreous forms (see Fig. 12.3). All forms, however, melt to give the same liquid which consists of symmetrical P tetrahedral molecules, P - P 225 pm. The same molecular form exists in the gas phase ( P - P 221pm), but at high temperatures (above ^800°C) and low pressures P is in equilibrium with the diatomic form P = P (189.5 pm). At atmospheric pressure, dissociation of P into 2 P reaches 50% at M 8 0 0 ° C and dissociation of P into 2P reaches 50% at - 2 8 0 0 ° . 4

4

4

2

2

The commonest form of phosphorus, and the one which is usually formed by condensation from the gaseous or liquid states, is the waxy, cubic, white form a - P (d 1.8232 g c m at 20°C). This, paradoxically, is also the most volatile and reactive solid form and thermody­ namically the least stable. It is the slow phos­ phorescent oxidation of the vapour above these crystals that gives white phosphorus its most characteristic property. Indeed, the emission of yellow-green light from the oxidation of P is one of the earliest recorded examples of chemiluminescence, though the details of the reaction - 3

4

The presence of silica to form slag which is vital to large-scale production was perceptively intro­ duced by Robert Boyle in his very early exper­ iments. Two apparently acceptable mechanisms have been proposed and it is possible that both may be occurring. In the first, the rock is thought to react with molten silica to form slag and P Oio which is then reduced by the carbon: 4

2Ca (P0 ) + 6Si0 3

4

2

2

> 6CaSi0 + P O 3

4

4

l 0 2 3

P O 4

1 0

+ 10C

• 10CO + P

4

D . E. C . CORBRIDGE, The Structural Chemistry phorus, Elsevier, Amsterdam, 1974, 542 pp.

of Phos­

480

Phosphorus

mechanism are still not fully understood: the pri­ mary emitting species in the visible region of the spectrum are probably (PO) and HPO; ultra­ violet emission from excited states of PO also At —76.9° and atmospheric pressure occurs. the α-form of P4 converts to the very similar white hexagonal /3-form (d 1.88 g c m ) , pos­ sibly by loss of rotational disorder; ΑΗ(α^β) —15.9 kJ (mol P 4 ) . White phosphorus is insol­ uble in water but exceedingly soluble (as P4) in C S ( - 8 8 0 g per 100 g C S at 10°C). It is also very soluble in PC1 , POCl , liquid S Q , liquid 2

(24)

- 3

- 1

2

2

3

3

2

R . J . VAN Z E E and A. U. KHAN, J. Am. Chem. Soc. 96, 6 8 0 5 - 6 (1974). 2 4

Ch. 12

NH3 and benzene, and somewhat less soluble in numerous other organic solvents. The β-form can be maintained as a solid up to 64.4°C under a pressure of 11 600 atm, whereas the α-form melts at 44.1°C. White phosphorus is highly toxic and ingestion, inhalation or even contact with skin must be avoided; the fatal dose when taken inter­ nally is about 50 mg. Amorphous red phosphorus was first obtained in 1848 by heating white P4 out of contact with air for several days, and is now made on a commercial scale by a similar process at 270°-300°C. It is denser than white P4 (—2.16 g c m ) , has a much higher m.p. - 3

§12.2.3

Allotropes

of

phosphorus

481

Figure 12.3 Interconversion of the various forms of elemental phosphorus (1 kbar = 10 Pa = 987.2 atm). 8

(~600°C), and is much less reactive; it is therefore safer and easier to handle, and is essentially non-toxic. The amorphous material can be transformed into various crystalline red modifications by suitable heat treatment, as summarized on the right hand side of Fig. 12.3. It seems likely that all are highly polymeric and contain three-dimensional networks formed by breaking one P - P bond in each P tetrahedron and then linking the remaining P4 units into chains or rings of Ρ atoms each of which is pyramidal and 3 coordinate as shown schematically below: 4

This is well illustrated by the crystal structure of Hittorfs violet monoclinic allotrope (d 2.35 g c m " ) which was first made in 1865 by crystallizing phosphorus in molten lead. The and consists structure is exceedingly c o m p l e x of Pg and P groups linked alternately by pairs of Ρ atoms to form tubes of pentagonal cross-section and with a repeat unit of 2 I P (Fig. 12.4). These tubes, or complex chains, are stacked (without direct covalent bonding) to form sheets and are linked by P - P bonds to similar chains which lie at right angles to the first set in an adjacent parallel layer. These pairs of composite parallel sheets are then stacked to form the crystal. The average P - P distance is 222 pm (essentially the 3

(25)

9

VON H . THURN and H . KREBS. Acta Cryst. B25, 1 2 5 - 3 5 (1969). 2 5

482

Figure 12.4

Ch. 12

Phosphorus

Structure of Hittorf s violet monoclinic phosphorus showing (a) end view of one pentagonal tube, (b) the side view of a single tube (dimensions in pm).

same as in P ) but the average P - P - P angle is 101° (instead of 60°). Black phosphorus, the thermodynamically most stable form of the element, has been prepared in three crystalline forms and one amorphous form. It is even more highly polymeric than the red form and has a correspondingly higher density (orthorhombic 2.69, rhombohedral 3.56, cubic 3.88 g c m ) . Black phosphorus (orthorhombic) was originally made by heating white P to 200° under a pressure of 12000 atm (P. W. Bridgman, 1916). Higher pressures convert it successively to the rhombohedral and cubic forms (Fig. 12.3). Orthorhombic black Ρ (mp —610°) has a layer structure which is based on a puckered hexagonal net of 3-coordinate Ρ atoms with 2 interatomic angles of 102° and 1 of 96.5° ( P - P 223 pm). The relation of this form to the rhombohedral and cubic forms is shown in Fig. 12.5. Comparison with the rhombohedral forms of As, Sb and Bi is also instructive in showing the increasing tendency towards octahedral coordination and metallic properties (p. 551). Black Ρ is semiconducting but its electrical properties are probably significantly affected by impurities introduced during its preparation. 4

- 3

4

12.2.4 Atomic and physical properties (26)

Phosphorus has only one stable isotope, ^ P , and accordingly (p. 17) its atomic weight is known with extreme accuracy, 30.973 762(4). Sixteen radioactive isotopes are known, of which P is by far the most important; it is made on the multikilogram scale by the neutron irradiation of S(n,p) or P ( n , y ) in a nuclear reactor, and is a pure ^-emitter of half life 14.26 days, £ 1.709 MeV, £ 0.69 MeV. It finds extensive use in tracer and mechanistic studies. The stable isotope Ρ has a nuclear spin quantum number of j and this is much used in nmr spectroscopy. Chemical shifts and coupling constants can both be used diagnostically to determine structural information. In the ground state, Ρ has the electronic configuration [Ne]3s 3p]3p[,3pξ with 3 unpaired 3 2

32

31

m a x

m e a n

3 1

(27)

2

Mel lor's Comprehensive Treatise on Inorganic and The­ oretical Chemistry, Vol. 8, Suppl. 3, Phosphorus, Longman, London, 1971, 1467 pp. D . G . GORENSTEIN (ed.) Phosphorus-31 NMR; Principles and Applications Academic Press, London, 1984, 604 pp. J. G . VERKADE and L. D . QUIN (eds.), Phosphorus-31 NMR Spectroscopy in Stereochemical Analysis, VCH Publishers, Weinheim, 1987, 717 pp. 2 7

§12.2.5

Chemical

reactivity

and

stereochemistry

483

9

(a)

y y

y

y

y

238 pm

y

y

y

y

yy (d)

y (c)

Figure 12.5

y\

y

yy

y* y y

y

(e)

yy

The structures of black phosphorus: (a) portion of one layer of orthorhombic Ρ (idealized), (b) rhombohedral form, portion of one hexagonal layer, (c) cubic form, 4 unit cells, (d) distortion of (a) to the cubic form, and (e) distortion of (b) to the cubic form.

electrons; this, together with the availability of low-lying vacant 3d orbitals, accounts for the pre­ dominant oxidation states III and V in phosphorus chemistry. Ionization energies, electronegativity, and atomic radii are compared with those of N, As, Sb and Bi on p. 550. White phosphorus (aP ) has mp 44. Γ (or 44.25° when ultrapure), bp 280.5° and a vapour pressure of 0.122mmHg at 40°C. It is an insulator with an electrical resistiv­ ity of M O ohm cm at 11 °C, a dielectric constant of 4.1 (at 20°) and a refractive index n (29.2°) 1.8244. The heat of combustion of P to P O i is —2971 kJ m o l and the heat of transition to amorphous red phosphorus is —29 kJ (mol P ) . 4

1 1

D

4

4

of phosphorus depends markedly upon which allotrope is being studied and that increasing catenation of the polymeric red and black forms notably diminishes both reactivity and solubility. The preference of phosphorus for these forms rather than for the gaseous form P , which is its most obvious distinction from nitrogen, can be rationalized in terms of the relative strengths of the triple and single bonds for the 2 elements. Reliable values are hard to obtain but generally accepted values are as follows: 2

0

- 1

_ 1

4

12.2.5 Chemical reactivity and stereochemistry The spontaneous chemiluminescent reaction of white phosphorus with moist air was the first property of the element to be observed and was the origin of its name (p. 473); its spontaneous ignition temperature in air is ^ 3 5 ° . We have already seen (p. 481) that the reactivity

£(N=N)/kJ per mol of Ν £(>N-N<)/kJ per mol of Ν Ratio

946 159 (or 296) 5.95 (or 3.20)

£(P=P)/kJ per mol of Ρ £(>P-P<)/kJ per mol of Ρ Ratio

490 200 2.45

It is clear that, for nitrogen, the triple bond is pre­ ferred since it has more than 3 times the energy of a single bond, whereas for phosphorus the triple-bond energy is less than 3 times the singlebond energy and so allotropes having 3 single bonds per Ρ atom are more stable than that with a triple bond.

484

Ch. 12

Phosphorus

Table 1 2 . 2 CN

Stereochemistry of phosphorus

Geometry

0 1

Examples —

P(g) — in equilibrium with P2(g) above 2200°C P (g) — in equilibrium with P (g) above 800°C; H C = P ; F C = P ; M e C = P (p. 544) HP=CH , [P(CN) ]-, [{C H S(NR)C} P] X- (p. 544), cyclo-C U P, 2,4,6-Ph C H P; M e P = P C F ; P ~ anion (isoelectronic with P S ) in S r P i ; P n ~ anion in N a P ; diazaphospholes [PhP{Mn(rη -C H )(CO) } ] , [(fluorenyl)=P{=C(SiMe ) }]P , PH , PX , P 0 , [PhP{Co(CO) } ] P H , Cl PO, P Oio, P 0 ~ , polyphosphates, MP (zinc-blende type, M = B, Al, Ga, In), [Co (CO) (M -PPh)], [(P )Ni{(Ph PCH CH ) N}]; many complexes of PR etc., with metal centres PBr -, [PBr (CN) ]-. [M(^ -P ){Ni(triphos)} ] PF , PPh [Co (CO) (M-CO) (M -PPh) ], [Os (CO) (M -POMe)] ) PF ", PC1 ", MP (NaCl-type, M = La, Sm, Th, U etc.) Rh P , Hf P (also contains seven- and eight-fold coordination of Ρ by M), [( -P){Os(CO) } ]-( °) [Co (CO) (/z-CO) P]Ta P, Hf P (contains Ρ in seven-, eight-, and nine-fold coordination by M) M P (antifluorite type (p. 118), M = Ir, Rh) Hf P

—

2

Bent<

2

28)

4

( 2 9 )

(30)

2

+

2

6

4

2

3

5

5

3

5

2

3

3

(31)

7

3

4

3

3

4

3

n

(32)

3

5

Planar

(33)

5

5

2

2

(33a)

3

(34)

Pyramidal Tetrahedral

4

2

4

3

3

4

6

4

+

2

3

4

3

4

4

(35)

3 (36)

4

6

2

2

4

2

5

3

3

3

(37)

Local Civ Trigonal bipyramidal Square pyramidal Octahedral Trigonal primsatic

5

2

9

3

2+(38)

2

3

2

5

(39

4

8

2

6

4

2

5

15

4

6

4

3

3

2

4

M6

Irregular (4 + 2) Capped trigonal prismatic Cubic Bicapped trigonal prismatic Tricapped trigonal prismatic Monocapped square antiprismatic

7 8

9

6

3

14

2

2

2

2

2

M P (M = Ti, V, Cr, Mn, Fe, Ni, Zr, Nb, Ta) M P (PbCl -type, M = Fe, Co, Ru) [Rh (CO) P] -( > 3

2

2

2

9

41

21

2 8

E . FLUCK, Topics in Phosphorus

2 9

H.

3 0

W.

S . SHELDRICK, J . KRONER, F. ZWASCHKA and A . SCHMIDPETER, Angew.

3 1

W.

D A H L M A N N and H . G . VON SCHNERING, Naturwissenschaften

Chemistry

6

10, 1 9 3 - 2 8 4 ( 1 9 8 0 ) .

W . K R O T O , J . F. N I X O N , K . O H N O and N . P . C . S I M M O N S , J. Chem. Soc,

Chem.

Commun.,

7 0 9 (1980).

Chem. Int. Edn. Engl.

18, 9 3 4 - 5 ( 1 9 7 9 ) .

59, 4 2 0 ( 1 9 7 2 ) . W . W I C H E L H A U S and H . G . VON SCHNERING,

ibid. 60, 1 0 4 ( 1 9 7 3 ) . 3 2

J.

H . W E I N M A I E R , A . SCHMIDPETER, et al,

Organometallic 3 3

G.

3 3 a

3 4

Angew.

HUTTNER, H . - D . M U L L E R , A . F R A N K , and H . L O R E N Z , Angew.

R . A P P E L , Ε . GAITZSCH and F. K N O C H , Angew.

J.

Chem. Int. Edn. Engl.

18, 4 1 2 ( 1 9 7 9 ) ; Chem. Ber. 113, 2 2 7 8 - 9 0 ( 1 9 8 0 ) ; J.

Chem. 185, 5 3 - 6 8 ( 1 9 8 0 ) .

C . B U R T and G . SCHMID, J. Chem. Soc,

Chem. Int. Edn. Engl.

14, 7 0 5 - 6 ( 1 9 7 5 ) .

Chem. Int. Edn. Engl. 24, 5 8 9 - 9 0 ( 1 9 8 5 ) .

Dalton

Trans.,

1 3 8 5 - 7 (1978).

3 5

L . M A R K O and B . M A R K O , Inorg. Chim. Acta 14, L 3 9 ( 1 9 7 5 ) .

3 6

P . DAPPORTO, S . MIDOLLINI and L . SACCONI, Angew.

3 7

W.

Chem. Int. Edn. Engl.

18, 4 6 9 ( 1 9 7 9 ) .

S . SHELDRICK, A . SCHMIDPETER, F. ZWASCHKA, Κ . B . D I L L O N , A . W . G . PLATT and T . C . W A D D I N G T O N , J. Chem.

Soc,

Dalton Trans., 4 1 3 - 8 ( 1 9 8 1 ) see also Angew. Chem. Int. Edn. Engl. 18, 9 3 5 - 6 ( 1 9 7 9 ) . 3 8

M.

Di V A I R A , S . MIDOLLINI and L . SACCONI, J. Am.

Chem.

Soc.

101, 1 7 5 7 - 6 3 ( 1 9 7 9 ) . For analogous complexes in which

3

μ - ( ? 7 - Ρ ) bridges RhCo, RhNi, IrCo, and RhRh, see C. BIANCHINI, M . D I VAIRA, A . MELI and L . SACCONI, Angew. Chem. Int. 3

Edn.

Engl.

19, 4 0 5 - 6 ( 1 9 8 0 ) .

3 9

J.

4 0

S . B . C O L B R A N , C . M . H A Y , B . F. G . JOHNSON, F. J . L A H O Z , J . L E W I S and P . R. RAITHBY, J. Chem.

M . FERNANDEZ, B . F. G . JOHNSON, J . L E W I S and P . R. RAITHBY, J. Chem. Soc,

Chem.

Commun.,

1 0 1 5 - 6 (1978). Soc,

Chem.

Commun.,

1 7 6 6 - 8 (1986). 4 ,

J.

L . V I D A L , W . E . W A L K E R , R. L . PRUETT and R. C. SCHOENING, [ R h P ( C O ) i ] 9

2

2 -

. Inorg. Chem.

18, 1 2 9 - 3 6 ( 1 9 7 9 ) .

Chemical

§12.2.5

reactivity

Phosphorus forms binary compounds with all elements except Sb, Bi and the noble gases. It reacts spontaneously with 0 and the halogens at room temperature, the mixtures rapidly reaching incandescence. Sulfur and the alkali metals also react vigorously with phosphorus on warming, and the element combines directly with all metals (except Bi, Hg, Pb) frequently with incandes­ cence (e.g. Fe, Ni, Cu, Pt). White phosphorus (but not red) also reacts readily with heated aqueous solutions to give a variety of products (pp. 493 and 513ff), and with many other aqueous and nonaqueous reagents. 2

and

stereochemistry

485

The P2 group is isoelectronic with ethyne (p. 932) and with N (pp. 4 1 4 - 6 ) and A s . It has emerged as a versatile ligand with several well characterized coordination modes as shown schematically in Fig 12.7. The first compound containing the P ligand, [ ( C o ( C O ) ] ( ^ , ^ - P ) ] , was isolated as a red oil in 1973 and was clearly similar to the already known alkyne and A s complexes [ { C o ( C O ) } { ^ - ( C R ) } ] and [{Co(CO) } (^,?7 -As )]. It was formed by reaction of Na[Co(CO) ] with PC1 or PBr in thf. The tetrahedrane-like core (Fig. 12.7a) was confirmed by X-ray analysis on the related P P h derivative [Co (CO) (PPh )(M,rη -P )]. Direct action of P4 with appropriate carbonyl, cyclopentadienyl or alkoxide derivatives of Cr, Mo, W, etc. has yielded a wide range of such compounds of P acting as a 4e-donor, in all of which the two M L vertices can be considered as 15e-acceptors (i.e d + 5e, "isoelectronic" with Ρ in Group 15) e.g., {Cr(Cp)(CO) }, > {Mo(Cp)(CO) }, {W(py)(OPr ) (M-OPr )} , etc., where Cp is (rη5-C5H5) or one of its derivatives. With 14e or 16e metal-vertex acceptors the core adopts the more open "butterfly" configuration (Fig 12.7b) without direct M - M bonding, e.g [{Ni(Et PCH CH PEt )} (M,^ -P )]( ) and its {Ni(PEt ) } and [Pt(PEt ) ] analogues. Further electron-pair donation from one or both of the Ρ atoms can also occur to give compounds such as [Cr (rη -C5H5) (CO) ^,rη P ){M(CO) h ] (M = Cr, Mo, W) (see Figs. 12.7 c, d ) . In these, the P group acts as a 6e or 8e donor, and bridges 3 or 4 M atoms respectively. See below — p. 488 — for examples cf. bis-P , i.e. pseudo-P complexes.) 2

2

2

2

3

2

2

2

2

3

2

2

2

3

2

2

4

3

3

2

The stereochemistry and bonding of Ρ are very varied as will become apparent in later sections: the element is known in at least 14 coordination geometries with C N up to 9, though the most frequently met have CN 3, 4, 5 and 6. Some typ­ ical coordination geometries are summarized in Table 12.2 and illustrated in Fig. 12.6. Many of these compounds will be more fully discussed in later sections. The great propensity of Ρ atoms to catenate into chains, rings and clusters, P , has already been noted during the discussion on allotropy (pp. 4 7 9 - 8 3 ) . These groupings and other similar ones also feature in the structures of metal phosphides (p. 489), polyphosphanes (p. 492) and organopolyphosphanes (p. 542). Moreover, neutral or charged groupings, P„, (n = 2 - 6 , 10) can also serve as l i g a n d s , as can isolated Ρ atoms in anions such as [ ( M 6 - P ) { O s ( C O ) } 6 ] ~ and other structures shown at the foot of Fig. 12.6. Two decades ago virtually nothing was known about this aspect of phosphorus chemistry, but it is now a burgeoning field, and the substantial progress which has been made in recent years now permits a general overview to be given. r t

( 4 2 _ 4 4 )

(40)

3

3

M . DI VAIRA and P . STOPPIONI, Polyhedron 6, 3 5 1 - 8 2 (1987). (Review) O. J. SCHERER, Angew. Chem. Int. Edn. Engl. 24, 9 2 4 - 4 3 (1985); 29 1 1 0 4 - 2 2 (1990). (Reviews) O. J. SCHERER (and 9 others), in R. STEUDEL (ed.), The Chemistry of Inorganic Ring Systems, Elsevier, Amsterdam, 1992, pp. 1 9 3 - 2 0 8 .

5

(45)

3

2

2

n

1 0

(46

2

i

,

2

(47)

2

2

2

2

3

2

2

48

2

2

2

3

2

5

2

2

2

5

2

4

o r 2

( 4 9 )

2

2

4

5

4

C . F . CAMPANA, A . V I Z I - O R O S Z , G . PLYI, L . M A R K O and

L . F . D A H L , Inorg. Chem. 18, 3 0 5 4 - 9 (1979). 4

4 2

2

6

L . Y . G O H , C . K . C H U , R. C . S . W O N G and T . W . H A M B -

LEY, J. Chem. Soc.,-Chem. 4

7

M . H . CHISHOLM,

Commun.,

1 9 5 1 - 6 (1979).

Κ . FOLTING,

J . C . HUFFMAN

4 3

J. J . K O H , Polyhedron

4 4

Int. Edn. Engl. 24, 5 2 2 - 4 (1985).

4 8

4 9

and

4, 8 9 3 - 5 (1985).

H . SCHΒFFER, D . BINDER and D . FENSKE, Angew.

Chem.

L . Y . G O H , R. C . S . W O N G and T. C . W . M A K , J. Organo­ metallic Chem. 364, 3 6 3 - 7 1 (1989) and 373, 7 1 - 6 (1989).

486

Phosphorus

Figure 12.6

S c h e m a t i c r e p r e s e n t a t i o n of s o m e of t h e c o o r d i n a t i o n g e o m e t r i e s of p h o s p h o r u s .

Ch. 12

§12.2.5

Chemical

Figure 12.7

reactivity

and

stereochemistry

487

(a) (μ,η -Ρ ) 4e-donor to 15e ML vertices, (b) ^,rη -P )4e-donor to 14e or 16e ML . (c) Triply bridging (μ ,η -Ρ ), a formal 6e-donor. (d) Quadruply bridging (μ ,?7 -Ρ ) 8e-donor. 2

2

3

n

2

2

2

n

4

2

2

2

as an η and η donor (see text), (b) Ηyc/o-P as an η donor; addition of η donation to (a) Cyclo-Pi 1, 2 or 3 further metal centres is possible, (c) Bis-?? ligation of cyclo-P^ to coordinated metal centres M(L ). (d) More open η ,η coordination of P to different metal centres, e.g. M = {Ni(triphos)} , M' = {Pt(PPh ) } (see text).

Figure 12.8

]

3

2

3

1

3

n

2

3

3

2,

cyclo-Pi ligand can act in either the or η mode as shown schematically in Fig. 1 2 . 8 ( a ) - ( c ) . - Each of the three Ρ atoms in 8(b) can also have a further pendant M L group attached thereby making the cyclo-P^ ligand μ , μ or μ . In addition, the more open structure 8(d) is known in the binu­ clear cation [(triphos)Ni{P Pt(PPh ) }]" , where triphos is 1,1,1 -tris(diphenylphosphinomethyl)ethane, { C H C ( C H P P h ) } . The η and η modes in Fig. 12.8(a) have only recently been established (in [ { ( r η - C M e ) ( C O ) F e - P } Cr(CO) ]) but the η mode in Fig. 12.8(b) has been known since 1976 when it was found that one of the main products of the reaction between P and [Co (CO)g] was the reactive

The

η ,η χ

2

3

(42

50)

n

2

3

4

3

3

2

2

3

3

2

f

1

(42)

2

5

4

4

5

5

2

3

3

(50)

+

2

2

pale-yellow solid [ C o ( C O ) ( r η - P ) ] . Numer­ ous other examples featuring Co, Rh and Ir, and the isoelectronic cationic metal centres with Ni, Pd and Pt are now known. Metals in ear­ lier groups require more electron donation from pendant ligands to achieve the 15-electron ver­ tex configuration isolobal with the subrogated Ρ atom in P , e.g. { M o ( ^ - C M e ) ( C O ) } . The bi­ nuclear η ,η mode of cyclo-P^ (Fig. 12.8c) and its A s homologues were extensively studied by L. Sacconi and others in the early 1 9 8 0 s . ' As a ligand, P can adopt various geo­ m e t r i e s / ' - ^ including the P tetrahedron, planar cyclo-P (both square and trapezoidal), and a planar zig-zag chain. In principle, the tetrahedral cluster P could ligate in η \ η and η modes, 3

4

3

5

3

5

5

(51)

2

3

3

( 3 M 2

4 3 )

4

42

4

4

4

2

4

L . WEBER, U. SONNENBERG, H . - G . STAMMLER and B . N E U M A N N , Z. anorg. allg. Chem. 605, 8 7 - 9 9 ( 1 9 9 1 ) .

3

3

5 0

5 1

A . V I Z I - O R O S Z J. Organomet.

Chem.

Ill, 6 1 - 4 (1976).

Ch. 12

Phosphorus

488

Figure 12.9

Schematic representation of various coordination modes: (a) *7 -P2)2 (see text).

(b) ?7 -Ρ ; (c) ^-cyclo-P^

η -?4\

2

Ι

(d) (μ,

4

2

though only the first two have so far been established (Fig. 12.9 (a), (b)). [Note, however, the face-coordinated η configuration in the B i complex [ ( C O ) F e ( ^ 4 ^ - B i 4 ) { F e ( C O ) } 3 ] ~ . ] The first example of what turned out to be a complex involving the η mode was the unstable red-brown compound [ { F e ( C O ) } 3 ^ 3 - P ) ] which was made in 1977 by reacting P with Fe2(CO)η in benzene at room temperature: one vertex of the P tetrahedron was coordinated η to one of the {Fe(CO) } groups while opposite edges of the P cluster were bonded η to the other two {Fe(CO) } groups. The first Ξ7 -P complex to be characterized by X-ray structural analysis was [(?7 -np3)Ni(?7 -P )], formed by direct reaction of white P with the Ni° complex [Ni(j7 -np )] in thf at 0°C where np3 is N(CH2CH2PPh2)3. Coordination results in a slight elongation of the tetrahedron with 3

4

3

3

2

(52)

1

4

4

4

(53)

4

1

4

4

2

4

!

4

3

1

4

(54)

4

4

3

Pbasal-Papical 220 pm and Pbasal-Pbasal 209 pm (cf. 221pm in a - P . The rη -P mode of coordination is featured in many complexes with Rh, Ir, etc., for example [RhCl(rη P )(PPh3) ], formed by direct reaction of P with [RhCl(PPh ) ] in C H C 1 at - 7 8 ° C . The coordinated edge is almost perpendicular to the {RhClL } plane and is lengthened by 4

2

4

2

4

2

(55)

4

3

3

2

2

4

about 25 pm to 246.2 pm, whereas the other P - P distances are essentially unchanged from those in uncoordinated P . Square planar cyclo-?^ features as a ligand and the in [Nb(y/ -C5H3Bu -l,3)(CO)2(r? -P )] The compounds corresponding Ta a n a l o g u e . are formed by uv photolysis of P with [M(cp*)(CO) ] and the square-pyramidal nido structure of the M P cluster (Fig. 12.9c) is con­ sistent with its 14e (2n + 4) cluster-electron count (p. 161). The P - P distances in the copla­ nar P ligand are in the range 2 1 4 - 2 1 8 pm for the Nb complex, with N b - P ( c e n t r e ) being 142 pm and the basal PPP angles being 92.6° and 88.4°. In the Ta complex, the P - P dis­ tances are 2 1 5 - 2 1 7 pm. A co-product of the pho­ tolysis reaction is the related bis-(P2) complex [ { T a ( C H B ^ ) ( C O ) ( ^ - P ) } 2 ] , Fig. 12.9d, in which the P - P distance is 212 pm within each P ligand and 357 pm between the coplanar P ligands. Several similar binuclear bis-(P2) com­ plexes are known, including Rh/Rh, and mixed metal species involving Nb/Ta and T a / C o . A still more open configuration occurs in the zig-zag P chain shown in Fig. 12.10(a). This was found in the dianion of the deep 4

5

( 5 6 )

r2

4

4

(57)

(58)

4

4

4

4

4

5

3

2

2

2

2

(58)

4

(59)

2

5

5

2

K . H . WHITMIRE,

M . R . CHURCHILL

and

T . A . ALBRIGHT, J . C . FETTINGER,

S . K . KANG, lnorg.

Chem.

25,

2 7 9 9 - 8 0 5 (1986). G . SCHMID and H . P . KEMPNY, Z. anorg. allg. Chem. 432, 1 6 0 - 6 (1977). 5 3

5

4

P . DAPPORTO,

S . MIDOLLINI

and

L . SACCONI,

Angew.

Chem. Int. Edn. Engl. 18, 469 (1979). 5

5

Am.

W . E . LINDSELL, K . J . M C C U L L O U G H and A . J . W E L C H , J.

Chem. Soc. 105, 4 4 8 7 - 9 (1983).

6

A . P . GINSBERG,

C . R . SPRINKLE

W . E . LINDSELL,

and A . J . W E L C H ,

K . J. MCCULLOUGH,

J. Am.

Chem.

Soc.

108,

4 0 3 - 1 6 (1986). 5

7

O . J . SCHERER,

J. VONDUNG

and

G . WOLMERSHUSER,

Angew. Chem. Int. Edn. Engl. 28, 1 3 5 5 - 7 (1989). 5

8

O . J . SCHERER,

R . WINTER

and

G . WOLMERSHUSER,

Ζ.

anorg. allg. Chem. 619, 8 2 7 - 3 5 (1993). 5

9

G . FRITZ,

E . LAYHER,

H . KRAUTSCHEID,

B . MAYER,

E . M A T E R N , W . H Τ N L E and H . G . VON SCHNERING, Z . anorg.

allg. Chem. 611, 5 6 - 6 0 (1992).

§12.3.1

Phosphides

489

Figure 12.10 (a) Zig-zag P chain, M = {Cr(CO) }; ( b ) ^-cyclo-P , text). 5

4

red crystalline compound [Li(dme)3] 2[(SiMe3){Cr(CO) } P-P=P-P{Cr(CO) } (SiMe )] which was obtained by reacting Li[P(SiMe3)2{Cr(CO) }] with B r C H C H B r in 1,2-dimethoxyethane (dme). The interatomic distances P - P 221.9pm and P = P 202.5pm reflect the bond orders indicated. Because cyclo-Ps and cyclo-Ρβ can be con­ sidered as isoelectronic with C5H5 and C^He their appearance as ligands is not entirely unexpected, but the recent synthesis and char­ acterization of such complexes was never­ theless a noteworthy achievement. Typical examples are [ ( M n ( C O ) ( r η - P ) ] (formed by the direct action of K P on [Mn(CO) Br] in dmf at 155°C) and [Fe(rη -C H )(/x:rη ,rη P )Fe(rη -C Me R)] (Fig. 12.10(b)) for cyclo+

5

2

5

5

2

2

3

5

5

(60)

5

5

5

5

4

5

5

5

5

(43)

Ps; and [{Mo(^-C5Me )} (M:^,^-P6)] (Fig. 5

(43)

2

12.10(c)) for planar cyclo-P^. Several cyclo-As$ and -As6 analogues are also known. The complex [{Ti(y/ -C Me )} (M:^ ,^ -P )] features a puck­ ered Ρό ring in the chair conformation, so that the overall cluster core has a distorted cubane geometry/ The most complex P ligand so far character­ ized is the astonishing hexadentate P\ unit 5

5

5

2

12.3.1

3

3

6

0

in [{Cr(rη -C H )(CO) }5Pio] (see ref. 62 for 2

5

various (see

Phosphides {63

65)

Phosphorus forms stable binary compounds with almost every element in the periodic table and those with metals are called phosphides. Like borides (p. 145) they are known in a bewilderingly large number of stoichiometries, and typical formulae are M P , M P , M i P , 4

3

2

5

M7P3, M P , M P , M5P3, M3P2, M P , M P , 2

7

4

4

3

5

M P , M P , M P , M P , M P , M3P7, 6

5

3

4

2

3

2

4

M P , 2

5

M P , M3P11, M P , M P , M3P16, M P , M P , 3

3

i 4

5

4

2 6

7

M Pi6 and M P . Many metals (e.g. Ti, Ta, W, 2

i 5

Rh) form as many as 5 or 6 phosphides and Ni has at least 8 (Ni P, N i P , N i i P , N i P , N i P , NiP, N i P and N1P3). Ternary and more complex metal phosphides are also known. The most general preparative route to phos­ phides (Faraday's method) is to heat the metal with the appropriate amount of red Ρ at high temperature in an inert atmosphere or an evac­ uated sealed tube: 3

5

2

2

5

2

5

4

2

heat

n M + mP

n

5

6

12.3 Compounds

61 )

5

M

^-cyclo-P ,

2

(43)

5

various; (c)

2

3

5

M

5

>

MP n

m

An alternative route (Andrieux's method) is the electrolysis of fused salts such as molten

details). A. WILSON, The metal phosphides, Chap. 3 (pp. 2 8 9 - 3 6 3 ) in ref. 23, see also p. 256. A. D . F . T O Y , in Comprehensive Inorganic Chemistry, Vol. 2, Pergamon Press, Oxford, 1973 (Section 20.2, Phosphides, pp. 4 0 6 - 14). 6 3

6 0

M . BAUDLER and T . E T Z B A C H , Angew.

Chem.

Int.

Edn.

Engl. 30, 5 8 0 - 2 (1991). 6 1

O . J . SCHERER,

H . SWAROWSKY,

W . K A I M and S . K O H L M A N N , Angew.

6 4

G . WOLMERSHUSER, Chem.

Int. Edn.

Engl.

26, 1 1 5 3 - 5 (1987). 6 2

L . Y . G O H , R . C . S . W O N G and E . S I N N ,

12, 8 8 8 - 9 4 (1993).

D. E. C . CORBRIDGE, Phosphorus (3rd edn.), Elsevier, Amsterdam, 1985, Section 2.2 Metallic Phosphides, pp. 5 6 - 6 9 . (See also 5th edn. 1995.) 6 5

Organometallics

490

Phosphorus

alkali-metal phosphates to which appropriate metal oxides or halides have been added: electrol

{ ( N a P 0 ) / N a C l / W 0 } fused 3

rt

• W P

3

3

Variation in current, voltage and electrolyte composition frequently results in the formation of phosphides of different stoichiometries. Lessgeneral routes (which are nevertheless extremely valuable in specific instances) include: (a) Reaction of P H with a metal, metal halide or sulfide, e.g.: 3

800°

P H + 2Ti

• Ti P

3

2

Ho 2

2PH + 3Ni(0 CMe) 3

2

• Ni P

2

3

2

further

+ 6HOAc

• Ni P 5

2

reaction

(b) Reduction of a phosphate such as apatite with C at high temperature, e.g.: 1200°

C a ( P 0 ) + 8C 3

4

> C a P + 8CO

2

3

2

(c) Reaction of a metal phosphide with further metal or phosphorus to give a product of different stoichiometry, e.g.: 900°

4ThP

T h P + Th 3

4

650°

4RuP + P (g)

4RuP

4

2

Ch. 12

thermal stability and general chemical inertness. Phosphorus is often in trigonal prismatic coordination being surrounded by 6 M, or by 7, 8 or 9 M (see Fig. 6.7 on p. 148 and Fig. 12.6). The antifluorite structure of many M P also features eightfold (cubic) coordination of Ρ by M. The details of the particular structure adopted in each case are influenced predominantly by size effects. (b) Monophosphides adopt a variety of structures which appear to be influenced by both size and electronic effects. Thus the Group 3 phosphides MP adopt the zinc-blende structure (p. 1210) with tetrahedral coordination of P, whereas SnP has the NaCl-type structure (p. 242) with octahedral coordination of P, VP has the hexagonal NiAs-type structure (p. 556) with trigonal prismatic coordination of isolated Ρ atoms by V, and MoP has the hexagonal WC-type structure (p. 299) in which both Mo and Ρ have a trigonal prismatic coordination by atoms of the other kind. More complicated arrangements are also encountered, e . g . : 2

(65)

TiP, ZrP, HfP: half the Ρ trigonal prismatic and half octahedral; M P (M = Cr, Mn, Fe, Co, Ru, W): distorted trigonal prismatic coordination of Ρ by M plus two rather short contacts to Ρ atoms in adjacent trigonal prisms, thus building up a continuous chain of Ρ atoms; NiP is a distor­ tion of this in which the Ρ atoms are grouped in pairs rather than in chains (or isolated as in VP).

1150°

4IrP 2 ~

^2Ir P+lip (g)

(low press)

2

4

Δ

Phosphides resemble in many ways the metal borides (p. 145), carbides (p. 297), and nitrides (p. 417), and there are the same difficulties in classification and description of bonding. Perhaps the least-contentious procedure is to classify according to stoichiometry, i.e. (a) metalrich phosphides (M/P > 1), (b) monophosphides (M/P = 1), and (c) phosphorus-rich phosphides (M/P < 1): (a) Metal-rich phosphides are usually hard, brittle, refractory materials with metallic lustre, high thermal and electrical conductivity, great

(c) Phosphorus-rich phosphides are typified by lower mps and much lower thermal stabilities when compared with monophosphides or metalrich phosphides. They are often semiconductors rather than metallic conductors and feature increasing catenation of the Ρ atoms (cf. boron rich borides, p. 148). P units occur in F e P , R u P and O s P (marcasite-type, p. 680) and in P t P (pyrites type, p. 680) with P - P 217 pm. Planar P rings (square or rectangular) occur in several M P (M = Co, Ni, Rh, Pd, Ir) with P - P typically 223 pm in the square ring of R h P . Structures are also known in which the Ρ atoms form chains (PdP , N i P , C d P , BaP ), 2

2

2

2

2

4

3

3

2

2

2

3

491

Phosphides

§12.3.1

double chains (ZnPbPi , C d P b P , H g P b P ) , or layers (CuP , A g P , C d P ) ; in the last 3 phosphides the layers are made up by a regular fusion of puckered 10-membered rings of Ρ atoms with the metal atoms in the interstices. The double-chained structure of M P b P is closely related to that of violet phosphorus (p. 482). In addition, phosphides of the electropositive elements in Groups 1, 2 and the lanthanoids form phosphides with some degree of ionic bonding. The compounds Na3Pn and Sr3P]4 have already been mentioned (p. 484) and other somewhat ionic phosphides are M3P (M = Li, Na), M P (M = Be, Mg, Zn, Cd), M P (M = La, Ce) and T h 3 P . However, it would be misleading to consider these as fully ionized compounds of P and there is extensive metallic or covalent interaction in the solids. Such compounds are characterized by their ready hydrolysis by water or dilute acid to give PH3. Recent extensive structural studies by Xray crystallography and by Ρ nmr spec­ troscopy have revealed an astonishing vari­ ety of conjuncto-po\y phosphides with quasi-ionic 4

2

2

H

14

4

ξ4

3

4

2

Thus, the yellow com­ cluster s t r u c t u r e s / pound L13P7 (which has been known since 1912) and its N a - C s analogues have been found to contain the P ~ cluster shown schematically in Fig. 12.11(a). The cluster can be regarded as being related to the P tetrahedron (p. 479) by the notional insertion of three 2-connected P~~ atoms (cf. the structure of P S 3 , p. 507, with which it is precisely isoelectronic). Substitution of Ρ by As leads to a series of closely related anions [ P ^ A s J " χ = 1 - 5 , ( ? 6 ) , and A s " is also known for Na, Rb, Cs). Catenation of the P ~ unit, as shown in Fig. 12.11(b), leads to the stoichiometry M P ~ . The repeating unit = P = , which is clearly related to a segment in the structure of Hittorf s allotrope (p. 482), is shown in Fig. 12.11(c). A more complex 66,67}

7 3

4

4

3

(68)

7 3

7 3

+

7

8

3 _

3 1

6

6

H . G . VON SCHNERING,

in

A. H . C O W L E Y

(ed.)

Rings,

Clusters and Polymers of the Main Group Elements, ACS Symposium Series N o . 232, Washington D . C. 1 9 8 3 , pp. 6 7

69-80.

M . BAUDLER, Angew. Chem. Int. Edn. Engl. 21, 4 9 2 - 5 1 2

( 1 9 8 2 ) ; 26, 4 1 9 - 4 1 6 8

(1987).

W. HONLE and H . G . VON SCHNERING, Angew. Chem. Int.

Edn.

Engl.

25, 3 5 2 - 3

(1986).

492

cluster occurs in the yellow/orange compounds M P - (Fig. 12.1 Id): P u can be thought of as comprising two axial P P tetrahedra joined by a central belt of three 2^connected P~ atoms, so that the sequence of cluster planes contains 1,3,(3),3,1 Ρ atoms, respectively. Even more complex conjuncto-polyphosphiae anions can be constructed, such as those of stoichiometry P i 6 ~ \ P 2 i ~ and P 6 , Fig. 12.12(a)(b)(c). ' These bear an obvious structural relationship t o = P g = (Fig. 12.11c) and to Hittorf s phosphorus (Fig. 12.4) and can be viewed as ladders of Ρ atoms with alternate P - P and P(P~)P rungs, terminated at each end by a ring-closing P(P~) unit. The P - P distances and PPP angles in these various species are much as expected. These cluster anions, and those mentioned in the preceding paragraphs, can be partially or completely protonated (see next subsection) and they also occur in neutral organopolyphosphanes (p. 495). A completely different structural motif has very recently been found in the red-brown phosphide CasPs, formed by direct fusion of Ca metal and red Ρ in the correct atom ratio in a corundum crucible at 1000°C. The structure comprises C a cations and P s ~ anions, the latter adopting a staggered ethane conformation. (Note that P is isolobal with C and P " with Η so that C H = [ ( P ) ( P - ) ] = P " . ) The internal P - P distance is 230.1pm and the terminal P - P distances 214.9-216.9pm, while the internal PPP angles are 104.2-106.4° and the outer angles are 103.4-103.7°. Few industrial uses have so far been found for phosphides. "Ferrophosphorus" is produced on a large scale as a byproduct of P4 manufacture, and its uses have been noted (p. 480). Phosphorus is also much used as an alloying element in iron and steel, and for improving the workability of Cu. Group 3 monophosphides are valuable semi­ conductors (p. 255) and C a P is an important ingredient in some navy sea-flares since its reac­ tion with water releases spontaneously flammable +

3

Ch. 12

Phosphorus

3

3 -

n

3

2

3

4 _

2

(66

12.3.2 Phosphine and related compounds

67)

(69)

2 +

1 0

+

2

+

2

phosphines. By contrast the phosphides of Nb, Ta and W are valued for their chemical inertness, particularly their resistance to oxidation at very high temperatures, though they are susceptible to attack by oxidizing acids or peroxides.

6

2

2

3

1 0

6

8

2

The most stable hydride of Ρ is phosphine (phosphane), P H . It is the first of a homologous open-chain series P„H„+ (n = 1-9) the mem­ bers of which rapidly diminish in thermal sta­ bility, though P2H4 and P3H5 have been iso­ lated pure. There are ten other (unstable) homol­ ogous series: P„H„ ( n = 3 - 1 0 ) , P H _ (n = 4 - 1 2 ) , and P„H _ (n = 5 - 1 3 ) and so on up to P„H„__ (n = 1 9 - 2 2 ) ; in all of these there is a tendency to form cyclic and condensed polyphosphanes at the expense of open-chain structures. Some 85 phosphanes have so far been identified and structurally characterized by nmr spectroscopy and other techniques, although few have been obtained pure because of prob­ lems involving thermal instability, ready dispro­ portionation, light-sensitivity and great chemi­ cal r e a c t i v i t y . Phosphorane, PH5, has not been prepared or even detected, despite numer­ ous attempts, although H P F , H P F and H P F have recently been well c h a r a c t e r i z e d / 3

2

W

A7

( 6 7 )

(67,70,7I)

4

C. HADENFELDT and F. BARTELS, Z. anorg. allg. 1 2 4 7 - 5 2 (1994).

Chem.

2

3

3

2

72,73}

PH3 is an extremely poisonous, highly reactive, colourless gas which has a faint garlic odour at concentrations above about 2 ppm by volume. It is intermediate in thermal stability between NH3 (p. 421) and A s H (p. 557). Several convenient routes are available for its preparation: 3

1. Hydrolysis of a metal phosphide such as A1P or Ca3P2; the method is useful even 7 0

M . BAUDLER and K . G L I N K A , Chem.

Rev.

93, 1623-67

Rev.

94, 1273-97

(1993). 7 1

M . BAUDLER and K . G L I N K A , Chem.

( 1 9 9 4 ) . See also Z. anorg. allg. Chem. 7 2

6 9

2

18

A . J. DOWNS

G . S. M C G R A D Y ,

D . W . H . R A N K I N , J. 620,

W

4

Chem.

Soc,

621, 1 1 3 3 - 9 (1995). E. A . BARNFIELD

Dalton

Trans.,

and

545-50

(1989). 7 3

A . BECHERS, Z. anorg. allg. Chem.

6 1 9 , 1 8 6 9 - 7 9 (1993).

§12.3.2

Phosphine

and related

up to the 10-mole scale and can be made almost quantitative Ca P + 6H 0 3

2

• 2 P H 4- 3Ca(OH)

2

3

493

compounds

organic liquids, and particularly so in C S C C 1 C 0 H . Some typical values are: 3

2. Pyrolysis of phosphorous acid at 2 0 5 - 2 1 0 ° ; under these conditions the yield of P H is 97% though at higher temperatures the reac­ tion can be more complex (p. 512) 3

and

2

Solvent ( r C )

2

2

H 0 (17°) 26

CH C0 H (20°) 319

CS (21°) 1025

CC1 C0 H 1590

2

Solubility/ml PH (g) per 100 ml solvent 3

Solvent ( r C ) Solubility/ml PH (g) per 100 ml solvent

3

2

3

CH (22°) 726

2

3

6

6

2

200°

4H P0 3

> PH + 3 H P 0

3

3

3

[ N o t e : l m l P H ( g ) ^ 1.5 mg] 3

4

3. Alkaline hydrolysis of P H I (for very pure phosphine): 4

P + 2I + 8 H 0 4

2

3

+

PH + H 0

• 2 P H I + 2HI

2

Aqueous solutions are neutral and there is little tendency for P H to protonate or deprotonate: 3

PH - + H 0 ;

2

2

3

4

+ 2H P0 3

P H I + KOH(aq)

K PH + H 0

> P H + KI + H 0

4

3

3

PH

2

+ 4

+ OH"; K =4x

2

3

10~

B

28

4

E t 0 / < 0°

PC1 + LiAlH

2 9

2

4. Reduction of PC1 with LiAlH or LiH: 3

= 1.6 χ 1 0 "

A

4

+

• PH + ...

4

warm

2

2

> P H + 3LΞC1

3

-

4

3

PC1 + 3LiH

In liquid ammonia, however, phosphine dissolves to give N H P H and with potassium gives K P H in the same solvent. Again, phosphine reacts with liquid HCl to give the sparingly soluble P H C 1 and this reacts further with BC1 to give P H B C 1 . .The corresponding bromides and P H I are also known. More generally, phosphine readily acts as a ligand to numerous Lewis acids and typical coor­ dination complexes are [ B H ( P H ) ] , [ B F ( P H ) ] , [A1C1 (PH )], [Cr(CO) (PH ) ], [Cr(CO) ( P H ) ] , [Co(CO) (NO)(PH )], [ N i ( P F ) ( P H ) ] and [CuCl(PH )]. Further details are in the Panel and other aspects of the chemistry of P H have been extensively r e v i e w e d . Phosphine is also a strong reducing agent: many metal salts are reduced to the metal and PCI5 yields PC1 . The pure gas ignites in air at about 150° but when contaminated with traces of P H it is spontaneously flammable:

3

+

5. Alkaline hydrolysis of white P process):

4

(industrial

-

4

3

4

4

4

P H- 3KOH + 3 H 0 4

• PH + 3KH P0

2

3

2

2

Phosphine has a pyramidal structure, as expected, with P - H 142 pm and the H - P - H angle 93.6° (see p. 557). Other physical properties are mp —133.5°, bp —87.7°, dipole moment 0.58 D, heat of formation AH° —9.6 kJ m o l (uncertain) and mean P - H bond energy 320 kJ m o l . The free energy change (at 25°C) for the reaction ±P (a-white) + §H (g) = PH (g) is —13.1 kJ m o l , implying a tendency for the elements to combine, though there is negligible reaction unless H is energized photolytically or by a high-current arc. The inversion frequency of P H is about 4000 times less than for N H (p. 423); this reflects the substantially higher energy barrier to inversion for P H which is calculated to be M 5 5 kJ m o l rather than 24.7 kJ m o l for N H . {

- 1

- 1

4

2

- 1

3

2

3

3

3

3

2

3

2

3

3

3

3

2

3

4

3

3

3

2

3

3

(74)

3

2

4

PH + 2 0 3

2

> H P0 3

4

3

3

- 1

3

When heated with sulfur, P H yields H S and a mixture of phosphorus sulfides. Probably the most important reaction industrially is 3

2

- 1

3

Phosphine is rather insoluble in water at atmospheric pressure but is more soluble in

7 4

E . FLUCK, Chemistry of phosphine, Topics in Current Chem. 35, 1-64 (1973). A review with 493 references.

494

Phosphorus

Chapter 5 in ref. 2 , Phosphorus(III) ligands in transitionmetal complexes, pp. 1 7 7 - 2 0 7 . 7 5

C. A. M C A U L I F F E and W . LEVASON, Phosphine, Arsine and Stibine Complexes of the Transition Elements, Elsevier, A m s ­ terdam, 1 9 7 9 , 5 4 6 pp. A review with over 2 7 0 0 references. See also C. A. M C A U L I F F E (ed.), Transition-Metal Complexes of Phosphorus, Arsenic and Antimony Donor Ligands, Macmillan, London, 1 9 7 2 . 7 6

Ch. 12

O . STELZER, Topics in Phosphorus Chemistry 9, 1 - 2 2 9 ( 1 9 7 7 ) . An extensive review with over 1 7 0 0 references arranged by element and by technique but with no assessment or generalizations. 7 7

7 8

17,

R. M A S O N and D. W. M E E K , Angew. Chem. Int. Edn. Engl. 1 8 3 - 9 4 (1978).

G. PARSHALL, Homogeneous catalytic activation of C - H bonds, Acc. Chem. Res. 8, 1 1 3 - 7 ( 1 9 7 5 ) . 7 9

§12.3.3

Phosphorus

its hydrophosphorylation of formaldehyde aqueous hydrochloric acid solution:

in

halides

495

with HX, though other routes are also available. Thus, treatment of P yields P H ~ , P H " and P H by sucessive protonation of the three 2-connected P~ sites. The alkyl derivatives are more stable than the parent polycyclic phosphanes and provide many examples of the elegant solu­ tion of complex conformational problems by the use of nmr s p e c t r o s c o p y . 3 _

7

PH

3

+ 4HCHO + HCl

• [P(CH 0H) ]C1 2

4

The tetrakis(hydroxymethyl)phosphonium chlo­ ride so formed is the major ingredient with ureaformaldehyde or melamine-formaldehyde resins for the permanent flame-proofing of cotton cloth. Of the many other hydrides of phosphorus, diphosphane (diphosphine), P H , is the most studied. It is best m a d e by treating CaP with cold oxygen-free water. Passage of P H through an electric discharge at 5 - 1 0 kV is an alternative method for small amounts. P H is a colourless, volatile liquid (mp —99°) which is thermally unstable even below room temperature and is decomposed slowly by water. Its vapour pressure at 0°C is 70.2 mmHg but partial decomposition precludes precise determination of the bp (63.5° extrap); d ~ 1.014 g c m " at 20°C. Electrondiffraction measurements on the gas establish the gauche-Ci configuration (p. 428) with P - P 222 pm, P - H 145 pm, and the angle H - P - H 91.3°, though vibration spectroscopy suggests a trans-C2h configuration in the solid phase. These results can be compared with those for the halides P X on p. 498. 2

2

7

4

7

7

2

3

(67,70)

12.3.3 Phosphorus halides

(71)

3

2

4

3

2

4

The next member of the open-chain series P„H„+ is P H , i.e. P H P H P H , a colourless liq­ uid that can be stored in the dark at —80° for several d a y s . It can be made by dispropor­ tionation ( 2 P H > P H + P H ) but it is dif­ ficult to purify because of its own fairly ready dis­ proportionation and reactivity, e.g. 2 P H > P H + P H ; and P H + P H >P H + PH . Tetraphosphane(6), P H , exists as an equi­ librium mixture of the two structural isomers H P P H P H P H (n) and P ( P H ) ( / ) , and itself reacts with P H at —20° according to the ide­ alized stoichiometry P H + P H > 2PH + P5H5, i.e. cyclo-(PH)$. All members of the series cyclo-P H (n = 3 - 1 0 ) have been detected mass spectrometrically in the thermolysis products from P H . Polycyclic polyphosphanes are often best pre­ pared by direct protonation of the correspond­ ing polyphosphide anions (Figs. 12.11 and 12.12) 2

3

5

2

2

( 6 7 , 7 1 )

2

4

3

5

3

3

4

6

2

4

3

5

3

2

4

2

2

n

n

( 7 0 )

2

4

6

4

( 8 0 )

2

3

3

2

3

3

4

7

2

3

7

5

4

6

3

5

4

2

3

Phosphorus

trihalides

6

2

3

4

Phosphorus forms three series of halides P X , P X and PX5. All 12 compounds may exist, although there is considerable doubt about P l 5 . Numerous mixed halides P X Y and P X Y are also known as well as various pseudohalides such as P(CN) , P(CNO) , P(CNS) and their mixed halogeno-counterparts. The compounds form an extremely useful extended series with which to follow the effect of progressive substitution on various properties, and the pentahalides are particularly significant in spanning the "ioniccovalent" border, so that they exist in various structural forms depending on the nature of the halogen, the phase of aggregation, or the polarity of the solvent. Some subhalides such as P X and P X , and some curious polyhalides such as PBr and PBrn have also been characterized. Physical properties of the binary halides are summarized in Table 12.3 (on the next page). Ternary (mixed) halides tend to have properties intermediate between those of the parent binary halides.

3

5

3

All 4 trihalides are volatile reactive compounds which feature pyramidal molecules. The fluoride is best made by the action of C a F , Z n F or A s F on PC1 , but the others are formed by direct halogenation of the element. P F is colourless, odourless and does not fume in air, but is very hazardous due to the formation of a complex with blood haemoglobin (cf. 2

3

2

3

3

8 0

1.

TORNIEPORTH-OETTING and T. KLAPΤTKE, J. Chem.

Chem. Commun.,

1 3 2 - 3 (1990).

Soc.,

496

Phosphorus

Table 12.3 Compound

Ch. 12

Some physical properties of the binary phosphorus halides MP/°C

Physical State at 25°C

BP/°C

P-X/pm

Angle X - P - X

-151.5 -93.6 -41.5 61.2

-101.8 76.1 173.2 decomp > 200

Colourless gas

-86.5

-6.2

159 (P-P 228)

99.1° ( F - P - P 95.4°)

P2CI4

Colourless oily liquid

-28

- 1 8 0 (d)

P Br

?

— —

— —

248 (P-P 221)

102.3° ( I - P - P 94.0°)

PF PC1 PBr

Colourless gas Colourless liquid Colourless liquid Red hexagonal crystals

3

3 3

PI3 P2F4

2

4

P2I4

Red triclinic needles

PF

Colourless gas

5

Off-white tetragonal crystals Reddish-yellow rhombohedral crystals Brown-black crystals

PCI5 PBr

5

PI ? 5

—

—

125.5

decomp

-93.7

-84.5

167 <100 (d)

(8l)

(82)

4

3

4

3

4

2

M

2

2

3

4

6

3

3

3

P F , unlike the other trihalides of phosphorus, hydrolyses only slowly with water, the products being phosphorous acid and HF: P F -f 3 H 0 - > H P 0 + 3HF. The reaction is much more rapid in alkaline solutions, and in dilute aqueous K H C 0 solutions the intermediate monofluorophosphorous acid is formed: 3

3

3

See text However, see ref. 80

—

2

3

106 (d)

41

3

2

3

3

96.3° 100° 101° 102°

153 (eq) 120° (eq-eq) 158 (ax) 90° (eq-ax) See text

160 (subi)

CO, p. 1101). It is about as toxic as C O C l . The similarity of P F and CO as ligands was first noted by J. C h a t t and many complexes with transition elements are now k n o w n , e.g. [Ni(CO)„(PF ) _„] (n = 0 - 4 ) , [ P d ( P F ) ] , [Pt(PF ) ], [CoH(PF ) ], [ C o ( - P F ) ( P F ) ] , etc. Such complexes can be prepared by ligand replacement reactions, by fluorination of PC1 complexes, by direct reaction of P F with metal salts or even by direct reaction of P F with metals at elevated temperatures and pressures. 3

156 204 222 243

PC1 is the most important compound of the group and is made industrially on a large scale^ by direct chlorination of phosphorus suspended in a prιcharge of PC1 — the reaction is carried out under reflux with continuous take-off of the PC1 formed. PC1 undergoes many substitution reac­ tions, as shown in the diagram, and is the main source of organophosphorus compounds. Partic­ ularly notable are P R , PR„C1 _„, PR„(OR) _„, (PhO) PO, and (RO) PS. Many of these com­ pounds are made on the 1000-tonne scale pa, and the major uses are as oil additives, plasticizers, flame retardants, fuel additives and intermediates in the manufacture of insecticides. PC1 is also readily oxidized to the important phosphorus(V) derivatives PC1 , POCl and PSC1 . It is oxi­ dized by A s 0 to P 0 5 though this is not the commercial route to this compound (p. 505). It fumes in moist air and is more readily hydrolysed (and oxidized) by water than is P F . With cold N 0 (—10°) it undergoes a curious oxida­ tive coupling reaction to give C l P = N - P O C l , 3

3

3

3

3

3

3

3

3

(83)

3

5

2

3

3

3

2

3

2 % aq K H C 0

PF + 2 H 0 3

3

• 0 = P H ( O H ) F + 2HF

2

2

4

3

8 1

J. CHATT,

Nature

165,

637-8

(1950);

J. CHATT

and

A. A. W I L L I A M S , 7. Chem. Soc. 3 0 6 1 - 7 ( 1 9 5 1 ) . 8 2

T . KRVCK,

Angew. Chem. Int. Edn. Engl. 6, 5 3 - 6 7 ( 1 9 6 7 ) ;

J. F . NIXON, Adv. lnorg.

Chem.

Radiochem.

13, 3 6 3 - 4 6 9

( 1 9 7 0 ) ; R . J. CLARKE and M. A. B U S C H , ACC. Chem. Res. 6, 246-52

(1973).

2

' World production exceeds one third of a million tonnes pa; of this U S A produces M 55 0 0 0 tonnes, Western Europe M 1 5 0 0 0 and Japan ~ 3 5 0 0 0 tonnes pa. 8 3

D . H . CHADWICK

pp. 1 2 2 1 - 7 9 .

and R . S. W A T T , Chap. 19 in ref. 11,

Phosphorus

§12.3.3

mp 35.5°; (note the presence of two different 4coordinate P a t o m s ) . Other notable reactions of PCI3 are its extensive use to convert alcohols to RC1 and carboxylic acids to R C O C 1 , its reduction to P2I4 by iodine, and its ability to form coordi­ nation complexes with Lewis acids such as BX3 and Ni°. PI3 is emerging as a powerful and versatile For example solutions deoxygenating a g e n t . of PI3 in CH2CI2 at or below room temperature v

(84)

497

halides

convert sulfoxides (RR'SO) into diorganosulfides, selenoxides (RR'SeO) into selenides, alde­ hyde oximes ( R C H = N O H ) into nitriles, and pri­ mary nitroalkanes (RCH2NO2) into nitriles, all in high yield ( 7 5 - 9 5 % ) . The formation of nitriles, RCN, in the last two reactions requires the pres­ ence of triethylamine in addition to the PI3.

(85)

8 4

M . BECKE-GOEHRING,

A . DEBO,

W . G O E T Z E , Chem. Ber. 94, 1 3 8 3 - 7 8 5

J . N . D E N I S and A . KRIEF, J. Chem. Soc,

5 4 4 - 5 (1980).

E . FLUCK

and

(1961). Chem.

Commun.,

Diphosphorus halides of

tetrahalides phosphorus

and other

lower

The physical properties of P2X4, in so far as they are known, are summarized in Table 12.3. P2F4 was first made in other than trace amounts in

Ch. 12

Phosphorus

498

1966, using the very effective method of coup­ ling two PF2 groups at room temperature under reduced pressure: 8 6 % yield

2PF I + 2Hg

• P F + Hg I

2

2

4

2

2

been detected in CS2 solutions by Ρ nmr spectroscopy. It has also been found that reactions CS2 solution between P4 and half a mole-equivalent of B r 2 yielded not only P 2 B r 4 but also small amounts of the new "butterfly" molecules exo,exo-P4Bv2 and exo,endo-? Q The structure of these can be viewed as being formed by the scission of one P - P bond in the P tetrahedron by B r (cf. the structure of B4H10, p. 154) which is also a 22 valenceelectron species). The molecules P4BrCl and P4C1 were also identified, following chlorination of the bromide solution using Me3SnCl. Other products of the initial reactions included Ρ γ Β ^ and P7I3 which are structurally related to P7H3 (p. 495). None of these novel subhalides has been isolated p u r e . 3 1

(87)

4

The compound hydrolyses to F P O P F which can also be prepared directly in good yield by the 2

reaction of 0

2

2

on P F 4 . 2

P2CI4 can be made (in low yield) by passing an electric discharge through a mixture of PCI3 and H2 under reduced pressure or by microwave dis­ charge through PCI3 at l - 5 m m H g pressure. The compound decomposes slowly at room tempera­ ture to PCI3 and an involatile solid, and can be hydrolysed in basic solution to give an equimolar mixture of P H and P ( O H ) . Little is known of P 2 B r 4 , said to be produced by an obscure reaction in the system C H PBr -Al Br . By contrast, P I is the most stable and also the most readily made of the 4 tetrahalides; it is formed by direct reaction of I and red Ρ at 180° or by I and white P4 in C S solution, and can also be made by reducing PI3 with red P, or PCI3 with iodine. Its X-ray crystal structure shows that the molecules of P2I4 adopt the trans-, centrosymmetric (C /,) form (see N H , p. 428, N F , p. 439). Reaction of P I with sulfur in C S yields P l 4 S , which probably has the symmetrical structure 2

4

2

4

2

4

2

2

(87)

4

( 8 6 )

3

2

6

2

2

4

2

2

2

2

4

2

4

2

2

2

4

2

Phosphorus

pentahalides

Considerable theoretical and stereochemical interest attaches to these compounds because of the variety of structures they adopt; PCI5 is also an important chemical intermediate. Thus, P F is molecular and stereochemically non-rigid (see below), PCI5 is molecular in the gas phase, + ionic in the crystalline phase, [PCi4] [PCl6]~, and either molecular or ionically dissociated in solution, depending on the nature of the solvent. PBr5 is also ionic in the solid state but exists as [ P B r ] [ B r ] ~ rather than [ P B r ] [ P B r ] - . The pentaiodide does not e x i s t (except perhaps as PI3.I2, but certainly not as P L ^ I " as originally claimed ). PF5 is a thermally stable, chemically reac­ tive gas which can be made either by fluorinat­ ing PCI5 with A s F (or CaF ), or by thermal decomposition of N a P F , B a ( P F ) or the cor­ responding diazonium salts. Single-crystal X-ray analysis (at — 164°C) indicates a trigonal bipyra­ midal structure with P - F ^ (158.0 pm) being 5

+

+

4

4

6

(80)

(88)

but most reactions of P2I4 result in cleavage of the P - P bond, e.g. B r gives PBrI in 90% yield. Hydrolysis yields various phosphines and oxoacids of P, together with a small amount of hypophosphoric acid, ( H O ) ( 0 ) P P ( 0 ) ( O H ) . Several ternary diphosphorus tetrahalides, P 2 X « Y 4 - „ , (X, Y = C1, Br, I) have recently 2

2

2

2

3

8 7 8 6

R. I. PYRKIN, Y A . A . LEVIN and

Ε . I. GOLDFARB, J.

2

6

Gen.

B . W . TATTERSHALL and

1517-21

20,

J. Gen. Chem. USSR 48, 1 9 5 - 6 ( 1 9 7 8 ) .

8 8

2

N . L . KENDALL, Polyhedron

Chem. USSR 43, 1 6 9 0 - 6 ( 1 9 7 3 ) . See also A . HINKE, W . KUCHEN and J. KUTTER, Angew. Chem. Int. Edn. Engl. 1060 (1981).

6

13,

(1994).

N. G. FESHCHENKO V. G. KOSTINA and

Α. V.

KIRSANOV,

Phosphorus

§12.3.3

Figure 12.13

(89)

ax

eq

1 9

1 9

(90)

l

499

Interchange of axial and equatorial positions by Berry pseudorotation (BPR).

significantly longer than P - F ^ (152.2 p m ) . This confirms the deductions from a gas phase electron-diffraction study (D^: P-¥ 158 pm, P-F 153 pm). However, the F nmr spec­ trum, as recorded down to — 100°C, shows only a single fluorine resonance peak (split into a doublet by P - F coupling) implying that on this longer time scale (milliseconds, as distinct from "instantaneous" for electron diffraction) all 5 F atoms are equivalent. This can be explained if the axial and equatorial F atoms interchange their positions more rapidly than this, a process termed "pseudorotation" by R. S. Berry (1960); indeed, PF5 was the first compound to show this effect. The proposed mechanism is illus­ trated in Fig. 12.13 and is discussed more fully in ref. 91 ; the barrier to notation has been calculated as 1 6 ± 2 k J m o r . 3 1

halides

( 9 2 )

The mixed chlorofluorides PCI4F (mp —59°, bp +67°) and PC1 F (mp - 6 3 ° ) are also trigonal bipyramidal with axial F atoms; likewise PCI2F3 (mp - 1 2 5 ° , bp +7.1°) has 2 axial and 1 equatorial F atoms and PC1F (mp —132°, 3

2

4

D . M O O T Z and M . WIEBCKE, Z. anorg. allg. Chem. 545, 3 9 - 4 2 (1987).

bp —43.4°) has both axial positions occupied by F a t o m s . These compounds are obtained by addition of halogen to the appropriate phosphorus(III) chlorofluoride, but if PCI5 is fluorinated in a polar solvent, ionic isomers are formed, e.g. [ P C 1 ] [ P C 1 F ] ~ (colourless crystals, subi 175°) and [ P C l ] + [ P F r (white crystals, subi 135° with decomposition). The crystalline hemifluoride [ P C 1 ] [ P C 1 F ] - has also been identified. The analogous parallel series of covalent and ionic bromofluorides is less well characterized but P B r F is known both as an unstable molecular liquid (decomp 15°) and as a white crystalline powder [ P B r ] + [ P F r (subi 135° decomp). It can be noted that P F ( N H ) is a trigonal bipyramidal molecule with C symmetry (i.e. equatorial N H groups), whereas the most stable form of tetra-arylfluorophosphoranes is ionic, [ P R ^ + F , although molecular monomers R P F and an ionic dimer [ P R ] [ P R ^ ] " also e x i s t . (93)

4

+

4

2

6

4

4

+

5

2

3

6

4

3

2

2

2 v

2

(94)

-

4

4

+

(95)

PCI5 is even

closer to the ionic-covalent bor­ derline than is P F , the ionic solid [ P C 1 ] [ P C 1 ] melting (or subliming) to give a covalent molecular 5

4

+

6

8 9

R . S. BERRY, / . Chem, Phys. 32, 9 3 3 - 8 (1960). R . LUCKENBACH, Dynamic Stereochemistry of Pentacoand Related Elements, G . THIEME, ordinate Phosphorus Stuttgart, 1973, 259 pp.

9 3

C . MACHO,

R . MINKWITZ,

J. ROHMAN,

B . STEGER,

9 0

W . W O L F E L and H . OBERHAMMER, lnorg. Chem. 25, 2 8 2 8 - 3 5

9 1

( 1 9 8 6 ) , and references cited therein.

C . J . MARSDEN, J. Chem. Soc, Chem. Commun., (1984). 9 2

401-2

9 4

C . J. MARSDEN,

K . D . G U P T A , lnorg. 9 5

S . J. BROWN,

K . HEDBERG, Chem.

J . M . SHREEVE

and

23, 3 6 5 9 - 6 2 ( 1 9 8 4 ) .

J. H . CLARK

and

D . J. MACQUARRIE,

Chem. Soc, Dalton Trans., 2 7 7 - 8 0 ( 1 9 8 8 ) .

J.

500

Phosphorus

liquid (or gas). Again, when dissolved in nonpolar solvents such as CCI4 or benzene, PCI5 is monomeric and molecular, whereas in ionizing solvents such as MeCN, MeN02 and PI1NO2 there are two competing ionizing equilibria:

Ch. 12

cation is known and can readily be formed by reacting PhPCU with a chlorine ion acceptor such as BC1 , SbCl , or even PC1 itself: (100)

3

5

5

PhPCl + PCl 4

(96)

+

• [PhPCl ] [PCl r

5

3

2

+

2PC1

[PC1 ] + [PC1 ]-

5

4

6

3

+

+

PCI5

[PC1 ] + CI­ 4

6

Likewise crystalline P h P C l is molecular whereas the corresponding Me and Et deriva­ tives are ionic [ R 2 P C i 2 ] C l . However, all 3 triorganophosphorus dihalides are ionic [ R P C 1 ] Cr(R = Ph, Me, Et). The pale-yellow, crys­ talline mixed halide P2BrCl9 appears to be [PCl ]Ξ[PCl Br] [PCl ] -[Br] (i.e. P B r Cl ). Phosphorus pentabromide is rather different. The crystalline solid is [ P B r ] B r ~ but this appears to dissociate completely to PBr and B r in the vapour phase; rapid cooling of this vapour to 15 Κ results in the formation of a disordered lattice of PBr and PBr (i.e. [ P B r ] [ B r D and this mixture reverts to [ P B r 4 ] B r ~ on being warmed to 1 8 0 K . The corresponding trichloride, [ P B r ] [ C l ] ~ is also k n o w n . [Pl4] has been identified only as its salt [PI ] [AsF ]-. PCI5 is made on an industrial scale by the reaction of CI2 on PC1 dissolved in an equal vol­ ume of CCI4. World production probably exceeds 20 000 tonnes pa. On the laboratory scale C l gas (or liquid) can be passed directly into PC1 . PCI5 reacts violently with water to give HCl and Η Ρ θ 4 but in equimolar amounts the reaction can be moderated to give P O C l : _

+

3

As might be expected, the former equilibrium predominates at higher concentrations of PCI5 (above about 0.03 mol Γ ) whilst the latter pre­ dominates below this concentration. The P - C l distances (pm) in these various species are: PCI5 214 (axial), 202 (equatorial); [PC1 ] 197; [PC1 ] 208 pm. Ionic isomerism is also known and, in addition to [PC1 ] [PC1 ]", another (metastable) crystalline phase of constitution [ P C l 4 # [ P C l 6 r c r can be formed either by application of high pressure or by crystallizing PCI5 from solutions of dichloromethane contain­ ing B r or S C l 2 . When gaseous PCI5 (in equi­ librium with P C I 3 + C I 2 ) is quenched to 15 Κ the trigonal-bipyramidal molecular structure is retained; this forms an ordered molecular crys­ talline lattice on warming to M30K, but fur­ ther warming towards room temperature results in chloride-ion transfer to give [PC1 ] [PC1 ]". The first alkali metal salt of [PC1 ]~, CsPCl , has only recently been m a d e . The delicate balance between ionic and covalent forms is influenced not only by the state of aggregation (solid, liquid, gas) or the nature of the solvent, but also by the effect of substituents. Thus PhPCl is molecular with Ph equatorial whereas the corresponding methyl derivative is ionic, [ M e P C l ] C r . Despite this the [ P h P C l ] 1

+

4

_

6

+

4

6

(97)

2

+

(98)

4

6

6

6

(99)

4

+

+

3

3

+

4

3

2

6

4

4

12

6

( , 0 1 )

5 4

+

4

3

2

+

3

7

4

3

+

(98)

+

(102)

4

3

+

+

4

( 8 0 )

6

3

2

3

3

3

PCI5 + H 0

> P O C l + 2HC1

2

3

PCI5 chlorinates alcohols to alkyl halides and carboxylic acids to the corresponding RCOC1. When heated with NH C1 the phosphonitrilic chlorides are obtained (p. 536). These and other reactions are summarized in the diagram. 4

(8)

9 6

R . W . SUTER, H . C . KNACHEL, V . P . PETRO, J . H . HOWAT-

SON and S. G . SHORE, J. Am. Chem. Soc. 95, 1 4 7 4 - 9 (1973). 9 7

A. FINCH,

P . N. GATES,

K. P . THAKUR, J. Chem.

H . D. B . JENKINS

Soc, Chem.

Commun.,

and

579-80

1 0 0

Κ . B . DILLON,

R . J. LYNCH,

R . N. R E E V E

T . C . WADDINGTON, J. Chem. Soc, Dalton Trans.,

and

1243-8

(1980). See also H . D. B . JENKINS, L . SHARMAN, A. FINCH

( 1 9 7 6 ) . See also M . A . H . A . A L - J U B O O R I , P . N . GATES and

and P . N. GATES, Polyhedron ences cited therein.

A . S. MUIR, / Chem. Soc, Chem. Commun.,

9 8

13, 1 4 8 1 - 2 (1994) and refer­

A. FINCH, P . N. GATES and A. S. M U I R , J. Chem.

Chem. Commun.,

Soc,

8 1 2 - 4 (1981). See also H . D. B . JENKINS,

K . P . T H A K U R , A. FINCH and P . N . GATES, Inorg.

4 2 3 - 6 (1982). A. S. MUIR, Polyhedron 9 9

1 0 1

Chem. 2 1 ,

F . F . BENTLEY,

A . FINCH,

P . Ν. GATES,

1 2 7 0 - 1 (1991). F . J . RYAN

and

Κ . B . DILLON. J. Inorg. Nucl. Chem. 36, 4 5 7 - 9 ( 1 9 7 4 ) . See also J. Chem. Soc, Dalton Trans., 1 8 6 3 - 6 ( 1 9 7 3 ) . 1 0 2

Κ . B . DILLON,

M . P . NISBET and

R . N . REEVE,

Polyhe­

dron 7, 1 7 2 5 - 6 ( 1 9 8 8 ) . See also H . D . B . JENKINS, Poly­ 10, 2 2 1 7 - 9 (1991).

hedron

15, 2 8 3 1 - 4

(1996).

§12.3.4

Oxohalides

and thiohalides

The chlorination of phosphonic and phosphinic acids and esters are of considerable importance. PCI5 can also act as a Lewis acid to give 6coordinate Ρ complexes, e.g. p y P G s , and pyzPCI5, where py = C5H5N (pyridine) and pyz = cyclo- 1,4-C H N ( p y r a z i n e ) . 4

4

2

(l03)

Pseudohalides of phosphorus(lll) Paralleling the various phosphorus trihalides are numerous pseudohalides and mixed pseudohalidehalides of which the various isocyanates and isothiocyanates are perhaps the best known. Most are volatile liquids, e.g. Compound MP/°C BP/°C

1 0 3

Β . N. MEYER,

P(NCO)

3

PF(NCO)

-2 169.3

J. N. ISHLEY,

-55 98.7

PF (NCO)

of

phosphorus

Compound PCl(NCO) MP/°C BP/°C Compound

501

PCl (NCO)

P(NCS)

-50 134.6

-99 104.5

-4 -120/1 mmHg

PF (NCS)

PC1 (NCS)

2

2

2

MP/°C BP/°C

3

2

-95 90.3

-76 148(decomp)

The corresponding phosphoryl and thiophosphoryl pseudohalides are also known, i.e. PO(NCO) , PS(NCO) , etc. Preparations are by standard procedures such as those on the diagram for PCI3 (p. 497). As indicated there, P ( C N ) has also been made: it is a highly reactive white crys­ talline solid mp 203° which reacts violently with water to give mainly phosphorous acid and HCN. 3

3

3

2

2

-

-108 12.3

Α. V. FRATINI

and

H . C. KNACHEL, Inorg. Chem. 1 9 , 2 3 2 4 - 7 (1980) and ref­ erences therein.

12.3.4 Oxohalides and thiohalides of phosphorus The propensity of phosphorus(III) compounds to oxidize to phosphorus(V) by formation of an additional P = 0 bond is well illustrated by the

502

Phosphorus

Table 12.4 Compound POF

POBr POI PO(NCO) PO(NCS) PSF PSC1 PSBr PSI PS(NCO) PS(NCS)

BP/ C

-39.1 1.25 55 53 5.0 13.8 -148.8 -35 37.8 48 8.8

POCI3 3

3

3

3

3

3

3

3

3

Compound

-39.7 105.1 191.7

POF Cl 2

POFCI2 2

193.1 300.1 -52.2 -125 212 (d) decomp 215 123/0.3 mmHg

ease with which the trihalides are converted to their phosphoryl analogues P O X . Thus, PCI3 reacts rapidly with pure 0 (less rapidly with air) at room temperature or slightly above and this reaction is used on an industrial scale. Alternatively, a slurry of P4O10 in PCI3 can be chlorinated, the PC1 so formed reacting instantaneously with the P4O10: 3

2

5

P4O10 + 6PCI5

• 10POC1

3

POBr3 can be made by similar methods, but POF3 is usually made by fluorination of POCI3 using a metal fluoride (e.g. M = Na, Mg, Zn, Pb, Ag, etc.). P O I was first made in 1973 by iodinating P O C l with Lil, or by reacting R O P I with iodine (ROPI + I RI + P O I ) . Mixed phosphoryl halides, POX„Y3_„, and pseudohalides (e.g. X = NCO, NCS) are known, as also are the thiophosphoryl halides P S X , e.g.: 3

3

2

2

( 1 0 4 )

2

3

3

P S + 3PC1 2

5

• 5PSC1

5

AlCh

PCI3

+ S

3

CS,/dark

+ PSCI3;

PI + s — = 3

>

PSI

3

Most of the phosphoryl and thiophosphoryl com­ pounds are colourless gases or volatile liquids though PSBr forms yellow crystals, mp 37.8°, P O I is dark violet, mp 53°, and PSI3 is redbrown, mp 48°. All are monomeric tetrahedral ( C ) or pseudotetrahedral. Some physical prop­ erties are in Table 12.4. The P - O interatomic 3

3

3u

1 0 4

Α. V. KIRSANOV,

CHENKO, PureAppl.

Z H . K. GORBATENKO

and

Chem. 4 4 , 1 2 5 - 3 9 (1975).

POF Br POFBr POCl Br POClBr PSF.Cl PSFC1 PSF Br PSFBr PO(NCO)FCl PS(NCS)F 2

—

—

3

12

Some phosphoryl and thiophosphoryl halides and pseudohalides

MP/°C

3

Ch.

N. G . FESH-

2

2

2

2

2

MP/°C

BP/°C

-96.4 -80.1 -84.8 -117.2 11 31 -155.2 -96.0 -136.9 -75.2

3.1 52.9 31.6 110.1 52/3 mmHg 49/12 mmHg 6.3 64.7 35.5 125.3 103 90

— —

2

distance in these compounds generally falls in the range 154-158 pm, the small value being con­ sistent with considerable "double-bond charac­ ter". Likewise the P - S distance is relatively short ( 1 8 5 - 1 9 4 pm). The phosphoryl and thiophosphoryl halides are reactive compounds that hydrolyse readily on contact with water. They form adducts with Lewis acids and undergo a variety of substitution reactions to form numerous organophosphorus derivatives and phosphate esters. Thus, alcohols give successively (RO)POCl , (RO) POCl and (RO) PO; phenols react similarly but more slowly. Likewise, amines yield (RNH)POCl , (RNH) POCl and (RNH) PO whereas Grignard reagents yield R„POCl3_, (n — 1-3). Many of these compounds find extensive use as oil additives, insecticides, plasticizers, surfactants or flame retardants, and are manufactured on the multikilotonne scale. 2

2

3

2

2

3

2

In addition to the monophosphorus phosphoryl and thiophosphoryl compounds discussed above, several poly-phosphoryl and -thiophosphoryl halides have been characterized. Pyrophosphoryl fluoride, 0 = P F — O — P ( = 0 ) F (mp —0. Γ , bp 72° extrap) and the white crys­ talline cyclic tetramer [ 0 = P ( F ) — 0 ] were 2

2

4

I

I

obtained by subjecting equimolar mixtures of P F and 0 to a silent electric dis­ charge at —70°. Pyrophosphoryl chloride, 0 = P C 1 — O — P ( = 0 ) C 1 is conveniently pre­ pared by passing C l into a boiling suspension 3

2

2

2

2

Phosphorus

§12.3.5

of P O 4

1 0

oxides, sulfides, selenides

in PC1 diluted with CC1 : 3

4

and related

503

compounds

By contrast to the plethora of simple oxo­ halides and thiohalides of P , the corresponding derivatives of P are fugitive species that require matrix isolation techniques for preparation and characterization: C1PO, BrPO, FPS and BrPS all form non-linear triatomic molecules, as expected. The corresponding oxosulfide, BrP(O)S, and its thio-analogue, F P ( S ) S , have also recently been isolated. v

P O 4

l0

+ 4PCl + 4Cl 3

2

• 2 P 0 C 1 + 4POCl 2

3

4

3

It is a colourless, odourless, non-fuming, oily liquid, mp —16.5°, bp 215° (decomp), with reactions similar to those of POCI3. Sealedtube reactions between P 4 O 1 0 and POCI3 at 200-230° give more highly condensed cyclic and open-chain polyphosphoryl chlorides. A rather different structural motif occurs in P2S4F4; this compound is obtained by fluorinating P 4 S 1 0 with an alkali-metal fluoride to give the anion [ S P F ] ~ which is then oxidized by bromine to P2S4F4 (bp 60° at 10 mmHg). Vibrational and nmr spectra are consistent with the structure F (S)PSSP(S)F . Bromination of P 4 S 7 in cold C S yields, in addition to PBr3 and PSBr3, two further thiobromides P S 6 B r (mp 118° decomp) and P S5Br4 (mp 90° decomp). The first of these has the cyclic structure shown in which the ring adopts a skew-boat configuration. An even more complex, bicyclic arrangement is found in the orange-yellow compound P 4 S I (mp 120° decomp) which is formed (together with several other products) when equiatomic amounts of P, S and I are allowed to react. The Ρ and S atoms are arranged in two 5-membered rings having a common P - S - P group as shown; in each there is a P - P group and the I atoms are bonded in ds-configuration to the Ρ atoms not common to the two rings. The orange compound P S l 4 (mp 94°) was mentioned on p. 498. 2

2

2

2

2

2

2

2

3

2

2

2

m

(105)

(106)

(107)

12.3.5 Phosphorus oxides, sulfides, selenides and related compounds The oxides and sulfides of phosphorus are amongst the most important compounds of the element. At least 6 binary oxides and 9 welldefined sulfides are known, together with a similar number of selenides and several oxosulfides. It will be convenient to discuss first the preparation and structure of each group of compounds and then to mention the chemi­ cal reactions of the more important members in so far as they are known. It is notable that, in contrast to the ubiquitous N O and its many complexes (pp. 445 ff), little is known about its analogue, PO (see p. 506), although it is probably the most abundant P-containing The first com­ molecule in interstellar c l o u d s . plex with a PO ligand was first synthesized as recently as 1991, when dark green crys­ tals of the square-based pyramidal hetero-atom cluster [W(CO) {Ni( 7 -C HPr )} (M:/ ,^ -P )] was oxidized with bis(trimethylsilyl) peroxide, ( M e 3 S i ) 0 , to yield black crystals of the cor­ responding [W(CO)4{Ni(^ -C5HPr )} (M:^ ,r PO) ].< > (I08)

4

2

y

5

5

4

2

2

2

2

5

2

?2

l4

2

2

?2

108

Oxides is obtained by controlled oxidation of P4 in an atmosphere of 7 5 % 0 and 2 5 % N at 90 mmHg and ^ 5 0 ° followed by distillation of the product from the mixture. Careful

P4O6

2

1 0 5

H. SCHNΤCKE L and S . SCHUNCK, Z. anorg.

548,

1 6 1 - 4 (1987);

552, 155-62

and

allg.

63-70

Chem. (1987).

M . BINNEWIES and H . BORRMANN, ibid. 5 5 2 , 1 4 7 - 5 4 ( 1 9 8 7 ) . 1 0 6

S . SCHUNCK, H.-J. GΤCK E and H. SCHNΤCKEL , Ζ. anorg.

allg. Chem. 5 8 3 , 7 8 - 8 4 1 0 7

2

Η. Βοκ,

M. KREMER,

(1990). B . SOLOUKI,

1 0 8

M. BINNEWIES

and

M . MEISEL, J. Chem. Soc, Chem. Commun., 9 - 1 1 ( 1 9 9 2 ) .

O . J. SCHERER,

J. B R A U N ,

P . WALTHER,

G . HECKMANN

and G . WOLMERSHUSER, Angew. Chem. Int. Edn. Engl. 3 0 , 852-4.

(1991).

504

Phosphorus

Figure 12.14

Ch. 12

Molecular structures, symmetries and dimensions of the 5 oxides Ρ4θ +„ (η = 0 - 4 ) compared with a-P . The Ρ· · ·Ρ distances in the oxides are ~280-290 pm, i.e. essentially nonbonding. 6

4

precautions are necessary if good yields are to be o b t a i n e d . It forms soft white crystals, mp 23.8°, bp 175.4°, and is soluble in many organic solvents. The molecular structure has tetrahedral symmetry and comprises 4 fused 6-membered P3O3 heterocycles each with the chair conformation as shown in Fig. 1 2 . 1 4 . When P 0 is heated to 200-400° in a sealed, evacuated tube it disproportionates into red phosphorus and a solid-solution series of composition P O depending on conditions. The or-phase has a composition in the range P4O8.1-P Û9.2 and comprises a solid solution of oxides in which one or two of the "external" Ο atoms in P Oio have been removed. The β-phase has a composition range Ρ θ8.ο-Ρ4θ7.7 (109)

(110)

4

6

4

n

4

4

4

and appears to be a solid solution of P Og and P C>7, the latter compound having only one Ο atom external to the P 0 6 cluster (Ci symmetry). P4O7 is now best prepared from P C>6 dissolved in thf, using P h P O as a catalyst (not an oxidant) at room temperature. The molecular structure and dimensions of P C>7 are given in Fig. 12.14 from which it is apparent that there is a gradual lengthening of P - 0 distances in the sequence P - O < P - Ο < Ρ - Ο . Similar trends are apparent in the dimensions of the other members of the series P 0 shown in Fig. 1 2 . 1 4 . In addition, ring angles at Ρ (96-103°) are always less than those at Ο (122-132°), as expected. 4

4

4

3

4

v

t

v

D . HEINZE, PureAppl.

Chem. 4 4 , 1 4 1 - 7 2

(1975).

M . JANSEN and M. Voss, Angew. Chem. Int. Edn. Engl. 2 0 , 1 0 0 - 1 , 9 6 5 ( 1 9 8 1 ) , and references therein to crystal structure determinations on the other members of the series Ρ4Ο6+Π· See also M . JANSEN and M . MOEBS, Inorg. Chem.

1 1 0

23,

4 4 8 6 - 8 (1984).

μ

4

Ι Π

μ

6+n

(110)

P4O6 hydrolyses in cold water to give H3PO3 i.e. H P ( 0 ) ( O H ) ; this is interesting in view of the structure of P C>6 and implies an oxidative rearrangement of {P-OH} to { H - P = 0 } (p. 514). The oxide itself ignites and burns when heated in air; the progress of the reaction depends very much on the 2

1 0 9

v

4

4

§12.3.5

Phosphorus

oxides,

Table 12.5

sulfides,

H: hexagonal P4O10

0 : metastable (P 0 )„ 0': stable ( P 0 ) 2

2

5

5

n

3

2.30 2.72 2.74-3.05

Pressure at triple pt/mmHg

A// ,/kJ (mol ΡιΟ,οΓ

420 562 580

3600 437 555

95 152 142

(109)

4

4

6

2

3

4

4

505

MP/°C

purity of the oxide and the conditions employed, and, when traces of elemental phosphorus are present in the oxide, the reaction is spontaneous even at room temperature. P 4 O 6 reacts readily (often violently) with many simple inorganic and organic compounds but wellcharacterized products have rarely been isolated until r e c e n t l y . It behaves as a ligand and successively displaces CO from [Ni(CO)4] to give compounds such as [ P 0 6 { N i ( C O ) } 4 ] , [ N i ( C O ) ( P 0 ) ] and [ N i ( C O ) ( P 0 ) ] . With diborane adducts of formula [ P 0 6 ( B H ) „ ] (n = 1-3) are obtained. 2

and related compounds

Some properties of crystalline polymorphs of P2O5

Density/gem

Polymorph

selenides

6

3

3

"Phosphorus pentoxide", P4O10, is the com­ monest and most important oxide of phosphorus. It is formed as a fine white smoke or powder when phosphorus burns in air and, when con­ densed rapidly from the vapour phase in this way, is obtained in the Η (hexagonal) form compris­ ing tetrahedral molecules as shown in Fig. 12.14. This compound and the other phosphorus oxides are the first we have considered that feature the {PO4} group as a structural unit; this group domi­ nates most of phosphate chemistry and will recur repeatedly during the rest of this chapter. The common hexagonal form of P4O10 is, in fact, metastable and can be transformed into several other modifications by suitable thermal or highpressure treatment. A metastable orthorhombic (O) form is obtained by heating Η for 2 h at 400° and the stable orthorhombic (Ο') form is obtained after 24 h at 450°. Both consist of extensive sheet polymers of interlocking heterocyclic rings composed of fused { P O 4 } groups. There is also a high-pressure form and a glass, which prob­ ably consists of an irregular three-dimensional network of linked { P O 4 } tetrahedra. These poly­ meric forms are hard and brittle because of the P - O - P bonds throughout the lattice and, as

1

sub

expected, they are much less volatile and reac­ tive than the less-dense molecular H form. For example, whilst the common H form hydrolyses violently, almost explosively, with evolution of much heat, the polymeric forms react only slowly with water to give, finally, H3PO4. Some proper­ ties of the various polymorphs are compared in Table 12.5. The limpid liquid obtained by rapidly heating the H form contains P4O10 molecules but these rapidly polymerize and rearrange to layer or three-dimensional polymeric forms with a concomitant drop in the vapour pressure and an increase in the viscosity and mp. Because of its avidity for water, P4O10 is widely used as a dehydrating agent, but its efficacy as a desiccant is greatly impaired by the formation of a crusty surface film of hydrolysis products unless it is finely dispersed on glass wool. Its largest use is in the industrial production of ortho- and poly-phosphoric acids (p. 520) but it is also an intermediate in the production of phosphate esters. Thus, triethylphosphate is made by reacting P 4 O 1 0 with diethyl ether to form ethylpolyphosphates which, on subsequent pyrolysis and distillation, yield the required product: heat

P O 4

+ 6Et 0

1 0

2

• 4PO(OEt)

3

Direct reaction with alcohols gives mixed monoand di-alkyl phosphoric acids by cleavage of the P - O - P bonds: P 4 O 1 0 + 6ROH - ^ U 2(RO)PO(OH) 2

+ 2(RO) PO(OH) 2

Under drates to the drated

less-controlled conditions P4O10 dehy­ ethanol to ethene and methylarylcarbinols corresponding styrenes. H S 0 4 is dehy­ to S 0 , H N 0 gives N O s and amides 2

3

3

2

506

(RCONH ) yield nitriles (RCN). In each of these reactions metaphosphoric acid H P O 3 is the main P-containing product. P4O10 reacts vigorously with both wet and dry N H 3 to form a range of amorphous polymeric powdery materials which are used industrially for water softening because of their ability to sequester Ca ions; composition depends markedly on the preparative conditions employed but most of the commercial products appear to be condensed linear or cyclic amidopolyphosphates which can be represented by for­ mulae such as: 2

Ο

HO

Ch.

Phosphorus

O

I I

H

O

I

Ρ

Ο-

ο

I I I

-Ρ

Ν

ONH4

Ρ

O-

ONH4

-Ρ

ΝΗ

2

I ΟΝΗ

NH40 ο ΝΗ 0

Ρ

4

ο ΝΗ

Ο

Ρ

4

ΟΝΗ

4

ΟΝΗ

4

Ο

I ΝΗ 0

Ρ

I ΝΗ

Ρ

I

I I

Ο

Ο

The annual production/consumption of Ρ4Ο10 in USA and Western Europe totals about 15 000 tonnes. Other oxides of phosphorus are less well char­ acterized though the suboxide PO and the per­ oxide P2O6 seem to be definite compounds. PO was obtained as a brown cathodic deposit when a saturated solution of Et3NHCl in anhydrous P O C I 3 was electrolysed between Pt electrodes at 0°. Alternatively it can be made by the slow reac­ tion of POBr with Mg in E t 0 under reflux: 2POBr + 3Mg • 2PO + 3MgBr 3

2

3

2

Its structure is unknown but is presumably based on a polymeric network of P - O - P links. It reacts with water to give P H 3 and is quantitatively oxidized to P2O5 by oxygen at 300°. The peroxide P O o is thought to be the active ingredient in the violet solid obtained when P4O10 and 0 are passed through a heated 2

2

discharge tube at low pressure. The compound has not been obtained pure but liberates I from aqueous Kl, hydrolyses to a peroxophosphoric acid, and liberates 0 when heated to 130° under reduced pressure. Its structure may be (0=) P-0-0-P(=0) or, in view of the variable composition of the product, it may be a mixture of P 4 O 1 1 and Ρ 4 Ο Π obtained by replacing P - O - P links by P - O - O - P in P O . 2

2

2

2

4

1 0

Sulfides^ The sulfides of phosphorus form an intriguing series of compounds which continue to present puzzling structural features. The compounds P 4 S 1 0 , P 4 S 9 , P 4 S 7 , CX-P4S5, £ - P S , CX-P4S4, βP 4 S 4 , P 4 S 3 and P 4 S are all based on the P4 tetrahedron but only P4S10 (and possibly P 4 S 9 ) is structurally analogous to the oxide. P 4 S 6 is conspicuous by its absence. Structural data are summarized in Fig. 12.15 and some physical properties are in Table 12.6. P 4 S 3 is the most stable compound in the series and can be prepared by heating the required amounts of red Ρ and sulfur above 180° in an inert atmosphere and then purifying the product by distillation at 420° or by recrystallization from toluene. The retention of a P3 ring in the structure is notable. Its reactions and commercial application in match manufacture are discussed on p. 509. The curious phase relations between phos­ phorus, sulfur and their binary compounds are worth noting. Because both P4 and Sg are sta­ ble molecules the phase diagram, if studied below 100°, shows only solid solutions with a simple eutectic at 10° (75 atom % P). By contrast, when the mixtures are heated above 200° the elements react and an entirely different phase diagram is obtained; however, as only the most stable compounds P 4 S 3 , P 4 S 7 and P4S10 4

4

12

5

2

1 1 1

H . HOFFMANN and M . BECKE-GOEHRING, Topics in Phos­ phorus Chemistry 8 , 1 9 3 - 2 7 1 (1976); J. G . RIESS in A. H . COWLEY (ed.), Rings, Clusters and Polymers of the Main Group Elements, ACS Symposium Series No. 2 3 2 , 1 7 - 4 7 (1983).

Phosphorus

§12.3.5

Figure 12.15

oxides,

a-P S

Colour MP/°C BP/°C Density/gemSolubility in CS ( 17°)/ g per 100gCS

Yellow green 174 408 2.03 100

4

2

and related

compounds

507

Physical properties of some phosphorus sulfides

Property

2

selenides

Structures of phosphorus sulfides and oxosulfides (schematic).

Table 12.6

3

sulfides,

3

a-P S 4

4

Pale yellow 230 (d) — 2.22 sol

a-P S 4

5

Bright yellow 170-220 (d) — 2.17 0.5

P4S7

P4S10

Very pale yellow 308 523 2.19 0.029

Yellow 288 514 2.09 0.222

508

Ch. 12

Phosphorus

melt congruently, only these three appear as compounds in equilibrium with the melt. Careful work at lower temperatures is needed to detect peritectic equilibria involving P 4 S 9 , P 4 S 5 (and possibly even P S 2 ) , and it is notable that these compounds are normally prepared by lowtemperature reactions involving addition of 2S to P 4 S 7 and P 4 S 3 respectively. Likewise there is no sign of P 4 S 4 on the phase diagram, and claims to have detected it in this way have been shown to be e r r o n e o u s / P 4 S 4 is one of the most recent binary sulfides to be isolated and characterized and it exists in two structurally distinct f o r m s / Each can be made in quantitative yield by reacting the appropriate isomer of P 4 S 3 I 2 (p. 503) with [(Me Sn) S] in C S solution: ( 1 1 2 )

4

113)

1 1 3 U 4 )

3

2

2

and P 4 S 7 ) when compared with corresponding distances in P 4 S 5 (225 pm) and P itself (221 pm). The structure of β-Ρ4$4 has not been determined by X-ray crystallography but spectroscopic data indicate the absence of P = S groups and the C structure shown in Fig. 12.15 is the only other possible arrangement of 3 coordinate Ρ for this composition. P 4 S 5 disproportionates below its mp ( 2 P 4 S 5 ν P 4 S 3 + P 4 S 7 ) and so cannot be obtained directly from the melt. It is best prepared by irradiating a solution of P 4 S 3 and S in C S 2 solution using a trace of iodine as catalyst. Its structure is quite unexpected and features a single exocyclic P = S group and 3 fused heterocycles containing, respectively, 4, 5 and 6 atoms; there are 2 short P - P bonds and the 4-membered P 3 S ring is almost square planar. P 4 S 7 is the second most stable sulfide (after P 4 S 3 ) and can be obtained by direct reaction of the elements. Perhaps surprisingly the structure retains a P - P bond and has two exocyclic P = S groups. P 4 S 9 is formed reversibly by heating P 4 S 7 -f 2P Sio and has the structure shown in Fig. 12.15. P 4 S 1 0 is commercially the most important sul­ fide of Ρ and is formed by direct reaction of liquid white P4 with a slight excess of sulfur above 300°. It can also be made from byproduct ferrophosphorus (p. 480). 4

s

s

4

p-—s'

Γ

fi-P&h

£-p s 4

heat

4 F e P + 18FeS 2

4 F e P + 18S 2

France

1972, 4 5 1 7 - 2 1 ;

R . FΤRTHMANN and A. SCHNEIDER, Ζ Phys. Chem. ( N F ) 4 9 , 22-37 1 1 3

(1966).

A. M . GRIFFIN,

P . C . MINSHALL

J. Chem. Soc, Chem. Commun., 1 1 4

C . - C . CHANG,

and G . M . SHELDRICK,

8 0 9 - 1 0 (1976).

R . C . HALTIWANGER

and A. D . NORMAN,

Inorg. Chem. 1 7 , 2 0 5 6 - 6 2 ( 1 9 7 8 ) . See also B . W . TATTERSHALL

J.

Chem.

Soc,

B . W . TATTERSHALL 2629-37

(1994).

-Dalton

Trans.,

and N . L . KENDALL,

1515-20

4

+ 26FeS

1 0

(1987);

Polyhedron

13,

•

+ 8FeS

P4S10

It has essentially the same structure as the Η form of P 4 O 1 0 and hydrolyses mainly according to the overall equation P4S10

H . VINCENT, Bull. Soc. Chim.

>P S

4

As seen from Fig. 12.15 the structure of a - P 4 S 4 resembles that of AS4S4 (p. 579) rather than N 4 S 4 (p. 723). The 4 Ρ atoms are in tetrahedral array and the 4 S atoms form a slightly distorted square. The 2 P - P bonds are long (as also in P 4 S 3 1 1 2

2

+ 16H 0 2

> 4 H P 0 + 10H S 3

4

2

Presumably intermediate thiophosphoric acids are first formed and, indeed, when the hydrolysis is carried out in aqueous NaOH solution at 100°, substantial amounts of the mono- and dithiophosphates are obtained. P - S bonds are also retained during reaction of P 4 S 1 0 with alcohols or phenols and the products formed are used extensively in industry for a wide variety of

§12.3.5

Phosphorus

oxides, sulfides,

applications (see Panel). P4S10 is also widely used to replace Ο by S in organic compounds to form, e.g., thioamides RC(S)NH , thioaldehydes RCHS and thioketones R C S . Methanolysis yields (MeO) P(S)SH plus H S , and the related anions ( R O ) P S ~ are known as versatile 2

2

2

2

2

1 1 5

P . BOURDAUDUCQ and

( 1 1 5 )

2

M . C. DΙMARCQ , J.

Dalton Trans., 1 8 9 7 - 9 0 0 ( 1 9 8 7 ) .

Chem.

Soc,

selenides

and related

compounds

509

ligands with a remarkable variety of coordination modes. A rather different series of cyclic thiophosphate(III) anions [ ( P S ) „ ] ~ is emerging from a study of the reaction of elemental phosphorus with polysulfidic sulfur. Anhydrous compounds 0 1 6 )

2

1 , 6

n

M . G . B . DREW, R . J. HOBSON, P . P . Ε . M . M U M B A

and

D . A . RICE, J. Chem. Soc, Dalton Trans., 1 5 6 9 - 7 1 ( 1 9 8 7 ) .

510

Ch. 12

Phosphorus

M\[cyclo-P S\ ] and M\[cyclo-P S\2] were obtained using red phosphorus, whereas white P yielded [NH4]4[cjcto-P S8].2H 0 as shiny platelets. This unique P 4 S s ~ anion is the first known homocycle of 4 tetracoordinated Ρ atoms and X-ray studies reveal that the Ρ atoms form a square with rather long P - P distances (228pm). The new planar anion P S 3 (cf. the nitrate ion, NC>3~) has been isolated as its tetraphenylarsonium salt, mp 183°, following a surpris­ ing reaction of P4S10 with K C N / H S in MeCN, in which the coproduct was the known dianion [(NC)P(S) -S-P(S) (CN)] The first sulfido heptaphosphane cluster anions, [ P ( S ) 3 ] ~ and [ H P ( S ) ] " (cf. P ~ , p. 491), have also recently been c h a r a c t e r i z e d / 5

0

6

4

4

2

4

( l 2 0 )

(n = 1-3), P 0 S e , P 0 S „ (n = l , 2 ) . the crystal and molecular structures of P 4 0 e S and P4O6S3 have recently been d e t e r m i n e d / Two isomers each of ^ - P 4 S S e I and /?-P SSe I , prepared by reaction of P4S3_„Se„ with I in C S have been structurally identified by nmr spectroscopy/ 4

7

4

8

2

121}

2

2

4

2

2

2

3 1 P

2

122)

(117)

-

2

2

2

(118)

2

3

7

2

7

3

2

7

119)

Oxosulfides

12.3.6 Oxoacids of phosphorus and their salts The oxoacids of Ρ are more numerous than those of any other element, and the number of oxoanions and oxo-salts is probably exceeded only by those of Si. Many are of great importance technologically and their derivatives are vitally involved in many biological processes (p. 528). Fortunately, the structural principles covering this extensive array of compounds are very simple and can be stated as follows:^

When P4O10 and P4S10 are heated in appropriate proportions above 400°, P4O6S4 is obtained as colourless hygroscopic crystals, mp 102°. 3P O 4

1 0

+ 2P S 4

• 5P 0 S

1 0

4

6

ο

4

The structure is shown in Fig. 12.15. The related compound P4O4S6 is said to be formed by the reaction of H S with P O C l at 0° (A. Besson, 1897) but has not been recently investigated. An amorphous yellow material of composition P4O4S3 is obtained when a solution of P4S3 in C S or organic solvents is oxidized by dry air or oxygen. Other oxosulfides of uncertain authenticity such as P6O10S5 have been reported but their structural integrity has not been established and they may be mixtures. However, the following series can be prepared by appropriate redistribution reactions: P 4 0 6 S (n = 1-4), P 0 S e „ (n = l - 3 ) , P 0 S S e , P 0 S „ 2

(i) All Ρ atoms in the oxoacids and oxoanions are 4-coordinate and contain at least one P - 0 unit (1).

ρ

3

(1)

(ii) All Ρ atoms in the oxoacids have at least one Ρ - O H group (2a) and this often occurs in the anions also; all such groups are ionizable as proton donors (2b).

2

n

4

6

4

6

4

7

1 2 0

M . L. WALKER,

1 2 1

and

J. L. MILLS,

F . FRICK and M . JANSEN, Z. anorg. allg.

Chem. 6 1 9 ,

2 8 1 - 6 ( 1 9 9 3 ) . See M . JANSEN and S. STROJEK, Ζ anorg. allg. Chem. 6 2 1 , 4 7 9 - 8 3 ( 1 9 9 5 ) for X-ray structures of P 0 S , i.e. 4

4

6

H.

FALIUS,

W . KRAUSE

and

W . S . SHELDRICK,

Angew.

1 1 8

H . W . ROESKY,

R . AHLRICHS

and

2070-6

(1994).

t

Angew.

Chem.

1 2 5 7 - 6 1 ( 1 9 9 3 ) . See also M . RUCK, ibid. 6 2 0 , 1 8 3 2 - 6

( 1 9 9 4 ) R. BLACHNIK, A . HEPP, P . LΤNNECKE, J. A . DONKIN and B . W . TATTERSHALL, ibid. 6 2 0 , 1 9 2 5 - 3 1

S . BRODE,

Chem. Int. Edn. Engl. 2 5 , 8 2 - 3 ( 1 9 8 6 ) M . BAUDLER and A . FLORUSS, Z. anorg. allg. Chem. 6 2 0 ,

1 1 9

t

P . LΤNNECKE and R. BLACHNIK, Ζ anorg. allg.

619,

Chem. Int. Edn. Engl 2 0 , 1 0 3 - 4 ( 1 9 8 1 ) .

7

P 0 (0) (S) . 1 2 2

1 1 7

D. E. PECKENPAUGH

Inorg. Chem. 1 8 , 2 7 9 2 - 6 ( 1 9 7 9 ) .

(1994).

^ Heteropolyacids containing Ρ fall outside this classifica­ tion and are treated, together with the isopolyacids and their salts, on pp. 1 0 1 0 - 1 6 . Organic esters such as P(OR)3 are also excluded.

Oxoacids

§12.3.6

of phosphorus

(iii) Some species also have one (or more) P - H group (3); such directly bonded H atoms are not ionizable.

(2a)

(2b)

(3)

(iv) Catenation is by P - O - P links (4a) or via direct P - P bonds (4b); with the former both open chain ("linear") and cyclic species are known but only corner sharing of tetrahedra occurs, never edgeor face-sharing. ο

ο

(4a)

9

9

(4b)

(v) Peroxo compounds feature either ^

P — O O H groups or

POOP

links. It follows from these structural principles that each Ρ atom is 5-covalent. However, the oxidation state of Ρ is 5 only when it is directly bound to 4 Ο atoms; the oxidation state is reduced by 1 each time a P - O H is replaced by a P - P bond and by 2 each time a P - O H is replaced by

and their salts

511

a P - H . Some examples of phosphorus oxoacids are listed in Table 12.7 together with their recommended and common names. It will be seen that the numerous structural types and the variability of oxidation state pose several problems of nomenclature which offer a rich source of confusion in the literature. The oxoacids of Ρ are clearly very different structurally from those of Ν (p. 459) and this difference is accentuated when the standard reduction potentials (p. 434) and oxidation-state diagrams (p. 437) for the two sets of compounds are compared. Some reduction potentials (E°fV) in acid solution are in Table 1 2 . 8 (p. 513) and these are shown schematically below, together with the corresponding data for alkaline solutions. The alternative presentation as an oxidation state diagram is in Fig. 12.16 which shows the dramatic difference to Ν (p. 438). The fact that the element readily dissolves in aqueous media with disproportionation into PH3 and an oxoacid is immediately clear from the fact that Ρ lies above the line joining PH3 and either HJPO2 (hypophosphorous acid), H3PO3 (phosphorous acid) or H3PO4 (orthophosphoric acid). The reaction is even i , 2 3 )

G. MILAZZO and S . CAROLI, Tables of Standard Electrode Potentials, Wiley, New York, 1978, 421 pp. A. J. BARD, R. PARSONS and J. JORDAN, Standard Potentials in Aqueous Solution, Marcel Dekker, New York, 1985, 834 pp. 1 2 3

512

Ch. 12

Phosphorus

Table 12.7 Some phosphorus oxoacids Structure^

Formula/Name H3PO4

Formula/Name

Structure^

H3PO5

Ο

(Ortho)phosphoric acid

(a)

Ο

Peroxomonophosphoric acid

II .p.

II

OH HO

HO

H P 0 Diphosphoric acid (pyrophosphoric acid) 4

2

7

H P O Triphosphoric acid 3

4

II HO

5

H P 0 Peroxodiphosphoric acid

Ο

Ο

l0

/ HO O

2

η+

Η Ρ 0 Hypophosphoric acid^ [diphosphoric(IV) acid]

II

(HO) P — Ο — Ρ — Ο — P ( O H )

2

OH

Η 2Ρηθ3„ ι Polyphosphoric acid (n up to 17 isolated)

4

o

ο

II

II

II

(HO) P- ο — p 2

I

Ρ

Ο—P(OH)

2

6

o

2

3

6

Ο

H3PO3 (2)

3

HO^I

OH OH

ο

II ρ

|^OH

OH HO

Ο

OH

o,

H4P2O5 (2)

Ο

n

-

HO /

\

Ο

(b)

Ο

Diphosphonic acid (diphosphorous or pyrophosphorous acid)

OH Ο

HO^\>

ο

II .ρx

II /

HO

p^OH

ρ.

\^

H

OH

O

Ο

OH

Ο

OH

Ρ

ρ

ρ

ρ

OH

\

(b)

Phosphonic acid (phosphorous acid)

O

(HP0 )„ Polymetaphosphoric acid (see text for salts)

II

HO n-2

4

Ο

II .ρ

V 3

Ο

2

3

(HP0 ) Cyc/o-tetrametaphosphoric acid (anions known in both "boat" and "chair" forms)

ο

v

HO—Ρ — Ρ—OH / \ HO OH

OH

(HP0 ) Cyc/o-trimetaphosphoric acid

ο

o

H P 0 Isohypophosphoric acid [diphosphoric(III,V) acid]

+

ο

OH Λ OH

\

II 4

II

ο

Ο

HO / HO

O

II

8

\ OH OH

Ο

O

2

OOH

HO

0

OH

0

H3PO2 ( l )

(b)

Phosphinic acid (hypophosphorous acid) H

OH

(a)

S o m e acids are known only as their salts in which one or more - O H group has been replaced by O". T h e number in parentheses after the formula indicates the maximum basicity, where this differs from the total number of H atoms in the formula. (b)

more effective in alkaline solution. Similarly, Η Ρ θ 6 disproportionates into H3PO3 and H3PO4. Figure 12.16 also illustrates that H P 0 and H3PO3 are both effective reducing agents, being readily oxidized to H3PO4, but this 4

2

3

2

latter compound (unlike HNO3) is not an oxidizing agent. A comprehensive treatment of the oxoacids and oxoanions of Ρ is inappropriate but selected examples have been chosen to illustrate

Oxoacids

§12.3.6

of phosphorus

and their

salts

513

Figure 12.16 Oxidation state diagram for phosphorus. (Note that all the oxoacids have a phosphorus covalency of 5.) interesting points of stereochemistry, reaction chemistry or technological applications. The treatment begins with the lower oxoacids and their salts (in which Ρ has an oxidation state less than -f 5) and then considers phosphoric acid, phosphates and polyphosphates. The peroxoacids H 3 P O 5 and H P 0 g and their salts will not be treated further (except peripherally) nor will the peroxohydrates of orthophosphates, which are obtained from aqueous H 0 solutions. 4

2

its insoluble calcium salt: 2Ca

P + 40H- + 2H 0 4

2

Reaction Ρ+ Ρ+ |P H H,P0 2

4

2

2

2

H3PO3 +

The recommended names for these compounds (phosphinic acid and phosphinates) have not yet gained wide acceptance for inorganic compounds but are generally used for organophosphorus derivatives. Hypophosphites can be made by heating white phosphorus in aqueous alkali: warm

P + 40H- + 4H 0 4

• 4H.PO,- + 2H

2

H P0 + H P0 + H P0 + H P0 |H P 0 3

2

2

[NaOH/Ca(OH) ] 2

Phosphite and phosphine are obtained as byprod­ ucts (p. 493) and the former can be removed via

2

4

3

3

3

4

3

4

4

2

6

3 H + 3e" 2 H + 2e" +H + e-^=± + H 4- e"

E°N

-0.063 -0.097 +0.006 -0.508 -0.502 3H+ + 3e" 5 H + 5e" -0.411 2H+ -f 2e" -0.499 2 H + 2e" -0.276 H3PO3 + H 0 | H P 0 + H 0 -0.933 + H -f e" + H - h e - ^ = ^ H3PO3 +0.380 +

+

+

+

PH (g) ^P H (g) PH Ρ + 2H 0 Ρ + 3H 0 Ρ + 4H 0 H P0 +H 0 3

2

4

3

2

2

+

2

3

2

2

+

2

4

+

2

6

2

+

'P refers to white phosphorus, | P ( s ) . 4

Free hypophosphorous acid is obtained by acidifying aqueous solutions of hypophosphites but the pure acid cannot be isolated simply by evaporating such solutions because of its ready oxidation to phosphorous and phosphoric acids and disproportionation to phosphine and phosphorous acid (Fig. 12.16). Pure H P 0 is obtained by continuous extraction from aqueous solutions into E t 0 ; it forms white crystals mp 3

1 2 4

1 . I. CREASER and J. O . EDWARDS, Topics in

Chemistry 7, 3 7 9 - 4 3 5 (1972).

Phosphorus

3

(a)

ί64)

Hypophosphorous acid and hypophosphites [H PO(OH) and H P 0 " ]

3

Table 12.8 Some reduction potentials in acid solu­ tion (pH 0 )

0 2 4 )

2

2 +

• C a ( H P 0 ) + 2PH

2

2

2

514

Ch.

Phosphorus

26.5° and is a monobasic acid pK

a

1.244 at

25°(125)

During the past few decades hydrated sodium hypophosphite, N a H P 0 . H 0 , has been increasingly used as an industrial reducing agent, particularly for the electroless plating of Ni onto This developed both metals and n o n - m e t a l s . from an accidental discovery by A. Brenner and Grace E. Riddel at the National Bureau of Standards, Washington, in 1944. Acid solutions (E 0.40 V at pH 4 - 6 and Τ > 90°) are used to plate thick Ni layers on to other metals, but more highly reducing alkaline solutions (pH 7 - 1 0 ; Τ 25-50°) are used to plate plastics and other non-conducting materials: 2

2

2

(l26)

12

On an industrial scale PC1 is sprayed into steam at 190° and the product sparged of residual water and HC1 using nitrogen at 165°. Phosphorous acid forms colourless, deliquescent crystals, mp 70. Γ , in which the structural units shown form four essentially linear Η bonds (Ο· · Η 155-160 pm) which stabilize a complex 3D network. The molecular dimensions were determined by lowtemperature single-crystal neutron diffraction at 15K.< > 3

127

HP0 ~ + 2 H 0 + 2e" = F = ± H P 0 ~ + 30H~; 32

2

2

2

Ε

1.57 V

Typical plating solutions contain 1 0 - 3 0 g/1 of nickel chloride or sulfate and 1 0 - 5 0 g/1 NaH2P02; with suitable pump capacities it is possible to plate up to 10 kg Ni per hour from such a bath (i.e. 45 m surface to a thickness of 25 μ m). Chemical plating is more expensive than normal electrolytic plating but is competitive when intricate shapes are being plated and is essential for non-conducting substrates. (See also the use of B H 4 " in this connection, p. 167.) 2

In aqueous solutions phosphorous acid is dibasic (pJ5T, 1.257, pK 6 . 7 ) and forms two series of salts: phosphites and hydrogen phosphites (acid phosphites), e.g. 2

(125)

[NH MHP0 ].H 0, Li [HP0 ], Na [HP0 ].5H 0, K [HP0 ] [NH ][HP0 (OH)], Li[HP0 (OH)], Na[HP0 (OH)].2^H 0, K[HP0 (OH)] and M[HP0 (OH)] (M = Mg, Ca, Sr).

"normal":

4

3

2

"acid":

3

4

2

2

2

2

2

2

2

Phosphorous acid and phosphites [HPO(OH) and ΗΡ0 ~] 2

32

Again, the recommended names (phosphonic acid and phosphonates) have found more general acceptance for organic derivatives such as R P O 3 " " , and purely inorganic salts are still usually called phosphites. The free acid is readily made by direct hydrolysis of PCI3 in cold CCI4 solution: 2

PCI3

+ 3H 0 2

H P O ( O H ) + 3HC1 2

J. W. LARSON and M. PIPPIN, Polyhedron 8 , 5 2 7 - 3 0 (1989). H . NIEDERPRUM, Angew. Chem. Int. Edn. Engl. 1 4 , 6 1 4 - 2 0 (1975); G. A. KRULIK, Kirk-Othmer Encyclopedia of Chemical Technology, 4th edn., Vol. 9, pp. 1 9 8 - 2 1 8 , Wiley, New York, 1994.

3

3

2

2

2

2

Dehydration of these acid phosphites by warming under reduced pressure leads to the correspond­ ing pyrophosphites M ! > [ H P ( 0 ) - 0 - P ( 0 ) H ] and M [HP(0) -0-P(0) H]. Organic derivatives fall into 4 classes RPO(OH) , HPO(OR) , R P O ( O R ) and the phosphite esters P(OR)3*, this latter class has no purely inorganic analogues, though it is, of course, closely related to PCI3. Some preparative routes have already been indicated. Reactions with alcohols depend on conditions: 2

n

2

2

2

PC1 + 3ROH

1 2 5

3

2

2

,

2

HPO(OR) + RC1 + 2HC1 2

1 2 6

1 2 7

G . BECKER,

H . - D . HAUSEN,

O. MUNDT,

W . SCHWARZ,

C. T. WAGNER and T. VOGT, Z. anorg. allg. Chem. 5 9 1 , 17-31 (1990).

Oxoacids

§12.3.6

P C 1 + 3R0H + 3R N 3

of phosphorus

• P(OR) + 3R3NHCI

3

3

515

and their salts

and the 2 different Ρ atoms are joined by a p i n _ 0 - p v link/ ) 23

Phenols give triaryl phosphites P(OAr) directly at M 60° and these react with phosphorous acid to give diaryl phosphonates: 3

2P(OAr) + H P O ( O H ) 3

> 3HPO(OAr)

2

2

Hypophosphoric acid, ( H O ) P ( 0 ) - P ( 0 ) ( O H ) , is usually prepared by the controlled oxidation of red Ρ with sodium chlorite solution at room temperature: the tetrasodium salt, N a P O . l 0 H O , crystallizes at pH 10 and the disodium salt at pH 5.2: 2

4

Trimethyl phosphite P(OMe) spontaneously isomerizes to methyl dimethylphosphonate MePO(OMe) , whereas other trialkyl phosphites undergo the Michaelis-Arbusov reaction with alkyl halides via a phosphonium intermediate: 3

2

P(OR) + R X 3

• {[R'P(OR) ]X} 3

6

2

2P + 2NaCl0 + 8 H 0 2

> Na H P 0 .6H 0

2

2

2

2

2

6

2

+ 2HC1 Ion exchange on an acid column yields the crystalline "dihydrate" H P 0 Τ . 2 H 0 which is actually the hydroxonium salt of the dihydrogen hypophosphate anion [ H 0 ] J [ ( H O ) P ( 0 ) - P ( 0 ) ( O H ) ] ~ ; it is isostructural with the corre­ sponding ammonium salt for which X-ray diffrac­ tion studies establish the staggered structure shown. 4

> R P O ( O R ) + RX ,

2

2

2

2

3

Further discussion of these fascinating series of reactions falls outside our present s c o p e . (2)

2

2

2

Hypophosphoric acid ( H P 0 ) and hypophosphates 4

2

6

There has been much confusion over the struc­ ture of these compounds but their diamagnetism has long ruled out a monomeric formulation, H P 0 . In fact, as shown in Table 12.7, iso­ meric forms are known: (a) hypophosphoric acid and hypophosphates in which both Ρ atoms are identical and there is a direct P - P bond; (b) isohypophosphoric acid and isohypophosphates in which 1 Ρ has a direct P - H bond 2

3

The anhydrous acid is obtained either by the vacuum dehydration of the dihydrate over P Oio 4

516

Phosphorus

or by the action of H S on the insoluble lead salt P b P 0 6 . As implied above, the first proton on each -PO(OH)2 unit is more readily removed than the second and the successive dissociation constants at 25° are pK\ 2.2, pK 2.8, ρΚτ, 7.3, pK 10.0. Both H P 0 and its dihydrate are stable at 0° in the absence of moisture. The acid begins to melt (with decomposition) at 73° but even at room temperature it undergoes rearrangement and disproportionation to give a mixture of isohypophosphoric, pyrophosphoric, and pyrophosphorous acids as represented schematically on the previous page. Hypophosphoric acid is very stable towards alkali and does not decompose even when heated with 80% NaOH at 200°. However, in acid solu­ tion it is less stable and even at 25° hydrolyses at a rate dependent on pH (e.g. t\ 180 days in 1 M 2

2

2

Ch. 12

The structural relation between the reacting anions and the product is shown schematically below:

2

4

4

2

6

HC1, ti < 1 h i n 4 M H C l ) : 2 pH < 0

(HO) P(0)-P(0)(OH) + H 0 2

2

>

2

HP(0)(OH) + P(0)(OH) 2

3

The presence of P - H groups amongst the products of these reactions was one of the earlier sources of confusion in the structures of hypophosphoric and isohypophosphoric acids. The structure of isohypophosphoric acid and its salts can be deduced from Ρ nmr which shows the presence of 2 different 4-coordinate Ρ atoms, the absence of a P - P bond and the presence of a P - H group (also confirmed by Raman spec­ troscopy). It is made by the careful hydrolysis of PCI3 with the stoichiometric amounts of phos­ phoric acid and water at 50°: 3 1

Other lower oxoacids of phosphorus The possibility of P - H and P - P bonds in phosphorus oxoacids, coupled with the ease of polymerization via P - O - P linkages enables innumerable acids and their salts to be synthesized. Frequently mixtures are obtained and these can be separated by paper chromatography, paper electrophoresis, thinlayer chromatography, ion exchange or gel chromatography. Much ingenuity has been expended in designing appropriate syntheses but no new principles emerge. A few examples are listed in Table 12.9 to illustrate both the range of compounds available and also the abbreviated notation, which proves to be more convenient than formal systematic nomenclature in this area. In this notation the sequence of P - P and P - O - P links is indicated and the oxidation state of each Ρ is shown as a superscript numeral which enables the full formula (including P - H groups) to be deduced. (128)

50°

PCI3 + H3PO4 + 2 H 0

•

2

The phosphoric acids

w o - [ H ( H P 0 ) ] + 3HCl 3

2

6

The trisodium salt is best made by careful dehy­ dration of an equimolar mixture of hydrated disodium hydrogen phosphate and sodium hydro­ gen phosphite at 180°: 180°

Na HP0 .12H 0.+ N a H P 0 . 2 | H 0 2

4

2

2

3

2

2

6

4

+ 2

2

+

•

Na [HP 0 ] + 15^H 0 3

This section deals with orthophosphoric acid (H3PO4), pyrophosphoric acid ( Η Ρ θ 7 ) and the polyphosphoric acids ( H „ P „ 0 3 „ i ) . Several of these compounds can be isolated pure but their facile interconversion renders this area of phosphorus chemistry far more complex

2

S . OHASHI, Pure Appl. Chem. 4 4 , 4 1 5 - 3 8 ( 1 9 7 5 ) .

«* ηa Table 12.9

Some lower oxoacids of phosphorus (Superscript numerals in the abbreviated notation indicate oxidation states)

I A.

I ι Q. 5 S.


518

Phosphorus

than might otherwise appear. The corresponding phosphate salts are discussed in subsequent sections as also are the cyclic metaphosphoric acids ( H P 0 ) „ , the poly metaphosphoric acids ( H P 0 ) „ , and their salts. Orthophosphoric acid is a remarkable sub­ stance: it can only be obtained pure in the crystalline state (mp 42.35°C) and when fused it slowly undergoes partial self-dehydration to diphosphoric acid: 3

3

2H3PO4 ^

=± H 0 2

+ H4P2O7

The sluggish equilibrium is obtained only after several weeks near the mp but is more rapid at higher temperatures. This process is accompanied by extremely rapid autoprotolysis (see below) which gives rise to several further (ionic) species in the melt. As the concentration of these various species builds up the mp slowly drops until at equilibrium it is 34.6°, corresponding to about 6.5 Slow crystallization mole% of d i p h o s p h a t e . of stoichiometric molecular H3PO4 from this isocompositional melt gradually reverses the equilibria and the mp eventually rises again to the initial value. Crystalline H P 0 4 has a hydrogenbonded layer structure in which each PO(OH)3 molecule is linked to 6 others by Η bonds which are of two lengths, 253 and 284 pm. The shorter bonds link OH and 0 = P groups whereas the longer Η bonds are between 2 OH groups on adjacent molecules. (l29)

3

Ch. 12

readily supercools. At 45°C (just above the mp) the viscosity is 76.5 centipoise (cP) and this increases to 177.7cP at 25°. These values can be compared with 1.00 cP for H 0 at 20° and 24.5 cP for anhydrous H S Û 4 at 25°. As shown trideuterophosphoric acid has an in the T a b l e even higher viscosity and deuteration also raises the mp and density. 2

2

(129)

H P0

Property

3

MP/°C Density (25°C); supercooled/g c m η (25°C)/centipoise /c/ohnr cm1

D P0

4

3

42.35 1.8683

4

46.0 1.9083

- 3

177.5 231.8 4.68 χ 10~ 2.82 χ 10~

1

2

Property

2

H P0 ^H 0 3

MP/°C Density (25°C); supercooled/g c m η (25°C)/centipoise /c/ohm^cm1

4

2

29.30 1.7548 - 3

70.64 7.01 χ 10~

2

Despite this enormous viscosity, fused H3PO4 (and D3PO4) conduct electricity extremely well and this has been shown to arise from exten­ sive self-ionization (autoprotolysis) coupled with a proton-switch conduction mechanism for the °) H P 0 - ion/ 2

4

1 2 9

2H P0 3

13

H P0

4

4

4 +

+ H P0 2

(1)

4

In addition, the diphosphate group is also deprotonated: 2H3PO4

H 0 + H P 0 2

4

2

7

^=±H 0+ + H P 0 3

H3P2O7"

+ H P0 3

3

H P0

4

4

2

7

4 H

+ H P 0 2

3H P0

i.e.

3

4

H 0 3

+

+ H P0 4

2

4 +

+ H P 0 2

Extensive Η bonding persists on fusion and phosphoric acid is a viscous syrupy liquid that Ν . N. GREENWOOD and A . T H O M P S O N , 3 4 8 5 - 9 2 and 3 8 6 4 - 7 (1959).

J. Chem.

7 2

' (2)

At equilibrium, the concentration of H 0 and H P 0 7 ~ are each ^ 0 . 2 8 m o l a l and H P04 is ^ 0 . 2 6 m o l a l , thereby implying a 3

2

2

1 2 9

2

27

2

2

_

Soc. }

R . A . MUNSON, J. Phys. Chem. 6 8 , 3 3 7 4 - 7 ( 1 9 6 4 ) .

+

Oxoacids

§12.3.6

Figure 12.17

of phosphorus

and their

519

salts

Schematic representation of proton-switch conduction mechanism involving [H P0 ] phosphoric acid. 2

concentration of 0.54molal for H 4 P O 4 " . These values are about 2 0 - 3 0 times greater than the concentrations of ions in molten H 2 S O 4 , namely [ H S O 4 - ] 0.0178 molal, [ H S 0 ] 0.0135 molal and [ H S 2 O 7 - ] 0.0088 molal (see p. 711). Be­ cause of the very high viscosity of molten H 3 P O 4 electrical conduction by normal ionic migration is negligible and the high conductivity is due almost entirely to a rapid proton-switch followed by a relatively slow reorientation involving the H P 0 4 ~ ion, Η-bonded to the solvent structure (Fig. 1 2 . 1 7 ) . Note that the tetrahedral H P 0 + ion, i.e. [P(OH) ]+, like the N H + ion in liquid N H , does not contribute to the proton-switch conduction mechanism in H P U 4 because, having no dipole moment, it does not orient preferentially in the applied electric field; accordingly any proton switching will occur randomly in all directions independently of the applied field and therefore will not contribute to the electrical conduction. Addition of the appropriate amount of water to anhydrous H PC>4, or crystallization from a concentrated aqueous solution of syrupy phosphoric acid, yields the hemihydrate 2 Η Ρ θ 4 . Η 2 θ as a congruently melting compound (mp 29.3°). The crystal s t r u c t u r e shows the presence of 2 similar H P 0 molecules which, together with the H 0 molecule, are linked into 4

3

4 +

2

(129)

4

4

4

4

3

3

in molten

4

a three-dimensional Η-bonded network: each of the nine Ο atoms participates in at least 1 relatively strong O - H - Ο bond ( 2 5 5 - 2 7 2 p m ) and the interatomic distances P = 0 (149 pm) and P - O H (155 pm) are both slightly shorter than the corresponding distances in H P 0 4 . Hydrogen bonding persists in the molten compound, and the proton-switch conductivity is even higher than in the anhydrous acid (See Table on p. 518). In dilute aqueous solutions H P 0 behaves as a strong acid but only one of the hydrogens is readily ionizable, the second and third ionization constants decreasing successively by factors of M 0 (see p. 50). Thus, at 25°: 3

3

4

5

H P0 + H 0 3

4

H 0+ + H P0 ";

2

3

K = 7.11 χ 10~ ; }

3

H2PO4"

+ H 0

3

K = 6.31 χ 1 0 " ; 2

8

+ H 0

3

+ HP0 ~;

+

pK

4

= 2.15

{

4 2

= 7.20

2

H 0

2

2

pK H 0

2

HPO4 "

2

+

+ P0 "; 4 3

3

3

031}

3

4

2

A . D . MIGHELL, J. P . SMITH, Cryst. B 2 5 , 7 7 6 - 8 1 (1969).

1 3 1

and

W . E. BROWN,

Acta

K = 4.22 χ 1 0 ~ ; 3

13

pAT = 12.37 3

Accordingly, the acid gives three series of salts, e.g. N a H P 0 , N a H P 0 , and N a P 0 (p. 523). A typical titration curve in this system is shown in Fig. 12.18: there are three steps with two inflexions at pH 4.5 and 9.5. The first inflexion, corresponding to the formation of N a H P 0 4 , can be detected by an indicator such as methyl 2

4

2

4

3

4

2

Phosphorus

520

2

P . BECKER, Phosphates

and Phosphoric

Acid, Marcel Dekker, New York, 1988, 760 pp.

Ch.

12

Oxoacids

§12.3.6

of phosphorus

and their

521

salts

Figure 12.18 Neutralization curve for aqueous orthophosphoric acid. For technical reasons the curve shown refers to 10 cm of 0.1 M N a H P 0 titrated (to the left) with 0.1 M aqueous HC1 and (to the right) with 0.1 M NaOH solutions. Extrapolations to points corresponding to 0.1 M H3PO4 (pH 1.5) and 0.1 M N a P 0 (pH 12.0) are also shown. 3

2

3

4

4

orange (pK( 3.5) and the second, corresponding to Na2HPC>4, is indicated by the phenolphthalein end point (pKi 9.5). The third equivalence point cannot be detected directly by means of a coloured indicator. Between the two inflexions the pH changes relatively slowly with addition of NaOH and this is an example of buffer action.^ Indeed, one of the standard buffer solutions used in analytical chemistry comprises an equimolar mixture of Na2HP04 and KH2PO4. Another important buffer, which has been designed to have a pH close to that of blood, consists of 0.03043 M N a H P 0 and 0.008 695 M K H P 0 , i.e. a mole ratio of 3.5:1 (pH 7.413 at 25°).

the multimillion-tonne scale for the production of phosphate fertilizers and for many other purposes (see Panel). Two main processes (the so-called "thermal" and "wet" processes) are used depending on the purity required. The "thermal" (or "furnace") process yields concentrated acid essentially free from impurities and is used in applications involving products destined for human consumption (see also p. 524); in this process a spray of molten phosphorus is burned in a mixture of air and steam in a stainless steel combustion chamber:

Concentrated H3PO4 is one of the major acids of the chemical industry and is manufactured on

Acid of any concentration up to 84 wt% P 4 O 1 0 can be prepared by this method (72.42% P4O10 corresponds to anhydrous H3PO4) but the usual commercial grades are 7 5 - 8 5 % (expressed as anhydrous H3PO4). The hemihydrate (p. 518) corresponds to 91.58% H3PO4 (66.33% P4O10). The somewhat older "wet" (or "gypsum") process involves the treatment of rock phosphate (p. 476) with sulfuric acid, the idealized stoichiometry being: ^

2

4

2

' A buffer solution is one that resists changes in pH on dilution or on addition of acid or alkali. It consists of a solution of a weak acid (e.g. H P 0 ~ ) and its conjugate base ( H P 0 ~ ) and is most effective when the concentration of the two species are the same. For example at 25° an equimolar mixture of N a H P 0 and K H P 0 has pH 6.654 when each is 0.2 M and pH 6.888 when each is 0.01M. The central section of Fig. 12.18 shows the variation in pH of an equimolar buffer of N a H P 0 and N a H P 0 at a concentration of 0.033 M (you should check this statement). Further discussion of buffer solutions is given in standard textbooks of volumetric analysis. 2

4

2

4

2

4

2

2

4

4

2

6Η θ > 2

4

P + 50 4

> P4O10

2

4H P0 3

4

4

C a ( P 0 ) F + 5 H S 0 + 10H O 5

4

3

2

4

• 3H P0

2

3

4

+ 5 C a S 0 . 2 H 0 + HF 4

2

Figure 12.19

Ch. 12

Phosphorus

522

The composition of the strong phosphoric acids shown as the weight per cent of P2O5 present in the form of each acid plotted against the overall stoichiometric composition of the mixture. The overall stoichiometrics corresponding to the three congruently melting species Η Ρ 0 . | Η 0 , H P 0 and H4P2O7 are indicated. Compositions above 82 wt P 0 are shown on an expanded scale in the inset using the mole ratio [P20 ]/[H 0] as the measure of stoichiometry. (For comparison, H P 0 corresponds to a mole ratio of 0.500, H P Oio to a ratio 0.600, H P 0 i to 0.667, etc.). In both diagrams the curves labelled 1,2,3, . . . refer to ortho-, di-, tri- . . . phosphoric acids, and "highpoly" refers to highly polymeric material hydrolysed from the column. 3

2

5

4

2

3

4

5

2

4

5

The gypsum is filtered off together with other insoluble matter such as silica, and the fluorine is removed as insoluble Na2SiF6- The dilute phosphoric acid so obtained (containing 3 5 - 7 0 % H3PO4 depending on the plant used) is then concentrated by evaporation. It is usually dark green or brown in colour and contains many metal impurities (e.g. Na, Mg, Ca, Al, Fe, etc.) as well as residual sulfate and fluoride, but is suitable for the manufacture of phosphatic fertilizers, metallurgical applications, etc. (see Panel on p. 520). Diphosphoric acid H4P2O7 becomes an increasingly prevalent species as the system P4O10/H2O becomes increasingly concentrated: indeed, the phase diagram shows that, in addition to the hemihydrate (mp 29.30°) and orthophosphoric acid (mp 42.35°) the only other congruently melting phase in the system is H4P2O7. The compound is dimorphic with a metastable modification mp 54.3° and a stable form mp 71.5°, but in the molten state it comprises an isocompositional mixture of various polyphosphoric acids and their autoprotolysis

3

6

4

2

7

3

products. Equilibrium is reached only sluggishly and the actual constitution of the melt depends sensitively both on the precise stoichiometry and For the nominal the temperature (Fig. 1 2 . 1 9 ) stoichiometry corresponding to H4P2O7 typical concentrations of the species Η 2 Ρ Κ 0 „ ι from η = 1 (i.e. H3PO4) to η = 8 are as follows: (133)

Π +

η mole%

35.0

1 2 3 4 42.6 14.6 5.0

5 1.8

3

6 0.7

+

7 0.3

8 0.1

Thus, although H 4 P 2 O 7 is marginally the most abundant species present, there are substantial amounts of H 3 P O 4 , H P O i , H P 0 I and higher polyphosphoric acids. Note that the table indicates mole% of each molecular species present whereas the graphs in Fig. 12.19 plot weight percentage of P 2 O 5 present as each acid shown. In dilute aqueous solution H 4 P 2 O 7 is a somewhat stronger acid than H 3 P O 4 : the 4 dissociation constants at 25° are: K\ ~ 1 0 , 5

3

0

6

4

3

- 1

R. F . JAMESON, J. Chem. Soc. 7 5 2 - 9 (1959).

§12.3.6

Oxoacids

Table 12.10 Factor

of phosphorus

and their salts

Factors affecting the rate of polyphosphate degradation

Effect on rate

Temperature PH Enzymes Colloidal gels

10 10 Up Up

5

3

6

- 1 0 faster - 1 0 faster to 1 0 - 1 0 to 1 0 - 1 0 4

5

6

4

5

from 0° to 100° from base to acid faster faster

2

7

K - 1.5 χ 1 0 " , K 2.7 χ 1 0 ' and K 2.4 χ 1 0 ~ , and the corresponding negative logarithms are: pK ~ 1.0, pK ~ 1.8, pK 6.57 and pK 9.62. The Ρ — Ο — Ρ linkage is kinetically stable towards hydrolysis in dilute neutral solutions at room temperature and the reaction half-life can be of the order of years. Such hydrolytic breakdown of polyphosphate is of considerable importance in certain biological systems and has been much studied. Some factors which affect the rate of degradation of polyphosphates are shown in Table 12.10. 2

3

4

10

{

2

523

3

4

Factor

Effect on rate

Complexing cations Concentration Ionic environment in solution

Often much faster Roughly proportional Several-fold change

mono- and di-sodium phosphates are prepared industrially by neutralization of aqueous H P 0 with soda ash (anhydrous N a C 0 , p. 89). How­ ever, preparation of the trisodium salts requires the use of the more expensive NaOH to replace the third H atom. Careful control of concen­ tration and temperature are needed to avoid the simultaneous formation of pyrophosphates (diphosphates). Some indication of the struc­ tural complexity can be gained from the com­ pound N a P 0 . 1 2 H 0 which actually crystallizes with variable amounts of NaOH up to the lim­ iting composition 4 ( N a P 0 . 1 2 H 0 ) . N a O H . The structure is built from octahedral [ N a ( H 0 ) 6 ] units which join to form "hexagonal" rings of 6 octahedra which in turn form a continuous two-dimensional network of overall composition { N a ( H 0 ) } ; between the sheets lie { P 0 } con­ nected to them by H b o n d s . Some industrial, domestic, and scientific applications of Na, Κ and N H orthophosphates are given in the Panel. 3

2

3

4

4

3

2

3

4

2

2

Orthophosphates

(23,64)

Phosphoric acid forms several series of salts in which the acidic H atoms are successively replaced by various cations; there is considerable commercial application for many of these compounds. Lithium orthophosphates are unimportant and differ from the other alkali metal phosphates in being insoluble. At least 10 crystalline hydrated or anhydrous sodium orthophosphates are known and these can be grouped into three series: N a P 0 . r t H 0 (n = 0, i , 6, 8, 12) N a H P 0 . n H 0 (n = 0, 2, 7, 8, 12) N a H P 0 . n H 0 (n = 0, 1, 2), N a H P 0 . H P 0 [i.e. N a H ( P 0 ) ] N a H P 0 . N a H P 0 [i.e. N a H ( P 0 ) ] and 2 N a H P 0 . N a H P 0 . 2 H 0 3

4

2

2

4

2

2

4

2

2

4

2

4

3

4

2

2

5

4

4

4

3

2

4

3

2

4

2

2

2

4

(134)

4

Calcium orthophosphates are particularly important in fertilizer technology, in the chem­ istry of bones and teeth, and in innumerable industrial and domestic applications (see Panel). They are also the main source of phospho­ rus and phosphorus chemicals and occur in vast deposits as apatites and rock phosphate (p. 475). The main compounds occurring in the C a O - H 0 - P O i o phase diagram are: Ca(H P0 ) , Ca(H P0 ) .H 0, Ca(HP0 ).nH 0 (n = 0, i , 2), C a ( P 0 ) , C a P 0 ( O H ) . 2 H 0 , C a ( P 0 ) O H (i.e. apatite), C a P 0 [probably C a ( P 0 ) . C a O ] and C a H ( P 0 ) . 5 H 0 . In all of these alkali-metal and alkaline earth-metal orthophosphates there are discrete, approximately regular tetrahedral P 0 units in 2

2

4

2

4

2

4

3

5

3

Likewise, there are at least 10 well-characterized potassium orthophosphates and several ammo­ nium analogues. The presence of extensive H bonding in many of these compounds leads to considerable structural complexity and frequently confers important properties (see later). The

4

4

4

2

4

2

4

2

2

3

2

4

4

8

2

4

2

2

2

9

6

2

4

1 3 4

E. TILLMANNS and W. H . BAUR, Inorg. Chem. 9 , 1 9 5 7 - 8 (1970).

524

Phosphorus

Ch. 12

Oxoacids

of phosphorus

and their

salts

Ch.

Phosphorus

526

and sheet aluminium phosphate anions of com­ position [ H 2 A I P 2 O 8 ] and [ A i 5 P 0 i 6 ] ~ , respec­ tively, have also recently been structurally characterized/

which P - 0 is usually in the range 153 ± 3 p m and the angle Ο - P - 0 is usually in the range 109 ± 5°. Extensive Η-bonding and M - 0 inter­ actions frequently induce substantial deviations from a purely ionic formulation (p. 81). This trend continues with the orthophosphates of tervalent elements Μ Ρ 0 (Μ = Β, Al, Ga, Cr, Mn, Fe) which all adopt structures closely related to the polymorphs of silica (p. 342). N a B e P 0 is similar, and Y P 0 adopts the zir­ con ( Z r S i 0 ) structure. The most elaborate anal­ ogy so far revealed is for A 1 P 0 which can adopt each of the 6 main polymorphs of sil­ ica as indicated in the scheme below. The analogy covers not only the structural relations between the phases but also the sequence of transformation temperatures (°C) and the fact that the α -β-transitions occur readily whilst the others are sluggish (p. 343). Similarly, the orthophosphates of B, Ga and Mn are known in the ^-quartz and the a- and β-cristobalite forms whereas F e P 0 adopts either the a- or ^-quartz structure. Numerous hydrated forms are also known. The A l - P 0 - H 2 0 system is used industrially as the basis for many adhesives, binders and c η m e n t s . Novel chain Π Ι

4

3

136)

Chain polyphosphates^

4

4

12

3

64)

A rather different structure-motif is observed in the chain polyphosphates: these feature cornershared { P 0 } tetrahedra as in the polyphosphoric acids (p. 522). The general formula for such ~, of which the anions is [ P , i 0 3 + i ] diphosphates, P 2 O 7 " " , and tripolyphosphates, P 3 O 1 0 " " , constitute the first two members. Chain polyphosphates have been isolated with η up to 10 and with η "infinite", but those of intermediate chain length (10 < η < 50) can only be obtained as glassy or amorphous mixtures. As the chain length increases, the ratio (3n + I)/η approaches 3.00 and the formula approaches that of the polymetaphosphates [ P 0 3 ~ ] o o .

4

4

4

4

r t

( n + 2 )

4

5

4

Diphosphates (pyrophosphates) are usually prepared by thermal condensation of dihydrogen

4

(135)

1 3 6

1 3 5

J. H . MORRIS,

P . G . PERKINS,

A . E . A . ROSE

J . M . THOMAS et al., J. Chem. Soc, Chem.

1170-2

and

(1992),

R . KNIEP, Angew.

W . E . SMITH, Chem. Soc. Revs. 6 , 1 7 3 - 9 4 ( 1 9 7 7 ) .

SiQ

and

1266-8

tndymite

v

^

v

(1992).

1713° cnstobalite

^ men

2

573°

117°

163°

220°

705° berlinite

1025 ^

tridymite-form

ν

Commun., See

also

Chem. int. Edn. Engl. 2 5 , 5 2 5 - 3 4 ( 1 9 8 6 ) .

1470°

867° quartz

929-31

r

^ cristobahte-form

>1600 ^ melt

§12.3.6

Oxoacids

of phosphorus

phosphates or hydrogen phosphates:

and their

salts

527

150.1pm (cf. 145.3 pm in H 0 and 1 4 8 - 1 5 0 p m in S 0 " ) . As diphosphoric acid is tetrabasic, four series of salts are possible though not all are always known, even for simple cations. The most studied are those of Na, K, N H and Ca, e.g.: 2

2

2

2

2MH P0 2

M2H2P2O7 +

4

2M2HPO4

M P 0 4

2

+

7

H 0 2

H 0 2

5

4

They can also be prepared in specialized cases by (a) metathesis, (b) the action of H3PO4 on an oxide, (c) thermolysis of a metaphosphate, (d) thermolysis of an orthophosphate, or (e) reductive thermolysis, e.g.:

N a 4 P O . 1 0 H O ( m p 79.5°), N a P 0 ( m p 985°) 2

7

2

4

3

Na HP 0 .9H 0 3

2

7

2

35

°~ >

2

7

Na HP 0 .H 0 3

2

7

2

150°

> Na HP 0 3

(a) N a 4 P 0 + 4 A g N 0 2

7

3

> A g P 0 | + 4NaN0 4

(b) 2 H P 0 + P b 0

2

(c)

4Cr(P0 )

3

(d)

2Hg (P0 )

2

(e)

2FeP0 + H

3

4

3

3

4

4

2

7

Na H P 0 .6H 0 2

2

7

2

3

7

3

2

5

2 H g P 0 + 2Hg+ 0 2

2

7

• Fe P 0 + H 0 2

2

7

2

ι ν

7

7

4

2

2

7

4

( l 3 7 )

4

4

2

2

2

7

2

7

4

μ

2

7

2

t

(,38)

4

3

μ

G. M . CLARK 2 6 9 - 9 5 (1976).

3

t

1 3 8

and R . MORLEY,

Chem.

Soc. Revs. 5 ,

W . P . GRIFFITH, R . D . POWELL and A . C . SKAPSKI,

hedron 7 , 1 3 0 5 - 1 0 (1988).

7

• Na H P 0

2

2

2

2

7

2

7

Before the advent of synthetic detergents, N a P 0 was much used as a dispersant for lime soap scum which formed in hard water, but it has since been replaced by the tripolyphosphate (see below). However, the ability of diphosphate ions to form a gel with soluble calcium salts has made N a P 0 a useful ingredient for starchtype instant pudding which requires no cooking. The main application of N a H P 0 is as a leavening acid in baking: it does not react with N a H C 0 until heated, and so large batches of dough or batter can be made up and stored. C a P 0 , because of its insolubility, inertness, and abrasive properties, is used as a toothpaste additive compatible with Sn and fluoride ions (see Panel on p. 525). 4

2

2

Poly­

7

4

2

7

2

2

2

7

3

2

2

7

11

Of the tripolyphosphates only the sodium salt need be mentioned. It was introduced in the mid19408 as a "builder" for synthetic detergents, and its production for this purpose is now measured in mιgatonnes per annum (see Panel on the next page). On the industrial scale Na5P Oio is usually made by heating an intimate mixture of pow­ dered N a H P 0 and N a H P 0 of the required stoichiometry under carefully controlled condi­ tions: 3

2

4

2

2Na HP0 + N a H P 0 2

1 3 7

2

N a H P 0 ( m p 185°)

Cr (P 0 ) + 3P 0 4

2

2

Many diphosphates of formula Μ Ρ 2 θ , M P 0 and hydrated Μ Ρ 0 are known and there has been considerable interest in the relative orientation of the two linked { P 0 } groups and in the P - O - P angle between t h e m . For small cations the 2 { P 0 } are approximately staggered whereas for larger cations they tend to be nearly eclipsed. The P - O - P angle is large and variable, ranging from 130° in N a P O . 1 0 H O to 156° in a - M g P 0 . The apparent colinearity in the higher-temperature (β) form of many diphosphates, which was previously ascribed to a P - O - P angle of 180°, is now generally attributed to positional disorder. Bridging P - 0 distances are invariably longer than terminal P - 0 distances, typical values (for N a P O . 1 0 H O ) being Ρ - Ο 161pm, P - O 152 pm. Note that bridging can also be via a peroxo group as in ammonium p e r o x o d i p h o s p h a t e which features the zig-zag anion [ 0 P - 0 - 0 - P 0 ] ~ with Ρ - Ο 165.8 pm, P - O 150.8 pm and 0 - 0 2

7

3

• PbP 0 | + 3H 0

2

2

-27°

4

2

4

4

• Na P O 5

3

10

+ 2H 0 2

The low-temperature form (I) converts to the high-temperature form (II) above 417°C and both forms react with water to give the crystalline hexahydrate. All three materials contain the

Ch. 12

Phosphorus

528

tripolyphosphate ion P 0 i o ~ with a trans­ configuration of adjacent tetrahedra and a twofold symmetry axis; forms (I) and (II) differ mainly in the coordination of the sodium ions and the slight differences in the dimensions of the ion in the three crystals are probably within experimental error. Typical values are: 3

5

The stoichiometric formula of a chainpolyphosphate can sometimes be an unre­ liable guide to its structure, for example, the crystalline compound " C a N b P 6 0 i " has been shown by X-ray crystal structure anal­ ysis to contain equal numbers of oxide(2—), diphosphate(4—) and tetraphosphate(6—) anions, By contrast, i.e. C a N b 0 [ P 0 7 ] [ P 4 0 i 3 ] . (M = V, Fe) does contain the CsM P 0i anticipated homologous catenfl-pentaphosphate [Ρ 0 „+ι] - anion (p. 512) with η = 5 . Long-chain polyphosphates, M ^ P 0 , approach the limiting composition M P 0 as η - » oo and are sometimes called linear metaphosphates to distinguish them from the cyclic metaphosphates of the same composition (p. 529). Their history extends back over 150 y to the time when Thomas Graham described the formation of a glassy sodium polyphosphate mixture now known as Graham's salt. Various heat treatments converted this to crystalline compounds known as Kurrol's salt, Maddrell's salt, etc., and it is now appreciated, as a result of X-ray crystallographic studies, that these and many related substances all feature unbranched chains of corner-shared {PO4} units which differ only in the mutual orientations and 2

2

2

η

3

5

2

2

(142)

6

( η + 2 )

( 1 4 3 )

+ 2

w

!

The complicated solubility relations, rates of hydrolysis, self-disproportionation and intercon­ version with other phosphates depends sensitively on pH, concentration, temperature and the pres­ ence of impurities. Though of great inter­ est academically and of paramount importance industrially these aspects will not be further con­ Triphosphates such as sidered h e r e . ' adenosine triphosphate (ATP) are also of vital importance in living organisms (see text books on biochemistry, and also ref. 141). (139)

( 1 1

2 3 , 6 4 l 4 0 )

I. S . KULAEV, The Biochemistry of Inorganic phates, Wiley, Chichester, 1980, 225 pp. 1 4 1

1 3 9

G . P . HAIGHT, T . W . HAMBLEY, P . HENDRY, G . A . LAW-

RANCE and A . M . SARGESON, J. Chem. Soc, Chem. 4 8 8 - 9 1 ( 1 9 8 5 ) , and references cited therein. 1 4 0

Commun.,

E . J. GRIFFITH, Pure Appl. Chem. 4 4 , 1 7 3 - 2 0 0 ( 1 9 7 5 ) .

3 w + 1

3

Polyphos­

M . - T . AVERBUCH-POUCHOT, Ζ anorg. allg. Chem. 5 4 5 , 1 1 8 - 2 4 (1987). B . KLINKERT and M . JANSEN, Z. anorg. allg. Chem. 5 6 7 , 8 7 - 9 4 (1988).

1 4 2

1 4 3

§12.3.6

Figure 12.20

Oxoacids

of phosphorus

529

and their salts

Types of polyphosphate chain configuration. The diagrams indicate the relative orientations of adjacent P 0 tetrahedra, extended along the chain axes, (a) (RbP0 )„ and (CsP0 )„, (b) (LiP0 ),, low temp, and (KP0 )„, (c) (NaP0 )„ high-temperature Maddrell salt and [Na H(P0 ) ]„, (d) [Ca(P0 ) ]„ and [Fb(P0 ) ]„, (e) (NaP0 )„, Kurrol A and (AgP0 )„, (f) (NaP0 )„, KUITOI B, (g) [CuNH. (P0 ) ]„ and isomorphous salts, (h) [CuK (P0 ) ]„ and isomoφhous salts. Each crystalline form of Kurrol salt contains equal numbers of right-handed and left-handed spiralling chains. 4

3

3

3

2

3

+

3

2

3

μ

t

μ

t

t

The complex preparative interrelationships occurring in the sodium polyphosphate sys­ tem are summarized in Fig. 12.21 (p. 531). Thus anhydrous NaH PC>4, when heated to 170° under conditions which allow the escape of water vapour, forms the diphosphate N a H P U 7 , and further dehydration at 250° yields either Maddrell's salt (closed system) or the cyclic trimetaphosphate (water vapour pressure kept low). Maddrell's salt converts from the lowtemperature to the high-temperature form above 300°, and above 400° reverts to the cyclic 2

2

J. MALING and F . HANIC, Topics in Phosphorus 1 0 , 3 4 1 - 5 0 2 (1980).

3

2

(144)

2

3

2

3

repeat units of the constituent t e t r a h e d r a . These, in turn, are dictated by the size and coordination requirements of the counter cations present (including H). Some examples are shown schematically in Fig. 12.20 and the geometric resemblance between these and many of the chain metasilicates (p. 350) should be noted. In most of these polyphosphates Ρ - Ο is 161 ± 5 pm, P - O 1 5 0 ± 2 p m , Ρ - Ο - Ρ 125-135° and O - P - O 115-120° (i.e. very similar to the dimensions and angles in the tripolyphosphate ion, p. 528).

1 4 4

3

3

3

3

3

3

4

trimetaphosphate. The high-temperature form can also be obtained (via Graham's and Kurrol's salts) by fusing the cyclic trimetaphosphate (mp 526°C) and then quenching it from 625° (or from 580° to give Kurrol's salt directly). All these linear polyphosphates of sodium revert to the cyclic trimetaphosphate on prolonged annealing at - 4 0 0 ° C . Fuller treatments of the phase relations and structures of polyphosphates, and their uses as glasses, ceramics, refractories, cements, plasters and abrasives, are a v a i l a b l e / 1 4 4 , 1 4 5 )

Cyc\o-polyphosphoric acids and cyclopolyphosphates (146)

2

Chemistry

These compounds were formerly called meta­ phosphoric acids and metaphosphates but the IUPAC cyclo- nomenclature is preferred as being structurally more informative. The only A . E . R . WESTMAN, Topics in Phosphorus Chemistry 9, 2 3 1 - 4 0 5 , 1977. A comprehensive account with 963 refe­ rences. S . Y . KALLINEY, Topics in Phosphorus Chemistry 7 , 2 5 5 - 3 0 9 , 1972.

1 4 5

1 4 6

two important acids in the series are cyclo triphosphoric acid H3P3O9 and cyc/0-tetraphosphoric acid H4P4O12, but well-characterized salts are known with heterocyclic anions [cyclo( Ρ 0 ) Γ - (n = 3 - 8 , 1 0 ) , and larger rings are undoubtedly present in some mixtures. The structural relationship between the cyclo phosphates and P4O10 (p. 504) is shown schematically below. In P O all 10 P - O ( - P ) bridges are equivalent and hydrolytic cleavage of any one leads to "Η2Ρ4Ο1Γ in which P - O ( - P ) bridges are now of two types. Cleavage of "type a" leads to cryc/<9-tetraphosphoric acid or its salts (as shown in the upper line of the scheme), whereas cleavage of any of the other bridges leads to a cyclo -triphosphate ring with a pendant - O P ( 0 ) O H group which can subsequently be hydrolysed off to leave (HP03)3 3

η

(147)

4

1 4 7

Ch. 12

Phosphorus

530

1 0

U. SCHULKE, M . T . AVERBUCH-POUCHOT and A . DURIF, Ζ

anorg. allg. Chem. 6 1 2 , 1 0 7 - 1 2 (1992).

or its salts (lower line of scheme). Cyclo-(HPO?)* can, indeed, be made by careful hydrolysis of hexagonal P 4 O 1 0 with ice-water, and similar treatment with iced NaOH or NaHC03 gives a 7 5 % yield of the corresponding salt cyclo(NaP03)4. The preparation of cyclo-(NaP03)3 by controlled thermolytic dehydration of NaH PÛ4 was mentioned in the preceding section and acidification yields cyclo-triphosphoric acid. The cyc/
3

4 _

3

2 4

Phosphorus-nitrogen

§12.3.7

Figure 12.21

Interrelationship of metaphosphates. (From CIC, Vol. 2, p. 521.)

chromatographic separation from Graham's salt in which they are present to the extent of ~ 1 % .

12.3.7

531

compounds

Phosphorus-nitrogen compounds

The P - N bond is one of the most intriguing in chemistry and many of its more subtle aspects

still elude a detailed and satisfactory description. It occurs in innumerable compounds, frequently of great stability, and in many of these the strength of the bond and the shortness of the interatomic distance have been interpreted in terms of "partial double-bond character". In fact, the conventional symbols P - N and P = N are more an aid to electron counting than a description of the bond in any given compound (see p. 538). Many compounds containing the P - N link can be considered formally as derivatives of the oxoacids of phosphorus and their salts (pp. 5 1 0 - 3 1 ) in which there has been isoelectronic replacement of: PH [or P(OH)]

by P ( N H )

or P ( N R ) ;

P = 0 [or P = S ]

by P = N H

or P = N R ;

2

2

532

Phosphorus

P-O-P

byP-NH-P

or P - N R - P , etc>

(148,149)

Examples are phosphoramidic acid, H N P ( 0 ) (OH) ; phosphordiamidic acid, ( H N ) P ( 0 ) ( O H ) ; phosphoric triamide, (H N)3PO; and their derivatives. There are an enormous number of compounds featuring the 4-coordinate group shown in structure (1) including the versatile nonaqueous solvent hexamethylphosphoramide (Me N>3PO; this is readily made by reacting POCI3 with 6Me NH, and dissolves metallic Na to give paramagnetic blue solutions similar to those in liquid N H 3 (p. 77). 2

2

2

2

2

2

2

Ch, 12

which features a short interatomic P - N distance and a planar Ν atom as indicated in the dia­ gram below. (In the absence of this additional π bonding the P - N single-bond distance is close to 177 pm.) Again, the proton nmr of such compounds sometimes reveals restricted rota­ tion about P - N at low temperatures and typical energy barriers to rotation (and coalescence tem­ peratures of the non-equivalent methyl proton signals) are P C l ( N M e ) S S k J m o r (-120°), P ( C F ) ( N M e ) S S k J m o l " (-120°), PClPh(NMe ) S O k J m o r ( - 5 0 ° ) . m

2

3

2

2

1

2

1

2

1

OH (2)

(1)

OH

[ —

N

=

P

—

]„

OH

Other unusual P/N systems which have recently been investigated include the crystalline com­ pound H P N , i.e. PN(NH), which is formed by ammonolysis of P 3 N 5 at 580°C and which has a β-cristobalite ( S i 0 ) type structure; PNO (cf. N 0 ) , which can be studied as a matrix-isolated s p e c i e s ; various phosphine azides, R R ' P N 3 , and their r e a c t i o n s ; and numerous substituted phosphonyl triphenylphosphazenes, P h P = N - P X , ( X = C 1 , F, OPh, SEt, NEt , e t c . ) . The iminophosphenium ion, [ A r N = P ] + (Ar = 2,4,6-Bu' C6H ) has been obtained as its pale yellow AlCU" salt by reac­ tion of the corresponding covalently bonded chlo­ ride, A r N = P C l , with A I C I 3 ; the ion is notable 2

(3)

2

Another series includes the cyc/0-metaphosphimic acids, which are tautomers of the cyclopolyphosphazene hydroxides (p. 541). Similarly, halogen atoms in P X or other P - X compounds can be successively replaced by the isoelectronic groups - N H , - N H R , - N R , etc., and some­ times a pair of halogens can be replaced by = N H or = N R . These, in turn, can be used to prepare a large number of other derivatives as indicated schematically opposite for P ( N M e ) 3 . Although such compounds all formally con­ tain P - N single bonds, they frequently display properties consistent with more extensive bond­ ing. A particularly clear example is P F ( N M e ) 3

2

2

2

(2)

2

2

(150)

2

(151)

(152)

3

2

2

(153)

3

1 5 0

2

W. SCHNICK and J. LOCKE, Z. anorg. allg. Chem. 6 1 0 ,

1 2 1 - 6 (1992). 1 5 1

R . AHLRICHS, S . SCHUNK and H . - G . SCHNΤCKEL ,

Angew.

Chem. Int. Edn. Engl. 2 7 , 4 2 1 - 2 ( 1 9 8 8 ) . 1 5 2

1 4 8

D . A. PALGRAVE, Section 2 8 , pp. 7 6 0 - 8 1 5 , in ref. 2 6

M . L . NIELSEN, Chap. 5 in C . B . COLBURN (ed.), Develop­ ments in inorganic Nitrogen Chemistry, Vol. 1, pp. 3 0 7 - 4 6 9 , Elsevier, Amsterdam, 1 9 6 6 . 1 4 9

J. BOSKE,

J.-P.

E . NIECKE,

E . OCANDO-MAVEREZ,

MAJORAL and G . BERTAND, Inorg.

Chem.

25, 2695-8

allg.

Chem. 6 0 4 ,

(1986). 1 5 3

L . RIESEL and R . FRIEBE, Z. anorg.

85-91

(1991).

Phosphorus-nitrogen

§12.3.7

as the first stable species having a P = N triple bond ( P - N 148 pm, angle C - N - P 1 7 7 ° ) . The coordination chemistry of phosphorane iminato complexes (containing the R P N ~ ligand) of transition metals has been r e v i e w e d . (154)

3

(155)

phosph(III)azane dimers are also known, e.g. (RPNR') . A more complex example, contain­ ing fused heterocycles of alternating P and Ν atoms, is the interesting hexamethyl deriva­ tive P4(NMe)6 mp 122°. This stable compound (Fig. 12.23a) is readily obtained by reacting PC1 with 6 M e N H ; it is isoelectronic with and isostructural with P4O6 (p. 504) and undergoes many similar reactions. The stoichiometrically similar compound P4(NPr')6 can be prepared in the non-adamantane-type structure shown in Fig. 12.23b, though it converts to structure-type A a on being heated at 157° for 12 d a y s . different sequence of atoms occurs in P (NMe)6 2

m

3

Cyclophosphazanes Many heterocyclic compounds contain for­ mally single-bonded P - N groups, the simplest being the cyc/o-diphosphazanes ( X P N R ) and {X(0,S)PNR} . These contain P and have the structures shown in Fig. 12.22. A few 3

2

2

v

533

compounds

2

(156)

2

1 5 4

E. NIECKE, M. NIEGER and F. REICHERT Angew. Chem. Int.

Edn. 1 5 5

Engl. 2 7 , 1 7 1 5 - 6

K . DEHNICKE

(1989).

and

(1988). J. STRHLE,

1 5 6

Polyhedron

8,

707-26

O . J. SCHERER, K . ANDRES, C . KRUGER, Y . - H . TSAY and

G . WOLMERSHUSER, Angew. 571-2

(1980).

Chem.

Int. Edn. Engl. 19,

534

Figure 12.22

Structures of (a) (Cl PNMe) , and (b) {Cl(S)PNMe} . Note the difference in length of the axial P - N and equatorial P - N bonds (and of the axial and equatorial Ρ-CI bonds) about the trigonal bipyramidal Ρ atoms in (a). 3

2

2

Structures of (a) P (NMe) , (b) P (NPr') , and (c) P (NMe) (see text).

Figure 12.23

4

6

(Fig. 12.23c) and many other "saturated" heteroor P have been cycles featuring either P made. A typical example, made by slow addi­ tion of PC1 to P h N H in toluene at 0°, is [PhNHP (NPh) ] NPh; the crystal structure of the 1:1 solvate of this compound with C H C 1 (mp 250°) reveals that all Ν atoms are essen­ tially planar with distances to Ρ as indicated in the following d i a g r a m / m

3

2

4

6

2

6

v

2

2

2

2

2

157)

1 5 7

Ch. 12

Phosphorus

M. L. THOMPSON,

R . C. HALTIWANGER,

and

A. D.

MAN, J. Chem. Soc, Chem. Commun., 6 4 7 - 8 ( 1 9 7 9 ) .

NOR­

Phosphazenes Formally "unsaturated" PN compounds are called phosphazenes and contain P in the v

§12.3.7

Phosphorus-nitrogen

grouping —-p P = N - . A few phosph(III)azenes

535

compounds

solvent

3PC1 + NH4CI

• 4HC1

5

are also known. Phosphazenes can be classified into monophosphazenes (e.g. X 3 P = N R ) , diphosphazenes (e.g. X P = N - P ( 0 ) X 2 ) , polyphosphazenes containing 2,3,4,... oo - X 2 P = N - units, and the cyclo-polyphosphazenes [-X2P=N-] , η = 3,4,5 . . . 17. Monophosphazenes, particularly those with organic substituents, R 3 P = N R ' , derive great interest from being the Ν analogues of (p. 545). They phosphorus ylides R P = C R were first made by H. Staudinger in 1919 by reacting an organic azide such as P h N 3 with P R ( R = C 1 , OR, N R , Ar, etc.), e.g.:

+ [C1 P==N-PC1 ] PC1 -; 3

3

+

mp 310°

6

NH4CI

3

4HC1 +

[Cl P==N-PCl -=N-PCl ] Cr 3

3

2

+

M

3

2

3

> N + P h P = N P h ; mp 132°

3

2

CH2CI2 or SO2 solution, e.g.:

2

3

PPh + PhN

The inverse of these compounds are the phosphadiazene cations, prepared by halide ion abstraction from diaminohalophosphoranes in

3

More recently they have been made via a reac­ tion associated with the name of Α. V. Kirsanov (1962), e.g.:

(R N) PC1 + AICI3 2

3

2

2

• P h P = N P h + 2HC1 3

As expected, the P - N distance is short and the angle at Ν is M 20°, e.g. (a) and (b) above. Over 600 such compounds are now known, especially those with the C 1 P = N — g r o u p . Diphosphazenes can be made by reacting PCI5 with NH4CI in a chlorohydrocarbon solvent under mild conditions: 3

1 5 8

M . BERMANN, Topics in Phosphorus

1972.

(158)

Chemistry

7, 311 - 78,

2

2

+

4

An X-ray crystal structure of the Pr^N-derivative shows the presence of a bent, 2-coordinate Ρ atom, equal P - N distances, and accurately pla­ In nar 3-coordinate Ν atoms as in (c) a b o v e . liquid ammonia ammonolysis also occurs: (159)

2PC1 + 16NH (liq.) 5

•

3

[ ( H N ) P = N - P ( N H 2 ) 3 ] C r + 9NH C1 2

Ph PCl + PhNH

• [(R N) P] [A1C1 ]-

2

3

+

4

The P = N and P - N bonds are equivalent in these compounds and they could per­ haps better be written as [ X P — N — P X ] + , etc. Like the parent phosphorus pentahalides (p. 498), these diphosphazenes can often exist in ionic and covalent forms and they are part of a more extended group of compounds which can be classified into several general series C1(C1 PN)„PC1 , [ C l ( C l P N ) P C l ] C r , 3

2

1 5 9

A. H. COWLEY,

J. Am.

Chem. Soc.

4

M. C . CASHNER 100, 7 7 8 4 - 6 (1978).

3

2

and

n

3

+

J. S. SZOBOTA,

536

Cl

Cl

I Cl

Cl

l

Cl

I

P=N—PC1 Cl Cl

Cl

2

Cl

I

I

P=N—P=N—PCl Cl Cl Cl

(a) n

3

6

2

5

2

Cl

Cl

Cl

I

I

I

I

P=N—P=N—P=N—PC1 Cl Cl Cl Cl

n

2

2

2

(c)

+

2

Cl

2

Cl

(b)

[Cl(Cl PN) PCl ] PCl -, Cl(Cl PN) POCl , etc., where n = 0, 1, 2, 3 . . . . Some examples of the first series are PC1 (i.e. η = 0), P N C 1 (a), P N C 1 (b), and P4N3CI11 (c) (above). Some of these can exist in the ionic form rep­ resented by the second series (d): 3

Ch. 12

Phosphorus

preparation, structure, applications/ ' ' 2

8

bonding

and potential

1 6 0 1 6 1 )

7

9

α

ci

1

-

+

[ P - C l ]

ci

I [ C

2

I

l(_p=N-) P-Cl] PCl " 4

6

2

ci

ci

n

5

+

Cl

Preparation and structure. Polyphosphazenes have a venerable history. ( N P C l ) oligomers were first made in 1834 by J. von Liebig and F. Wτhler who reacted PC1 with N H , but their stoichiometry and structure were not elucidated until much later. The fluoro analogues ( N P F ) were first made in 1956 and the bromo compounds (NPBr )„ in 1960. The synthesis of (NPCI2)/! was much improved by R. Schenk and G. Rτmer in 1924 and their method remains the basis for present-day production on both the laboratory and industrial scales:

Cl

3

n

2

(d)

(e)

Likewise, the third series runs from η = 0 (i.e. P C 1 P C 1 - ) through R N C I 1 2 , P N C l i , and P N C l i to P N C l i (e). In the limit, polymeric phosphazene dichlorides are formed ( - N P C l - ) „ , where η can exceed 1 0 and these polyphosphazenes and their cyc/oanalogues form by far the most extensive range PN compounds. +

4

5

3

6

4

6

6

4

2

4

8

4

2

Polyphosphazenes The grouping R

solvent

n?C\

5

+ nNH Cl

• (NPCl ) + 4nHCl

4

2

n

120-150°

Appropriate solvents are 1,1,2,2-tetrachloroethane (bp 146°), PhCl (bp 132°) and 1,2dichlorobenzene (bp 179°). By varying the conditions, yields of the cyclic trimer or tetramer and other oligomers can be optimized and the compounds then separated by fractionation. Highly polymeric (NPCl )oo can be made by heating cyclo-(NPC\ h to 150-300°, though heating to 350° induces depolymerization. Polycyclic compounds are rarely obtained in 2

I

2

— N = P — is isoelectronic with the R

1 6 0

R

I silicone grouping — Ο — S i — (p. 364)

H. R. ALLCOCK, Phosphorus Nitrogen Compounds, Aca­ demic Press, N e w York, 1972, 498 pp.; H. R. ALLCOCK, Chem. Rev. 7 2 , 3 1 5 - 5 6 (1972) (475 refs.). H. R. ALLCOCK, Chap. 3 in A. H. COWLEY (ed.) Rings, Clusters and Poly­ mers of the Main Group Elements, ACS Symposium Series No. 2 8 2 , Washington, DC, 4 9 - 6 7 (1982). H. R. ALLCOCK in

J. E. M A R K , R. W E S T and H. R. ALLCOCK, Inorganic

and, after the silicones, the polyphosphazenes form the most extensive series of covalently bonded polymers with a non-carbon skeleton. This section will describe their

Poly­

mers, Prentice Hall, 1991, 304 pp. H. R. ALLCOCK, Chap. 9 in R. STEUDEL (ed.) The Chemistry of Inorganic Ring Systems, Elsevier, Amsterdam, 1 4 5 - 6 9 (1992). 1 6 1

S. S. KRISNAMURTHY,

Inorg. Chem. Radiochem.

A. C. S A U and M . W O O D S ,

Adv.

2 1 , 4 1 - 1 1 2 (1978) (499 refs.).

§12.3.7

Phosphorus-nitrogen

these preparations, one exception being N7P6CI9, mp 237.5°, which can be obtained in modest yields from the direct thermolytic reaction of PCI5 and NH4CI. The tricyclic structure is strongly distorted from planarity though the central N P 3 group features an accurately planar 3coordinate Ν atom with much longer N - P bonds than those in the peripheral macrocycle. The 2 sorts of P - C l bonds are also noticeably different in length and the 3 central Cl atoms are all on one side of the N P plane with ZNPCl 104°. 3

537

compounds

replacement leads to geminal derivatives (in which both Cl atoms on 1 Ρ atom are replaced) and to non-geminal derivatives which, in turn, can exist as cis- or trans- isomers. The cyclic trimer ( N P F ) , mp 28°, has an accurately planar 6-membered ring (D^ symmetry) in which all 6 P - N distances are equal (156 pm) and the angles NPN and PNP are all 120 ± 1 ° . Most other trimers are also more-or-less planar with equal P - N distances: for example, (NPC1 )3 is almost planar (pseudochair with P - N 158 pm, P - C l 197 pm, ZNPN 118.4°, ZPNP 121.4°, ZC1PC1 102°. Perhaps surprisingly, the cyclic tetramer (NPF )4, mp 30.4°, is also a planar heterocycle {D^ symmetry) with even shorter P - N bonds (151 pm) and with ring angles of 122.7° and 147.4° at Ρ and Ν respectively. However, other conformations are found in other derivatives, e.g. chair (C /,), saddle ( D ^ ) , boat ( S 4 ) , crown tetrameric ( C 4 J and hybrid. Thus (NPC1 )4 exists in the metastable Κ form (in which it has the boat conformation) and the stable Τ form (chair configuration) as shown in Fig. 12.24. The remarkable diversity of molecular conformations observed for the 8membered hèterocycle {P4N4} suggests that the particular structure adopted in each case results from a delicate balance of intra- and intermolecular forces including the details of skeletal bonding, the orientation of substituents and their polar and steric nature, crystal-packing effects, etc. The mps for various series of cjcfo-(NPX ) frequently show an alternation, with values for η even being greater than those for adjacent η odd. Some examples are in Fig. 12.25. The crystal structures of the four compounds ( N P M e ) 9 - i have recently been d e t e r m i n e d / ) 2

3

2

2

h

2

2

Many details of the preparative reaction mechanism remain unclear but it is thought that NH4CI partly dissociates into N H 3 and HC1, and that PCI5 reacts in its ionic form P C 1 P C 1 - (p. 499). Nucleophilic attack by N H 3 on PCI4 " then occurs with elimination of HC1 and the { H N = P C 1 } attacks a second PCI4+ to give [ C 1 P = N - P C 1 ] and HC1. After l h the major (insoluble) intermediate product is [ C L 3 P = N - P C l ] P C l - (i.e. P3NCI12, p. 536) and this then slowly reacts with more N H 3 to give HC1 and { C 1 P = N - P C 1 = N H } , etc. It is probable that N H B r and P B r B r " react similarly to give ( N P B r ) but NH4F fluorinates PCI5 to NH4PF4 and the fluoroanalogues ( N P F ) are best prepared by fluorinating ( N P C l ) with K S 0 F / S 0 (i.e. K F in liquid S 0 ) . Similarly, standard substitution reactions lead to many derivatives in which all (or some) of the Cl atoms are replaced by OMe, OEt, O C H C F 3 , OPh, NHPh, N M e , N R , R, Ar, etc. Partial 4 +

6

4

3

3

3

3

+

+

6

3

4 +

n

2

2

2

n

2

2

162

2

4

2

2

2

Bonding. or chain,

n

All phosphazenes, whether cyclic contain the formally unsaturated γ x

n

2

I

group

N

y

/

with 2-coordinate Ν and

2

2

2

1 6 2

R . T. OAKLEY,

S. J. RETTIG,

J. TROTTER, J. Am. Chem. Soc.

N. L. PADDOCK

107, 6 9 2 3 - 3 6 (1985).

and

538

Phosphorus

Figure 12.24

Ch. 12

Molecular structure and dimensions of the two forms of (NPCl2)4 and of (NPC1 ) .

4-coordinate P. The experimental facts that have to be interpreted by any acceptable theory of bonding are: (i) the rings and chains are very stable; (ii) the skeletal interatomic distances are equal around the ring (or along the chain) unless there is differing substitution at the various Ρ atoms; (iii) the P - N distances are shorter than expected for a covalent single bond ( M 7 7 pm) and are usually in the range 158 ± 2 pm (though bonds as short as 147 pm occur in some compounds); (iv) the N - P - N angles are usually in the range 120 ± 2° but the P - N - P angles in various compounds span the range from 120-148.6°; (v) skeletal Ν atoms are weakly basic and can be protonated or form coordination com­ plexes, especially when there are electronreleasing groups on P; (vi) unlike many aromatic systems the phosphazene skeleton is hard to reduce electrochemically;

2

5

(vii) spectral effects associated with organic πsystems (such as the bathochromic ultra­ violet shift that accompanies increased electron derealization) are not found.

Figure 12.25

Melting points of various series of cvc/o-polyphosphazenes (NPX )„ showing the higher values for η even. 2

In short, the bonding in phosphazenes is not ade­ quately represented by a sequence of alternating double and single bonds - N = P - N = P - yet it

§12.3.7

Phosphorus-nitrogen

Figure 12.26

539

compounds

A possible description of bonding in phosphazenes.

differs from aromatic σ-π system in which there is extensive electron derealization via p ^ - p ^ bonding. The possibility of p ^ - d ^ bonding in N - P systems has been considered by many authors since the mid-1950s but there is still no consensus, and for nearly every argument that can be mounted in favour of P(3d)-orbital contributions another can be raised against it. It seems generally agreed that 2 electrons on

Ν occupy an s p lone-pair in the plane of the ring (or the plane of the local PNP triangle) as in Fig. 12.26a. The situation at Ρ is less clear mainly because of uncertainties concerning the d-orbital energies and the radial extent (size) of these orbitals in the bonding situation (as distinct from the free atom). In so far as symmetry is concerned, the s p lone-pair on each Ν can be involved in coordinate bonding in the xy plane 2

2

540

Ch. 12

Phosphorus

and HCIO4; compounds with alkyl or N R substituents on Ρ are more basic than the halides, as expected, and their adducts with HC1 have been well characterized. There is usually a substantial lengthening of the two N - P bonds adjacent to the site of protonation and a noticeable contraction of the nextnearest N - P bonds. For example, the relevant distances in [HN P Cl2(NHPr )4]Cl and the parent compound a r e : 2

3

3

L

( 1 6 4 )

Figure 12.27 (a) Schematic representation of pos­ sible 3-centre islands of π bond­ ing above and below the ring plane for (NPX )3. (b) experimental electron bonding density (see text). 2

and d orbitals on the Ρ to "vacant" d 2_ 2 (Fig. 12.26b); this is called 7r'-bonding. Involve­ ment of the out-of-plane d^ and d ^ orbitals on the phosphorus with the singly occupied p orbital on Ν gives rise to the possibility of heteromorphic ( N - P ) "pseudoaromatic" p ^ - d ^ bond­ ing (with d ), or homomorphic ( N - N ) ρ -ρ bonding (through d ) as in Fig. 12.26c. The con­ troversy hinges in part on the relative contribu­ tions of the π' in-plane and of the two π outof-plane interactions; approximately equal con­ tributions from these latter two π systems would tend to separate the π orbitals into localized 3centre islands of π character interrupted at each Ρ atom, and broad derealization effects would not then be expected. This is shown schematically in Fig. 12.27(a) and is consistent with the bonding electron density (b) as found by deformation den­ sity studies on the benzene clathrate of hexa(laziridinyl)cyclotriphosphazene, 2(CH CH N)6P N3.C H6. The possibility of exocyclic π bonding between P ( d ) and appropriate orbitals on the substituents X has also been envisaged. x

y

xy

z

xz

π

π

yz

2

3

6

2

( 1 6 3 )

z2

Typical basicities (pK' measured against HCIO4 in PI1NO2) for ring-TV protonation are: a

N P (NHMe) 8.2 3

3

N P Ph 1.5 3

3

6

N P (NEt ) 8.2 3

3

N P (OEt) -0.2 3

6

3

6

N P Et6

6

3

3

6.4 rra^-N3P Cl3(NMe )3 3

2

-5.4

Cyc/0-polyphosphazenes can also act as Lewis bases (N donor-ligands) to form complexes such as [ T i C l ( N P 3 M e ) ] , [ S n C l 4 ( N P M e ) ] , [AlBr3(N P Br )] and [2AlBr3.(N P Br )]. Not all such adducts are necessarily ring-TV donors and the 1:1 adduct of (NPC1 ) with A I C I 3 is thought to be a chloride ion donor, [ N P C l 5 ] [AICI4]". By contrast, the complex [Pt Cl (rη N4P4Meg)].MeCN features transannular bridg­ ing of 2 Ν atoms by the PtCl m o i e t y . An intriguing example of a cyc/o-polyphosphazene acting as a multidentate macrocyclic ligand occurs in the bright orange complex formed when N 6 P 6 ( N M e ) i reacts with equal amounts of C u C l and CuCl. The crystal structure of 4

3

3

3

6

3

6

3

2

3

3

6

6

3

3

3

n

2

2

Reactions. The Ν atom in cycfo-polyphosphazenes can act as a weak Br0nsted base (proton acceptor) towards such strong acids as HF

2

+

2

2

(165)

2

2

1 6 4

Ν . V . M A N I and A . J. WAGNER, Acta Cryst.

27B, 5 1 - 8

(1971). 1 6 3

T. S. CAMERON

and B . BORECKA,

Silicon and Related Elements,

Phosphorus,

64, 1 2 1 - 8 (1992).

Sulfur,

1 6 5

J. P . O ' B R I E N , R . W . ALLEN and H . R . ALLCOCK, Inorg.

Chem. 1 8 , 2 2 3 0 - 5 ( 1 9 7 9 ) .

§12.3.7

Phosphorus-nitrogen

Figure 12.28

compounds

541

Structure of (a) the free ligand N P (NMe )i , and (b) the η complex cation [CuCl{N P6(NMe2)i2}] showing changes in conformation and interatomic distances in the phosphazene macrocycle. The Cl is obscured beneath the Cu and can be regarded as occupying either the apical position of a square pyramid or, since ZN(1)-Cu-N(r) is large (160.9°), an equatorial position of a distorted trigonal bipyramid. Note that coordination tightens the ring, already somewhat crowded in the uncomplexed state, the mean angles at Ρ being reduced from 120.0° to 107.5°, and the mean angles at Ν being reduced from 147.5° to 133.6°. The lengthening of the 8 P-N bonds contiguous to the 4 donor Ν atoms from 156 to 162 pm is significant, the other P-N distances (mean 156 pm) remaining similar to those in the free ligand. 6

6

the resulting [ N P ( N M e ) i C u C l ] + [ C u C l ] - has been determined (Fig. 12.28b) and detailed com­ parison with the conformation and interatomic distances in the parent heterocycle (Fig. 12.28a) gives important clues as to the relative impor­ tance of the various π and π' bonding interac­ tions involving Ν (and P) a t o m s . Incidentally, the compound also affords the first example of the linear 2-coordinate Cu complex [CuCl ]~. The related (and more extensive) organometallic chemistry of the phosphazenes has been reviewed. As Ρ is isoelectronic with N, it has been found possible to prepare 8-membered diazahexaphos6

6

6

2

2

4

+

2

2

2

(166)

1

2

phocins such as N P P h P P P h N P P h P P P h , analogous to ( N P P h ) . The two subrogated Ρ atoms can chelate to PdCl to form a square planar complex. Many of the cyclic and chain dichloro deriva­ tives ( N P C l ) can be hydrolysed to η-basic acids and the lower members form well-defined salts frequently in the tautomeric metaphosphimic-acid form, e.g.: 2

2

4

2

2

2

( 1 6 8 )

2

( 1 6 9 )

2

n

(167)

1 6 6

W . C . MARSH,

N . L . PADDOCK,

C . J. STEWART

and

J. TROTTER, J. Chem. Soc, Chem. Commun., 1 1 9 0 - 1 ( 1 9 7 0 ) . 1 6 7

H . R . ALLCOCK, J. L . DESORCIE and G . H . RIDING,

hedron 6 , 1 1 9 - 5 7 ( 1 9 8 7 ) .

Poly­

A. SCHMIDPETER and G . BURGET, Angew. Chem. Int. Edn. Engl. 2 4 , 5 8 0 - 1 ( 1 9 8 5 ) .

1 6 8

1 6 9

A. SCHMIDPETER, F. STEINMULLER and W. S . SHELDRICK,

Z. anorg. allg. Chem. 5 7 9 , 1 5 8 - 7 2 ( 1 9 8 9 ) .

542

Ch. 12

Phosphorus

The dihydrate of the tetramer is particularly stable and is, in fact, the bishydroxonium salt of tetrametaphosphimic acid [ H 3 0 ] J [ ( N H ) 4 P 4 0 6 ( O H ) ] ~ the anion of which has a boat configuration and is linked by short Η bonds ( 2 4 6 pm) into a two-dimensional sheet 2

2

(Fig. 1 2 . 2 9 ) .

The related

salts

Μ^[ΝΗΡ0 ) ].2

4

/ ξH 0 show considerable variation in conforma­ tion of the tetrametaphosphimate anion, as do the 8-membered heterocyclic tetraphosphazenes 2

(NPX ) 2

4

(p. 5 3 7 ) , e.g.

[NH ]4[N4H P40 ].2H 0

boat conformation

Κ4[Ν Η4Ρ4θ8].4Η 0

chair conformation

Cs4[N H P 08].6H 0

saddle conformation.

4

4

4

8

2

2

4

4

4

2

( N P C 1 ) „ but these readily hydrolyse in moist air to polymetaphosphimic acids. Greater stability is displayed by amino, alkoxy, phenoxy and especially fluorinated derivatives, and these are attracting increasing interest as rigid plastics, elastomers, plastic films, extruded fibres and expanded f o a m s . Such materials (MW > 5 0 0 0 0 0 ) are water-repellent, solvent-resistant, flame-resistant and flexible at low temperatures (Fig. 1 2 . 3 0 ) . Possible applications are as fuel hoses, gaskets and O-ring seals for use in high-flying aircraft or for vehicles in Arctic climates. Their extraordinary dielectric strength makes them good candidates for metal coatings and wire insulation. Other applications of polyphosphazenes include their use to improve the high-temperature properties of phenolic resins and their use as composites with asbestos or glass for non-flammable insulating material. Some of the more reactive derivatives have been proposed as pesticides and even as ultra-high capacity fertilizers. 2

( 1 6 0 , 1 7 0 )

12.3.8

Organophosphorus compounds

A general treatment of the vast domain of organic compounds of p h o s p h o r u s falls out­ side the scope of this book though several important classes of compound have already been briefly mentioned, e.g. tertiary phosphine ligands (p. 4 9 4 ) , alkoxyphosphines and their derivatives (p. 4 9 6 ) , organophosphorus halides (p. 5 0 0 ) , phosphate esters in life processes (p. 5 2 8 ) and organic derivatives of P N com­ pounds (preceding section). There are also innumerable organic derivatives of the polycyclic polyphosphanes (p. 4 9 5 ) , < ' ) and vast numbers of heterocyclic organophosphorus 071}

Figure 12.29

Schematic representation of the boatshaped anion [(NH) P 0 (OH) ] showing important dimensions and the positions of H bonds. 4

4

6

2

2

( 6 7

Applications. Many applications have been proposed for polyphosphazenes, particularly the non-cyclic polymers of high molecular weight, but those with the most desirable properties are extremely expensive and costs will have to drop considerably before they gain widespread Use (cf. silicones, p. 3 6 5 ) . The cheapest compounds are the chloro series

7 0

1 7 2

H. R. ALLCOCK, Sci. Progr. Oxf. 6 6 , 3 5 5 - 6 9 (1980). R. S . EDMONDSON (ed.) Dictionary of Organophosphorus Compounds, Chapman and Hall, New York, 1988, 1347 pp. G . FRITZ, H.-G. VON SCHERING et al, Ζ. anorg. allg. Chem. 5 5 2 , 3 4 - 4 9 (1987); 5 8 4 2 1 - 5 0 , 5 1 - 7 0 (1990); 5 8 5 , 5 1 - 6 4 (1990); 5 9 5 , 6 7 - 9 4 (1991); and references cited therein.

1 7 0

1 7 1

1 7 2

I I 3Ο

I Ο Ο

1 Ο C

&

Figure 12.30

Potential uses of polyphosphazenes: (a) A thin film of a poly(aminophosphazene); such materials are of interest for biomedical applications, (b) Fibres of poly[bis(trifluoroethoxy)phosphazene]; these fibres are water-repellant, resistant to hydrolysis or strong sunlight, and do not burn, (c) Cotton cloth treated with a poly(fluoroalkoxyphosphazene) showing the water repellancy conferred by the phosphazene. (d) Polyphosphazene elastomers are now being manufactured for use in fuel lines, gaskets, O-rings, shock absorbers, and carburettor components; they are impervious to oils and fuels, do not burn, and remain flexible at very low temperatures. Photographs by courtesy of H. R. Allcock (Pennsylvania State University) and the Firestone Tire and Rubber Company.

544

Ch. 12

Phosphorus

compounds. ' Within the general realm of organic compounds of phosphorus it is convenient to distinguish organophosphorus compounds as a particular group, i.e. those which contain one or more direct P - C bond. In such compounds the coordination number of Ρ can be 1, 2, 3, 4, 5 or 6 (p. 484). Examples of coordination number 1 were initially restricted to the relatively unstable compounds HCP, F C P and MeCP (cf. HCN, FCN and MeCN). H C = P was first made in 1961 by subjecting P H 3 gas at 40 mmHg pressure to a low-intensity rotating it is arc struck between graphite e l e c t r o d e s ; a colourless, reactive gas, stable only below its triple point of —124° (30 mmHg). Monomeric HCP slowly polymerizes at —130° (more rapidly at - 7 8 ° ) to a black solid, and adds 2HC1 at — 110° to give M e P C b as the sole product. Both monomer and polymer are pyrophoric in air even at room temperature. More r e c e n t l y MeCP was made by pyrolysing M e C ^ P C h at 930° in a low-pressure flow reactor and trapping the products at —78°. Dramatic stabilization of a phospha-alkyne has been achieved by η complexation to a metal c e n t r e : ° 0 7 3

1 7 4 )

(175)

was B u ' C ^ P , ) and its chemistry has been The similarly extensively i n v e s t i g a t e d . bulky A r C = P (Ar = 2,4,6,-Bu' C H2) has been and the studied by X-ray c r y s t a l l o g r a p h y C - P distance found to be 152 pm, similar to the short C - P distance of 154pm deduced from the microwave spectrum of HCP and MeCP. The most studied reactions of phosphaalkynes are cyclo-additions to give organo-P and reactions with nucleheterocycles,° ophiles to give phospha-alkenes and 1,3diphosphabutadienes. 0 7 8

079,180)

3

6

082)

7 9 - 1 8 1 )

(

1 8 2 )

As with coordination number 1, the first 2-coordinate Ρ compound also appeared in 1961: M e P = P C F was made as a white 5 with P M e ; solid by cleaving c^c/o-[P(CF )] it is stable at low temperatures but readily dissociates into the starting materials above room temperature. More stable is the bent 2-coordinate phosphocation occurring in the orange s a l t ° ( 1 8 3 )

3

3

3

4 o r

3

8 4 )

(176)

2

77)

C H 6

6

B u ' C = P + [Pt(C H )(PPh ) ] 2

4

3

C2H4

2

room temp

The aromatic heterocycle phosphabenzene C 5 H 5 P (analogous to pyridine) was reported in 1 9 7 1 , ° some years after its triphenyl derivative and 2 , 4 , 6 - P h C H P . See also H P = C H [P(CN) ](p. 484). The burgeoning field of heterocyclic phosphorus compounds featuring 85)

+ [Pt(rη -Bu'CP)(PPh ) ].C H 2

3

2

6

6

The translucent, cream-coloured benzene sol­ vate was characterized by single-crystal X-ray analysis and by P n m r spectroscopy. The first free phospha-alkyne stable to polymerization

3

2

5

2

2 ( 2 9 )

( 3 0 )

3 1

1 7 8

G. BECKER, G. GRESSER and W . U H L , Z. Naturforsch.,

Teil

Β 3 6 , 16 (1981). 1 7 3

E. FLUCK and B . NEUMULLER,

in H . W . ROESKY

(ed.),

Rings, Clusters and Polymers of Main Group and Transition Metals, Elsevier, Amsterdam, 1989, pp. 1 9 3 - 5 . 1 7 4

A. SCHMIDPETER and K . KARAGHIOSOFF, in H . W . ROESKY

(ed.), Rings, Clusters and Polymers of Main Group and Transition Metals, Elsevier, Amsterdam, 1989, pp. 3 0 7 - 4 3 . 1 7 5

T. E. GIER, J. Am. Chem. Soc. 8 3 , 1 7 6 9 - 7 0 (1961).

1 7 6

N . P . C . WESTWOOD,

H . W . KROTO,

N . P . C . SIMMONS, J. Chem. Soc, Dalton (1979). 1 7 7

J. C . T . R . BURKETT-ST. LAURENT,

J. F . NIXON

Trans.,

and

1405-8

1 7 9

1 8 0

J. F . NIXON, Chem. Rev. 8 8 , 1 3 2 7 - 6 2 (1988). M. REGITZ, Chem. Rev. 9 0 , 1 9 1 - 2 1 3 (1990). See also

M. REGITZ

and

O. J. SCHERER,

Coordination in Phosphorus Verlag, Stuttgart, (1990).

Multiple

Bonds

Chemistry,

1 8 1

R . BARTSCH and J. F . NIXON, Polyhedron,

1 8 2

A . M. ARIF,

A.F.BARRON,

Low

Thieme

8 , 2407 (1989).

A.H.COWLEY

S. W . HALL, J. Chem. Soc, Chem. Commun., 1 8 3

and

Georg

and

1 7 1 - 2 (1988).

A . BURG and W . MAHLER, J. Am. Chem. Soc

8 3 , 2388-9

(1961). P . B . HITCHCOCK,

H . W . KROTO and J. F . NIXON, J. Chem. Soc, Chem. mun., 1 1 4 1 - 3 (1981).

Com­

1 8 4

K . DIMROTH and P . HOFFMANN, Chem. Ber. 9 9 , 1 3 2 5 - 3 1

(1966); R . ALLMANN, Chem. Ber. 9 9 , 1 3 3 2 - 4 0 (1966). 1 8 5

A . J. ASHE, J. Am. Chem. Soc. 9 3 , 3 2 9 3 - 5 (1971).

§12.3.8

Organophosphorus

2-coordinate and 3-coordinate Ρ has been fully reviewed/ as has the equally active field of phospha-alkenes ^ - P — ^ and diphosphenes ( _ p _ p _ ) (179,180.186,187) 1 7 3 , 1 7 4 )

The most common coordination numbers for organophosphorus compounds are 3 and 4 as rep­ resented by tertiary phosphines and their com­ plexes, and quaternary cations such as [ P M e ] and [ P P h ] . Also of great significance are the 4-coordinate Ρ ylides^ R P = C H ; indeed, few papers have created so much activity as the report by G. Wittig and G. Geissler in 1953 that meth­ ylene triphenylphosphorane reacts with benzophenone to give P h P O and 1,1-diphenylethylene in excellent y i e l d . +

4

+

4

3

2

3

(188)

P h P = C H + Ph CO 3

2

• Ph PO + P h C = C H

2

3

2

2

compounds

545

and others and culminated in the award of the 1979 Nobel Prize for Chemistry (jointly with H. C. Brown for hydroboration, p. 166). The reaction of Ρ ylides with many inorganic compounds has also led to some fascinating new chemistry/ The curious yellow compound P h P = C = P P h should also be n o t e d : unlike aliène, H C = C = C H , which has a linear central carbon atom, the molecules are bent and the structure is strikingly unusual in having 2 crystallographically independent molecules in the unit cell which have substantially differing bond angles, 130. Γ and 143.8°. The short P = C distances (163 pm as compared with 183.5 pm for P-C(Ph)) suggest double bonding, but the nonlinear P = C = P unit and especially the two values of the angle, are hard to rationalize (cf. the isoelectronic cation [ P h P = N = P P h ] which has various angles in different compounds). Pentaorgano derivatives of Ρ are rare. The first to be made (by G. Wittig and M. Rieber in 1948) I89)

(190)

3

3

2

2

+

The ylide P h P = C H can readily be made by deprotonating a quaternary phosphonium halide with n-butyllithium and many such ylides are now known: 3

2

3

3

was PPI15:

LiBu" +

[Ph PCH ] Br~ 3

3

• Ph P=CH 3

(DLiPh

2

+ LiBr + Bu"H

H

+

> [PPh ] Cr

Ph PO (2)HC1

thf/0°

[PMe ]+Br" + N a N H 4

2

L i p h

• Me P=CH 3

2

+

[PPh ] r

• P P h (d. 124°)

4

+ NaBr + N H

3

The enormous scope of the Wittig reaction and its variants in affording a smooth, highyield synthesis of C = C double bonds, etc., has been amply delineated by the work of Wittig R . APPEL, F . KNOLL and I. RUPPERT, Angew.

Edn.

187N .

Chem.

Int.

Engl. 2 0 , 7 3 1 - 4 4 ( 1 9 8 1 ) . C . NORMAN, Polyhedron

Thus P h P = C H is triphenylphosphonium methylide (see pp. 2 7 4 - 3 0 4 of reference 2 , or textbooks of organic chem­ istry for a fuller treatment of the Wittig reaction). 1 8 8

3

2

3

2

3

4

12, 2 4 3 1 - 6 (1993).

' An ylide can be defined as a compound in which a carbanion is attached directly to a heteroatom carrying a high degree of positive charge:

3

5

Unlike SbPhs (which has a square-pyramidal structure p. 598), PPhs adopts a trigonal bipyramidal coordination with the axial P - C distances (199 pm) being appreciably longer than the equatorial P - C distances (185 pm). More recently (1976) P ( C F ) M e and P ( C F ) M e were obtained by methylating the corresponding chlorides with P b M e . There are also many examples of 5-coordinate Ρ in which not all the directly bonded atoms are carbon. One such is the dioxaphenylspiro-phosphorane shown in Fig. 12.31; the local symmetry about Ρ is essentially square pyramidal, and the factors which affect the choice between this geometry and trigonal bipyramidal is a topic of active 3

1 8 6

I

•

4

3

2

G . WITTIG and G . GEISSLER, Annalen

580, 4 4 - 5 7 (1953).

H. SCHMIDBAUR, Act: Chem. Res. 8 , 6 2 - 7 0 ( 1 9 7 5 ) . 1 9 0

A . T . VINCENT and P . J. WHEATLEY, J. Chem. Soc.

Chem. Commun., 5 9 2 ( 1 9 7 1 ) .

(D),

546

Figure 12.31

Schematic representation of the molecular structure of [P(C HMe5)(02C2H )Ph] showing the rectangular-based pyramidal disposition of the 5 atoms bonded to P; the Ρ atom is 44 pm above the C2O2 plane. 3

current interest/ ' It should also be noted and PI13PI2, that the compounds, P h P B r which might have been thought to involve 5coordinate P, feature instead 4-coordinate Ρ and an unusual end-on bonding of the dihalogen moiety, i.e. P h P - B r - B r / and P h P - I - I . 39,

9

3

3

1 9 1

2

1 9 2 )

W . ALTHOFF,

3

R. O . DAY,

( 1 9 3 )

R . K . BROWN

and

3

3

1 9 2

N . BRICKLEBANK, and

S . M . GODFREY,

A . G . MACKIE,

R . G . PRITCHARD,

Chem.

Soc,

Chem. Commun., 3 5 5 - 6 ( 1 9 9 2 ) . 1 9 3

A.

S . M . GODFREY, G . MACKIE,

Κ . B . DILLON and J. LINCOLN, Polyhedron,

(1989).

A . MCAULIFFE

D . G . KELLY,

R . G . PRITCHARD

and

C . S . MCAULIFFE, S . M . WATSON,

Chem. Soc, Chem. Commun., 1 1 6 3 - 4 ( 1 9 9 1 ) .

J.

1 9 4 )

9)

1 9 4

3276-85.

4

The corresponding interhalogen adducts Ph PIX (X = Cl, Br) appear to be 4-coordinate but ionic, i.e. [ P h P I ] + X - / Many organophosphorus compounds are highly toxic and frequently lethal. They have been actively developed for herbicides, pesticides and more sinister purposes such as nerve gases which disorient, harass, paralyse or k i l l /

R. R . HOLMES, Inorg. Chem. 1 7 , 3 2 6 5 - 7 0 ( 1 9 7 8 ) ; see also the immediately following two papers, pp. 3 2 7 0 - 6 and

C.

Ch. 12

Phosphorus

8,

1445-6