- Email: [email protected]
Pyrophosphite as a ligand
J. lnnrg. Nucl. Chem., 1964, Vol. 26, pp. 1985 to 1990. Pergamon Press Ltd. Printed in Northern Ireland
PYROPHOSPHITE AS A LIGAND D . GRANT* a n d D . S. PAYNE The University, Glasgow W.2, Scotland (Received 15 October 1963; in revised f o r m 10 March 1964)
PYROPHOSPHATE is k n o w n to f o r m s t r o n g m e t a l i o n c o m p l e x e s . (1) It is g e n e r a l l y c o n s i d e r e d t h a t six m e m b e r e d rings o c c u r in t h e s e c o m p l e x e s a n d t h e i n c r e a s e in the stability o f t h e m e t a l i o n c o m p l e x f r o m o r t h o to p y r o p h o s p h a t e is d u e to this c h e l a t i o n s t a b i l i s a t i o n . L i t t l e is h o w e v e r k n o w n a b o u t the c o m p l e x i n g ability o f the p y r o p h o s p h i t e unit, w h i c h at n e u t r a l p H differs o n l y f r o m p y r o p h o s p h a t e in t h e r e p l a c e m e n t o f P - - O H by P - - H , viz. 0
0
II
II
HO--P--O--P--OH 0
I
0
0
II
II
I
I
H--P--O--P--H
I
0
0
O__
It was t h e r e f o r e o f i n t e r e s t to s t u d y t h e c o m p l e x i n g o f p y r o p h o s p h i t e a n d c o m p a r e t h e results w i t h t h o s e o f p y r o p h o s p h a t e . EXPERIMENTAL The polarographic method was used to study the complexing of pyrophosphite and metal ions. The equation relating shifts in polarographic half wave potentials, AE~ (volts) to equilibrium constants in the formation of complex species ~2 M ~-pL ~ ML~, K AE~
~---
0"059 n
[MLp] [M][L],'
log]0K -- P- 0"059 Iogl0(L) II
n is the number of electrons involved in the electrode reaction is obeyed for electrode reactions where the polarographic wave is reversible, the curve obeying the relationship 0.059 i E - E~ -- - lOgl0 . E is the potential, i is the current and i~ the diffusion current. n ta -- i A number of metal ions have been studied at neutral pH using pyrophosphite as ligand (fro~rT Na2H~P20~) comparing the results with those for phosphite for Cd 2÷ and Cu 2-. Log K and p values obtained from the above equation are shown in Table 1. For Cd ~-, Cu 2~, Pb 2~ and TI ~ waves the plot of i/ia -- i vs. E was linear having the expected value of n for reduction to the metallic state in the electrode reaction. UO~ e+ showed a one electron reaction (E ! = 0.17l V). The shifts observed in the irreversible Ni z+ waves (the i/i a - i plots were linear around the centre with n values of one),. were treated mechanically, using these equations. A similar treatment was used for the effect of pyrophosphite on the Cr s+ wave. Ba ~+ gave an irreversible wave, however pyrophosphite produced no significant effect on it. * Present address: Monsanto Chemicals Ltd., Ruabon, North Wales. (Research Depamnent.) '~) J. R. VAN WAZER and C. F. CALL1S, Chem. Rev. 58, 1011 (1958). is) 1. M. KOLTHOFF and J. J. L I N G A N E , Polaro~raphy, 2nd Ed., Vol. I., p. 22l. lnterscience, New York (1952). 1985
1986
D. GRANT a n d D. S. PAYNE
C d 2+. SOUCHAY a n d FAUCHERRE in similar polarographic experiments observed p y r o p h o s p h a t e C d ~+ c o m p l e x f o r m a t i o n in 3'5 M K C I base electrolyte h a v i n g log K = 4.2 a n d p : 2. TM Little shift w a s h o w e v e r f o u n d by us in the C d 2+ wave as a function o f the p y r o p h o s p h i t e concentration in the presence o f excess chloride ion. E v e n with small a m o u n t s of chloride ions present smaller shifts were f o u n d t h a n t h o s e for nitrate media. This clearly indicates that p y r o p h o s p h i t e complexes with C d 3+ to a lesser extent t h a n the chloride ion, Values e s t i m a t e d f r o m the d a t a (Fig. 1) are Cd~+-Cl - interaction, p = 1, log K = 2"3 a n d Cd2+-H2P~O~ ~- interaction, p = 1, log K ~ 1.8. T h e m o s t recent investigations o f the complexing o f C I - by C d ~+ give log K v a l u e s for the 1 : 1 complex close to 2"0. ~4) TABLE 1.--EXPERIMENTAL RESULTS, COMPLEXING ACTION OF PYROPHOSPHITE AND PHOSPHITE AS INDICATED BY SHIFTS IN POLAROGRAPHIC HALF-WAVE POTENTIALS, COMPARED WITH PYROPHOSPHATE COMPLEXING
M PyrophosphiteCd2+ CdZ+ Cd2+ Cd2+ Cd2+ Cu z+ Cu2+ pb~+ TI + Ni2+ Co2+ CrY+ Ba2+
UO~2+ U022+ PhosphiteCd2+ Cu2+
lOgl0K
p
(L) Range studied
0.32 0.32 0.70
0.15 0.14 0.29
0-0.194 0-0.124 0-0.126
0.674 to 0.680 0.624 to 0.630 0.610 to 0.626
1.41 0.52 (1.5) 1.4 (0.7)
0-73 0.23 (1) 1.5 (1) -(0-45) (1) (1) -(4) (2)
0-0.096 04).120 0-0.096 0-0-588 0-0.686 0-0-423 0-0-347 0-0.498 ff4).274 0-0.297 0-0.116 0-0.138
0.582 to 0-601 0-579 to 0.608 +0.010 to --0.005 +0.013 to --0.019 0.413 to 0.428 0.483 to 0.497 1.035 to 1.053 1.368 to 1.424 0.893 to 0.971 (1.56) to (1.56) 0-171 to 0.420 0.182 to 0.360
0.49 0.2
0-0.038 0-0-102
0'582 to 0.587 +0"020 to +0.016
--
(0.5) (1.1) (1-9) -(12-7) (8.8) 0.95 0.35
Pyrophosphate cs~ Cd 2+ 4.2 Cu 2+ 12 Pb 2+ 5-3 TI 2+ 2.0
E½* ' range (V)
Medium
(M) × 10 -a
KCI 2.1 M KC1 3.5 M .fKNOa 0-34 M I.KCI 0.0033 M K N O a 0.34 M K N O 3 1.8 M K N O 3 0.34 M KNOz 1.8 M K N O 3 1.8 M KNOa 1.8 M KNO3 1.8 M K N O z 1.8 M K N O 3 1.8 M KNO3 1.8 M K N O 3 1.98 M NaCIOa 2.3 M
1-63 1.63 1.63
KNO 3 1 M K N O s 1.4 M
1.66 1-66 1.27 1.27 1.02 1.31 1.02 1-67 0.95 1.64 1.13 1"13 1'66 0.65
2 2 2 2
* E~ values are given in negative volts except where otherwise indicated. are estimates only.
Values given in brackets
R e s u l t s for the c o m p l e x i n g by p h o s p h i t e at p H 4"5 s h o w e d a similar shift in the half wave potential with ligand c o n c e n t r a t i o n to that o f the pyro species, Fig. 1 c u r v e f i suggesting that the complexing activity o f p h o s p h i t e is similar in m a g n i t u d e to that o f pyrophosphite. N o chelating effect which m i g h t h a v e been expected to lead to a n increase in the stability o f the pyro c o m p l e x is found. C u e+. T h e plot o f AE½ vs. the p y r o p h o s p h i t e concentration (log scale) is s h o w n in Fig. 2. T h e curve t e n d s to a line w i t h p = ca. 1-5 a n d log K = ca. 1'4, with increasing p y r o p h o s p h i t e concentration. T h e observed complexing action is t h u s smaller t h a n observed between C u 2+ a n d pyrop h o s p h a t e by several orders o f m a g n i t u d e , possibly u p to ten (a n u m b e r o f stability c o n s t a n t s have been reported for complexing between p y r o p h o s p h a t e a n d C u ~+, cf. Reference 5). O r t h o p h o s p h i t e is f o u n d to p r o d u c e similar shifts in the C u ~+ waves to the pyro species. Only relatively low p h o s p h i t e concentrations could be employed h o w e v e r owing to a tendency for chemical reduction o f the C u 2+ to C u + to occur with increasing C u e+ concentration. This reaction is detected by a c h a n g e in the C u 2+ wave. ~3~ P. SOUCHAY a n d J. FAUCHERRE, Bull. SOC. Chin. France, 14, 533 (1947). t4~ W. B. TREUMANN a n d L. M. FERRIS, J. /truer. Chem. Soc. 80, 5048 (1958).
Pyrophosphite as a ligand
1987
0.680 ( 3.670
~b
0-630
0.820
LIJ
0.810 (
i
~ d
j / 1 1 /
0.600
/
f,
/ i /
y
e
0"590
0"580t 0
1
I
0.04
I
I
I
0.08
Ligand
E
0,12
I
[
046
]
cone. M0
FIG. 1.--E~ values for the Cd2+-pyrophosphite system (a) KC12.1 M, (b) KCI 0"35 M, (c) KNOs 0.34 M, KC1 0.0033 M, (d) No base electrolyte, (e) KNOs 0"34 M, (g) KNO:~ 1-8 M. Curve (f) shows the corresponding values for the Cd~+-phosphite system KNO3 1 M. 0028-
0,020
>
0.012
-,~
///,,o 0.004
°I -0,004'
0.01
7//
~ . . . . . . . ~ t /
0.1
1,0
Ligand conc. M. FiG. 2.--Semi log plot of Cu ~+ E~ against pyrophosphite conc. (a) K N O a 0.34 M, (b) K N O 3 1.8 M. Ni =+. The Ni ~+ polarographic wave varies regularly with pyrophosphite concentration over a small range of voltage, Fig. 3. Although the absolute treatment of the results is made dimcult by the irreversible nature of the wave, it is considered that the general low values o f # and K obtained from the data are of the same order of magnitude to those obtained with C u 2+ and Cd =+. Pb 2+, C o ~+, Cr 3+, Tl + and Ba 2+. These ions were studied in lesser detail. The general type of interaction occurring was established, No significant change in the waves obtained with TI + or
1988
D. GRANT and D. S. PAYNE
Ba ~+ was found due to pyrophosphite in solution. Shifts occurred in the waves obtained with Pb z+, Co S+ and Cr a+ ions. Estimates of the K values assuming p = 1 were made, being significantly smaller than the values reported for pyrophosphate complexing, c5~ 1.055
I. 05 0
>
1.045
W I
1.040
1.035
I 0
0.1
I
I
I
0"2
0'3
0-4
Ligond conc.M.
FIG. 3.--Ni ~+ E½ plotted against the
pyrophosphitecone.
KNOa 1.8 M.
UO22+. A wave is obtained giving a straight line plot of ilia - - i vs. E having E½ = 0"171 V, (this value however was observed to change as the UOz 2+ solution "aged", after 16 hr the value was 0.195 V). Nitrate and perchlorate media were compared, different interactions between the pyrophosphite and uranyl species being shown in these media. There was a tendency for chemical reduction of UO~ 2+ and formation of a colloidal suspension to occur in solution during some of the experiments, particularly at high pyrophosphite concentrations, marked changes occurred in these waves. Fig. 4 shows a plot of the observed E½ values where little side reaction had occurred, for the two media as a function of the pyrophosphite concentration, comparing the experimental points with the theoretical curve for the p and log K values in Table 1.
O. 500
A
0300 LLI I
o.loo
0
I
0-02
I
0.04
I 0-06
I
0.08
I
o.Jo
I
~.iz
0-14
Ligond conc.M.
FIG. 4.--UO22+ E½ plotted against the pyrophosphite cone. (a) KNOz 1.98 M, (b) NaCIO4 2-3 M. (5~ j. B. BJERROM, G. SCHWARZENBACHand L. G. SILLEN, Special Publication No. 7, part II, S e e . (1958).
Chem.
Pyrophosphite as a ligand
1989
Instrumentation
The Tinsley recording polarograph was used (automatic recording and voltage sweep). The polarographic cell had a saturated calomel electrode thermostatted at 25°. Solutions were deoxygenated by bubbling nitrogen prior to the run. Reagents
Analytical grade reagents were employed for the metal ion solutions. Disodium pyrophosphite was studied since this is conveniently prepared by the thermal dehydration of NaH(HPO3)2~H20 at 220° without oxidation (checked by ceriometric analysis) of the pyrophosphite unit. Sodium pyrophosphite metal ion solutions had pH ca. 7 and hydrolysis of the pyrophosphite unit is sufficiently slow under these conditions to enable small amounts of ortho phosphite present to be ignored. Phosphite solutions pH ca. 4.5-5 were obtained from NaH(HPO)~2½H20. The concentration ranges of pyrophosphite and phosphite which could be conveniently studied were dictated by consideration of solubility and the tendency for side reactions to occur. DISCUSSION The complexing activity of pyrophosphite is much weaker than would be expected for this analogue of the pyrophosphate unit. The degree of interaction between Cu 2+ and pyrophosphite is of a similar order of magnitude to that of the interaction between alkali metal ions and pyrophosphate. The exception which has been found in the case of uranyl where strong pyrophosphite uranyl complexes occur is probably related to the unique properties of uranyl as a central metal unit. The chelation stabilization which is a characteristic feature of the complexing action of pyrophosphate appears to be absent with pyrophosphite. The colours of pyrophosphate complex ions with transition metals are often markedly different from those of the uncomplexed metal ion in solution. This is not the case with pyrophosphite where no colour change due to complex formation has been observed. Some insight may be thrown on the problem as to the cause of this difference in complexing ability by consideration of probable electronic differences in the structures of the two oxy-anions. From analogy with orthophosphorous acid and orthophosphoric acid where increased localization of bonding electrons in the isolated P- O bond occurs from phosphoric acid to phosphorous acid, (6) and from the known sequence where increased localization of bonding electrons in the isolated P - - O takes place on replacing P - - O by P - - O - - P in a structure, (7) it would be expected that in pyrophosphite, relative to pyrophosphate, increased localization of 7r-bonding would be found. It follows from this argument that there will be more ~r bonding in the P - - O - - P bond of pyrophosphate than pyrophosphite, possibly the latter case having essentially none. The hydrolytic stability of the two kinds of pyro P- O - - P bonds is markedly different, 18) pyrophosphite hydrolysing much faster, being comparable with the rate of hydrolysis of the branch P - - O - - P bond in condensed phosphates where again there is little ~rbonding character in the bond. Although it is therefore likely that the amount of ~r bonding in the two types of P - - O - - P bonds being considered will be different this is not directly relatable to the stability of the metal ion complexes which are formed by the anions containing these units unless the kind of bonding in the P - - O - - P bond (,~1S. FURBERGand P. LANDMARK,Aeta Chem. Stand. 11, 1505 (1957). c71 j . R . VAN WAZER, J. Amer. Chem. Soe.
78, 5709 (1956).
Isl j. R. VAN WASER,Phosohorus andits Compounds, Vol. I, p. 395; p. 452 et seq. lnterscience, New York (1958).
1990
D. GRANTand D. S. PAYNE
is involved in chelate bond formation, thus "resonance stabilisation" of a six membered chelate ring can be envisaged, delocalisation of electrons occurring around a six membered ring. It has been pointed out tg) that the difference between pyrophosphite and phosphite as regards the formation of complexes can be explained by the lesser electron donation by the P - - H relative to P - - O H towards the P - - O bond directly interacting with the metal ion. This would serve to explain decreased complexing by phosphite units in general, relative to phosphate, but does not seem to us to adequately explain the great increase in complexing action from phosphate to pyrophosphate, an increase which is not found from phosphite to pyrophosphite.
Acknowledgement--One of the authors (D. G.) thanks Albright & Wilson Ltd. for a maintenance grant. 19~by a referee.
PYROPHOSPHITE AS A LIGAND D . GRANT* a n d D . S. PAYNE The University, Glasgow W.2, Scotland (Received 15 October 1963; in revised f o r m 10 March 1964)
PYROPHOSPHATE is k n o w n to f o r m s t r o n g m e t a l i o n c o m p l e x e s . (1) It is g e n e r a l l y c o n s i d e r e d t h a t six m e m b e r e d rings o c c u r in t h e s e c o m p l e x e s a n d t h e i n c r e a s e in the stability o f t h e m e t a l i o n c o m p l e x f r o m o r t h o to p y r o p h o s p h a t e is d u e to this c h e l a t i o n s t a b i l i s a t i o n . L i t t l e is h o w e v e r k n o w n a b o u t the c o m p l e x i n g ability o f the p y r o p h o s p h i t e unit, w h i c h at n e u t r a l p H differs o n l y f r o m p y r o p h o s p h a t e in t h e r e p l a c e m e n t o f P - - O H by P - - H , viz. 0
0
II
II
HO--P--O--P--OH 0
I
0
0
II
II
I
I
H--P--O--P--H
I
0
0
O__
It was t h e r e f o r e o f i n t e r e s t to s t u d y t h e c o m p l e x i n g o f p y r o p h o s p h i t e a n d c o m p a r e t h e results w i t h t h o s e o f p y r o p h o s p h a t e . EXPERIMENTAL The polarographic method was used to study the complexing of pyrophosphite and metal ions. The equation relating shifts in polarographic half wave potentials, AE~ (volts) to equilibrium constants in the formation of complex species ~2 M ~-pL ~ ML~, K AE~
~---
0"059 n
[MLp] [M][L],'
log]0K -- P- 0"059 Iogl0(L) II
n is the number of electrons involved in the electrode reaction is obeyed for electrode reactions where the polarographic wave is reversible, the curve obeying the relationship 0.059 i E - E~ -- - lOgl0 . E is the potential, i is the current and i~ the diffusion current. n ta -- i A number of metal ions have been studied at neutral pH using pyrophosphite as ligand (fro~rT Na2H~P20~) comparing the results with those for phosphite for Cd 2÷ and Cu 2-. Log K and p values obtained from the above equation are shown in Table 1. For Cd ~-, Cu 2~, Pb 2~ and TI ~ waves the plot of i/ia -- i vs. E was linear having the expected value of n for reduction to the metallic state in the electrode reaction. UO~ e+ showed a one electron reaction (E ! = 0.17l V). The shifts observed in the irreversible Ni z+ waves (the i/i a - i plots were linear around the centre with n values of one),. were treated mechanically, using these equations. A similar treatment was used for the effect of pyrophosphite on the Cr s+ wave. Ba ~+ gave an irreversible wave, however pyrophosphite produced no significant effect on it. * Present address: Monsanto Chemicals Ltd., Ruabon, North Wales. (Research Depamnent.) '~) J. R. VAN WAZER and C. F. CALL1S, Chem. Rev. 58, 1011 (1958). is) 1. M. KOLTHOFF and J. J. L I N G A N E , Polaro~raphy, 2nd Ed., Vol. I., p. 22l. lnterscience, New York (1952). 1985
1986
D. GRANT a n d D. S. PAYNE
C d 2+. SOUCHAY a n d FAUCHERRE in similar polarographic experiments observed p y r o p h o s p h a t e C d ~+ c o m p l e x f o r m a t i o n in 3'5 M K C I base electrolyte h a v i n g log K = 4.2 a n d p : 2. TM Little shift w a s h o w e v e r f o u n d by us in the C d 2+ wave as a function o f the p y r o p h o s p h i t e concentration in the presence o f excess chloride ion. E v e n with small a m o u n t s of chloride ions present smaller shifts were f o u n d t h a n t h o s e for nitrate media. This clearly indicates that p y r o p h o s p h i t e complexes with C d 3+ to a lesser extent t h a n the chloride ion, Values e s t i m a t e d f r o m the d a t a (Fig. 1) are Cd~+-Cl - interaction, p = 1, log K = 2"3 a n d Cd2+-H2P~O~ ~- interaction, p = 1, log K ~ 1.8. T h e m o s t recent investigations o f the complexing o f C I - by C d ~+ give log K v a l u e s for the 1 : 1 complex close to 2"0. ~4) TABLE 1.--EXPERIMENTAL RESULTS, COMPLEXING ACTION OF PYROPHOSPHITE AND PHOSPHITE AS INDICATED BY SHIFTS IN POLAROGRAPHIC HALF-WAVE POTENTIALS, COMPARED WITH PYROPHOSPHATE COMPLEXING
M PyrophosphiteCd2+ CdZ+ Cd2+ Cd2+ Cd2+ Cu z+ Cu2+ pb~+ TI + Ni2+ Co2+ CrY+ Ba2+
UO~2+ U022+ PhosphiteCd2+ Cu2+
lOgl0K
p
(L) Range studied
0.32 0.32 0.70
0.15 0.14 0.29
0-0.194 0-0.124 0-0.126
0.674 to 0.680 0.624 to 0.630 0.610 to 0.626
1.41 0.52 (1.5) 1.4 (0.7)
0-73 0.23 (1) 1.5 (1) -(0-45) (1) (1) -(4) (2)
0-0.096 04).120 0-0.096 0-0-588 0-0.686 0-0-423 0-0-347 0-0.498 ff4).274 0-0.297 0-0.116 0-0.138
0.582 to 0-601 0-579 to 0.608 +0.010 to --0.005 +0.013 to --0.019 0.413 to 0.428 0.483 to 0.497 1.035 to 1.053 1.368 to 1.424 0.893 to 0.971 (1.56) to (1.56) 0-171 to 0.420 0.182 to 0.360
0.49 0.2
0-0.038 0-0-102
0'582 to 0.587 +0"020 to +0.016
--
(0.5) (1.1) (1-9) -(12-7) (8.8) 0.95 0.35
Pyrophosphate cs~ Cd 2+ 4.2 Cu 2+ 12 Pb 2+ 5-3 TI 2+ 2.0
E½* ' range (V)
Medium
(M) × 10 -a
KCI 2.1 M KC1 3.5 M .fKNOa 0-34 M I.KCI 0.0033 M K N O a 0.34 M K N O 3 1.8 M K N O 3 0.34 M KNOz 1.8 M K N O 3 1.8 M KNOa 1.8 M KNO3 1.8 M K N O z 1.8 M K N O 3 1.8 M KNO3 1.8 M K N O 3 1.98 M NaCIOa 2.3 M
1-63 1.63 1.63
KNO 3 1 M K N O s 1.4 M
1.66 1-66 1.27 1.27 1.02 1.31 1.02 1-67 0.95 1.64 1.13 1"13 1'66 0.65
2 2 2 2
* E~ values are given in negative volts except where otherwise indicated. are estimates only.
Values given in brackets
R e s u l t s for the c o m p l e x i n g by p h o s p h i t e at p H 4"5 s h o w e d a similar shift in the half wave potential with ligand c o n c e n t r a t i o n to that o f the pyro species, Fig. 1 c u r v e f i suggesting that the complexing activity o f p h o s p h i t e is similar in m a g n i t u d e to that o f pyrophosphite. N o chelating effect which m i g h t h a v e been expected to lead to a n increase in the stability o f the pyro c o m p l e x is found. C u e+. T h e plot o f AE½ vs. the p y r o p h o s p h i t e concentration (log scale) is s h o w n in Fig. 2. T h e curve t e n d s to a line w i t h p = ca. 1-5 a n d log K = ca. 1'4, with increasing p y r o p h o s p h i t e concentration. T h e observed complexing action is t h u s smaller t h a n observed between C u 2+ a n d pyrop h o s p h a t e by several orders o f m a g n i t u d e , possibly u p to ten (a n u m b e r o f stability c o n s t a n t s have been reported for complexing between p y r o p h o s p h a t e a n d C u ~+, cf. Reference 5). O r t h o p h o s p h i t e is f o u n d to p r o d u c e similar shifts in the C u ~+ waves to the pyro species. Only relatively low p h o s p h i t e concentrations could be employed h o w e v e r owing to a tendency for chemical reduction o f the C u 2+ to C u + to occur with increasing C u e+ concentration. This reaction is detected by a c h a n g e in the C u 2+ wave. ~3~ P. SOUCHAY a n d J. FAUCHERRE, Bull. SOC. Chin. France, 14, 533 (1947). t4~ W. B. TREUMANN a n d L. M. FERRIS, J. /truer. Chem. Soc. 80, 5048 (1958).
Pyrophosphite as a ligand
1987
0.680 ( 3.670
~b
0-630
0.820
LIJ
0.810 (
i
~ d
j / 1 1 /
0.600
/
f,
/ i /
y
e
0"590
0"580t 0
1
I
0.04
I
I
I
0.08
Ligand
E
0,12
I
[
046
]
cone. M0
FIG. 1.--E~ values for the Cd2+-pyrophosphite system (a) KC12.1 M, (b) KCI 0"35 M, (c) KNOs 0.34 M, KC1 0.0033 M, (d) No base electrolyte, (e) KNOs 0"34 M, (g) KNO:~ 1-8 M. Curve (f) shows the corresponding values for the Cd~+-phosphite system KNO3 1 M. 0028-
0,020
>
0.012
-,~
///,,o 0.004
°I -0,004'
0.01
7//
~ . . . . . . . ~ t /
0.1
1,0
Ligand conc. M. FiG. 2.--Semi log plot of Cu ~+ E~ against pyrophosphite conc. (a) K N O a 0.34 M, (b) K N O 3 1.8 M. Ni =+. The Ni ~+ polarographic wave varies regularly with pyrophosphite concentration over a small range of voltage, Fig. 3. Although the absolute treatment of the results is made dimcult by the irreversible nature of the wave, it is considered that the general low values o f # and K obtained from the data are of the same order of magnitude to those obtained with C u 2+ and Cd =+. Pb 2+, C o ~+, Cr 3+, Tl + and Ba 2+. These ions were studied in lesser detail. The general type of interaction occurring was established, No significant change in the waves obtained with TI + or
1988
D. GRANT and D. S. PAYNE
Ba ~+ was found due to pyrophosphite in solution. Shifts occurred in the waves obtained with Pb z+, Co S+ and Cr a+ ions. Estimates of the K values assuming p = 1 were made, being significantly smaller than the values reported for pyrophosphate complexing, c5~ 1.055
I. 05 0
>
1.045
W I
1.040
1.035
I 0
0.1
I
I
I
0"2
0'3
0-4
Ligond conc.M.
FIG. 3.--Ni ~+ E½ plotted against the
pyrophosphitecone.
KNOa 1.8 M.
UO22+. A wave is obtained giving a straight line plot of ilia - - i vs. E having E½ = 0"171 V, (this value however was observed to change as the UOz 2+ solution "aged", after 16 hr the value was 0.195 V). Nitrate and perchlorate media were compared, different interactions between the pyrophosphite and uranyl species being shown in these media. There was a tendency for chemical reduction of UO~ 2+ and formation of a colloidal suspension to occur in solution during some of the experiments, particularly at high pyrophosphite concentrations, marked changes occurred in these waves. Fig. 4 shows a plot of the observed E½ values where little side reaction had occurred, for the two media as a function of the pyrophosphite concentration, comparing the experimental points with the theoretical curve for the p and log K values in Table 1.
O. 500
A
0300 LLI I
o.loo
0
I
0-02
I
0.04
I 0-06
I
0.08
I
o.Jo
I
~.iz
0-14
Ligond conc.M.
FIG. 4.--UO22+ E½ plotted against the pyrophosphite cone. (a) KNOz 1.98 M, (b) NaCIO4 2-3 M. (5~ j. B. BJERROM, G. SCHWARZENBACHand L. G. SILLEN, Special Publication No. 7, part II, S e e . (1958).
Chem.
Pyrophosphite as a ligand
1989
Instrumentation
The Tinsley recording polarograph was used (automatic recording and voltage sweep). The polarographic cell had a saturated calomel electrode thermostatted at 25°. Solutions were deoxygenated by bubbling nitrogen prior to the run. Reagents
Analytical grade reagents were employed for the metal ion solutions. Disodium pyrophosphite was studied since this is conveniently prepared by the thermal dehydration of NaH(HPO3)2~H20 at 220° without oxidation (checked by ceriometric analysis) of the pyrophosphite unit. Sodium pyrophosphite metal ion solutions had pH ca. 7 and hydrolysis of the pyrophosphite unit is sufficiently slow under these conditions to enable small amounts of ortho phosphite present to be ignored. Phosphite solutions pH ca. 4.5-5 were obtained from NaH(HPO)~2½H20. The concentration ranges of pyrophosphite and phosphite which could be conveniently studied were dictated by consideration of solubility and the tendency for side reactions to occur. DISCUSSION The complexing activity of pyrophosphite is much weaker than would be expected for this analogue of the pyrophosphate unit. The degree of interaction between Cu 2+ and pyrophosphite is of a similar order of magnitude to that of the interaction between alkali metal ions and pyrophosphate. The exception which has been found in the case of uranyl where strong pyrophosphite uranyl complexes occur is probably related to the unique properties of uranyl as a central metal unit. The chelation stabilization which is a characteristic feature of the complexing action of pyrophosphate appears to be absent with pyrophosphite. The colours of pyrophosphate complex ions with transition metals are often markedly different from those of the uncomplexed metal ion in solution. This is not the case with pyrophosphite where no colour change due to complex formation has been observed. Some insight may be thrown on the problem as to the cause of this difference in complexing ability by consideration of probable electronic differences in the structures of the two oxy-anions. From analogy with orthophosphorous acid and orthophosphoric acid where increased localization of bonding electrons in the isolated P- O bond occurs from phosphoric acid to phosphorous acid, (6) and from the known sequence where increased localization of bonding electrons in the isolated P - - O takes place on replacing P - - O by P - - O - - P in a structure, (7) it would be expected that in pyrophosphite, relative to pyrophosphate, increased localization of 7r-bonding would be found. It follows from this argument that there will be more ~r bonding in the P - - O - - P bond of pyrophosphate than pyrophosphite, possibly the latter case having essentially none. The hydrolytic stability of the two kinds of pyro P- O - - P bonds is markedly different, 18) pyrophosphite hydrolysing much faster, being comparable with the rate of hydrolysis of the branch P - - O - - P bond in condensed phosphates where again there is little ~rbonding character in the bond. Although it is therefore likely that the amount of ~r bonding in the two types of P - - O - - P bonds being considered will be different this is not directly relatable to the stability of the metal ion complexes which are formed by the anions containing these units unless the kind of bonding in the P - - O - - P bond (,~1S. FURBERGand P. LANDMARK,Aeta Chem. Stand. 11, 1505 (1957). c71 j . R . VAN WAZER, J. Amer. Chem. Soe.
78, 5709 (1956).
Isl j. R. VAN WASER,Phosohorus andits Compounds, Vol. I, p. 395; p. 452 et seq. lnterscience, New York (1958).
1990
D. GRANTand D. S. PAYNE
is involved in chelate bond formation, thus "resonance stabilisation" of a six membered chelate ring can be envisaged, delocalisation of electrons occurring around a six membered ring. It has been pointed out tg) that the difference between pyrophosphite and phosphite as regards the formation of complexes can be explained by the lesser electron donation by the P - - H relative to P - - O H towards the P - - O bond directly interacting with the metal ion. This would serve to explain decreased complexing by phosphite units in general, relative to phosphate, but does not seem to us to adequately explain the great increase in complexing action from phosphate to pyrophosphate, an increase which is not found from phosphite to pyrophosphite.
Acknowledgement--One of the authors (D. G.) thanks Albright & Wilson Ltd. for a maintenance grant. 19~by a referee.