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Halogen exchange reactions of phosphorus(V) compounds in strongly acidic solvents—A 31P NMR study
J inorg, nucl Chem Vol. 41, pp. 1273-1275 Pergamon Press Ltd., 1979 Printed in Great Britain
HALOGEN EXCHANGE REACTIONS OF PHOSPHORUS(V) COMPOUNDS IN STRONGLY ACIDIC SOLVENTS---A 31p NMR STUDY K. B. DILLON, M. P. NISBET and T. C. WADDINGTON Chemistry Department, University of Durham, South Road, Durham DHI 3I_,E,England (Received 23 October 1978; received.for publication 23 January 1979) Abstract--Halogen exchange reactions of phosphorus(V) halides and oxyhalides in strongly acidic solvents have been investigatedby means of 31p spectroscopy. Neither PCI5 and PBrs, nor POCI3 and POBr3, undergo mutual exchange in an "inert" (non-exchanging)solvent such as 25 oleum at, or just above, room temperature. Exchange is observed when a suitable alkali metal halide is added to a solution of POBr3 in HSC103or HSFO3,but does not take place in the absence of the halide. The results confirm that exchange in these solvents does not involvedirect reaction of the phosphorus(V)compound with the solvent, but occurs via an indirect mechanism.
INTRODUCTION
present after 42 days and the PCI3OH ÷ signal had increased in intensity relative to the PCI4÷ peak. Slow solvolysis of the tetrahalophosphonium ions thus occurs, again in agreement with the data for PC15 and PBr~ separately, but there is no halogen exchange. The appearance of PCI3OH ~ in the initial spectrum is due to the instability of the hexachlorophosphate ion in strongly acidic media[l]. Very similar results were obtained for a PCIs-PBr~ mixture in 65 oleum; the initial 31p solution spectrum contained resonances at -88 (PCI4+), -23 (PCI3OH÷), 75 (PBr3OH ÷) and 85 (PBr4+) ppm. Some solvolysis occurs because the instability of PCIt- causes a marked local reduction in acid strength, as shown by eqn (1) for reaction with disulphuric acid. (65 oleums contains disulphuric and higher polysulphuric acids)J7].
During an investigation of the behaviour of some phosphorus(V) halides and oxyhalides in strongly acidic solvents, halogen exchange was observed in the reactions of PCIs, Et4NPCI~, PCLBCI4, PBr5 and PBr4BBr4 with HSFO3, and of PBr5 with HSCIO3[1]. No exchange took place, however, between partially protonated POCI3 or POBr3 and halosulphuric acids [1]. Mixed species have also been found in the products of reaction between phosphorus(Ill) halides and HSFO3 or HSC10312]. The results were interpreted in terms of an indirect exchange mechanism, with HF or HC1, formed either as byproducts or in side reactions, acting as the source of the exchanging halogen in HSFO3 and HSC103 respectively, rather than the solvents themselves. We have extended this work to other phosphorus(V) systems with the potential for halogen exchange, firstly to see whether two different solutes would undergo mutual exchange in an "inert" (non-exchanging) acidic solvent such as 25 oleum, and secondly to try to confirm the mechanism for exchange proposed previously. 3tp NMR solution spectroscopy has been used to identify the reaction products.
PCIt- + 3H2S207 ~ PCI3OH + + HC1 + 2HS207 + 2HSCIO,.
EXPERIMENTAL
All manipulations, including NMR sample preparation, were carried out under an inert atmosphere of dry nitrogen. Chemicals of the best available commercial grade were used, generally without further purification except for the alkali metal halides which were re-dried before use. ~P NMR spectra were recorded at 307.2K as described previously[l,2], using the F.T. spectrometer and 5ram o.d. sample tubes. Chemical shifts were measured relative to external 85% H3PO4, and are expressed with the upfielddirection taken as positive. RESULTS AND DISCUSSION A mixture of PCI5 and PBr5 was prepared in 25 oleum and its 31p NMR spectrum recorded. Signals were found at -88 (PC14+)[1.3-51, -27 (PC13OH+)[1, 2] and 85 ppm(PBr, ÷)[1-3]. (Phosphoryl compounds except H3PO4 will be only partially protonated [1, 6], but are represented in the protonated form here and subsequently for clarity.) The results are in very good agreement with those for PC15 and PBr5 separately in 25 oleum [1]. The spectrum was unchanged after 7 days, but a new weak signal at 60ppm (PBr3OH+)[I,2] was
(1)
Both HC1 and HSCIO3 are weaker acids than H25207. Once the system is homogeneous, however, further solvolysis is very slow. The spectrum showed no change after 50 days, but the PX~OH + peaks had increased somewhat in relative intensity after 10 months. No halogen exchange was detected, however, even after this length of time. A mixture of POCI3 and POBr3 in 25 oleum was also prepared. The initial 3~p spectrum showed signals at --25 and 65 ppm, corresponding to the (partially) protonated species PC13OH+ and PBr3OH+[I]. No change was apparent after 42 days, but additional weak signals at 4 and 15 ppm were found after 10 months, indicating that slow solvolysis to P(OH)4 + and condensed phosphate species occurs, as found for inorganic phosphates in 25 oleum[8]. Intermediate halophosphates such as PBr2(OH)2÷ remained below the detection limit in this system. The results are again similar to those for the components separately in 25 oleum and indicate that no halogen exchange occurs. Although POBr3 is readily soluble in POC13, the compounds do not exchange at room temperature or just above, as shown by 3rp NMR spectroscopy[9]. Rapid exchange takes place at 573 K, however, and may be used to obtain the mixed species[10]. Exchange can be
1273
1274
K.B. DILLON et al.
brought about at 307.2 K by addition of a catalytic quantity of the lanthanide shift reagent Eu(fod)3, either to a POCI3-POBr3 mixture in an organic solvent such as 1,2-dichloroethane, or to a mixture of the pure components[9]. This property was utilised to obtain NMR data for the (partially) protonated mixed species in HSCIO3. A small amount of Eu(fod)3 was added to a solution of POBr3 in POC13, and the mixture allowed to stand for one day. A portion of the mixture was then placed in an NMR tube, while a similar portion was dissolved in HSCIO3 in a second tube. The spectroscopic results for both solutions are represented diagramatically in Fig. 1 and the chemical shifts for the protonated forms and their precursors are given in Table 1. (The signals derived from POC13 are more intense because it is present in relative excess). The values for the parent compounds are in excellent agreement with literature data[11] and protonation causes the expected downfield shift in all cases[l, 6, 12]. Assignments made previously [1, 2] for PCI2BrOH÷ and PCIBr2OH÷ are thus confirmed. One interesting point (Fig. 1) is that while slow exchange continues in the parent sample over a 1 month period to increase the concentration of the mixed halides at the expense of POBr3, no further exchange takes place in HSCIOa. It is quite probable that the Eu(fod)3 reagent is destroyed, or at least rendered inactive, in such a highly acidic medium, thus preventing further exchange. Protonation of the phosphoryl oxygen may also hinder the approach of an Eu(fod)3 molecule, inhibiting the formation of weak complexes in which the phosphorus-halogen bonds are more labile than in the uncomplexed compounds [9]. As mentioned in the introduction, partially protonated POBr3 and POC13 do not undergo halogen exchange in HSFO3 or HSC103, even over a 4-month period[l]. Exchange was observed when PX5 or PX3 molecules were dissolved in these solvents, and this difference in behaviour was ascribed to indirect exchange via HF or HCI, for which plausible mechanisms of formation could be suggested in each case[l, 2]. To test this hypothesis, solutions of POBr3 in HSFO3 and HSC103 were reacted with both NaCI and KF separately. The alkali halides dissolved with effervescence. The 3tp NMR spectrum of the POBr3-NaCI mixture in HSFO3 showed a strong signal at 83 ppm (PBr3OH+), together with a strong signal at -2 ppm (P(OH)4+)[8, 12] and a weak doublet at 8 ppm, ~J'P-F 990 Hz, due to PF(OH)3 + [2, 6]. After 14 days the same resonances were present, together with a new peak at
Table 1. 8 31p (ppm) for some mixed phosphorus(V)oxyhalides in POCI3and HSC103 Species Solvent
POCI3 POC12Br POC1Br2
POCI3 HSCIO3
-2 -22
66 41
C1- + HSFO3--, HCI + SFO3-.
HC! + HSFO3~HF + HSCIO3.
H F + H S F O 3 ~ H 2 F + + SFO3-.
(4)
Thus the formation of both HCI and HF can be rationalised and these are likely to be the active species in exchange reactions with PBr3OH +. The POBr3-KF mixture in HSFO3 initially showed a'strong peak at -2 ppm, due to P(OH)4+ and a weaker signal at 78ppm (PBr3OH+). After 14 days an additional weak doublet at 8 ppm, ~Jp-v 990 Hz, was present, assigned to PF(OHh +. The results show that KF can act as a source of fluorine for exchange in HSFO3, although substitution in this instance appears to be quite slow. The data from HSC103 were even more convincing. The POBr3-NaCI mixture gave a strong 3~p signal at 83 ppm (PBr3OH+), with weaker signals at - 2 (P(OH)4+), 15 (PC12BrOH+) and 36 (PC1Br2OH+) ppm. Mixed species were thus rapidly formed. After 14 days the peak at 36ppm had disappeared and a new weak resonance at -15ppm, due to either PCI3OH+ or PC12(OH)2+, was apparent. The chloride ion presumably reacts with the HSCt0 3 Solution
i
lapin.
(3)
ARhough this equilibrium is expected to lie over to the left, protonation of HF, which is a stronger base than HC1, eqn (4), will pull it further to the right (1).
,
I
I
i
1 month
t
(2)
This reaction will reduce the acid strength locally, accounting for the appearance of solvolysis products such as P(OH)4+. HCI can then equilibriate with the solvent, eqn (3).
,i
-30
103 79
36 ppm, assigned to PCIBr2OH+. NaCI can therefore act as a source of both fluorine and chlorine for exchange in HSFO3. The initial reaction of chloride ion with the solvent will produce undissociated HCI, eqn (2).
POCI3 Solution
Initially
30 10
POBr3
120
t
-30
i
p.pm.
120
Fig. 1. Relative peak heights in the 31p NMR spectra of POCIr-POBr3mixtures with Eu(fod)3present.
1275
Halogen exchange reactions of P(V) compounds solvent to form undissociated HC1, eqn (5), simultaneously causing local reductions in acid strength and some solvolysis of PBr3OH + to P(OH). +.
mechanism of exchange is indirect, and does not involve a primary reaction of the phosphorus compound with the solvent.
CI + HSCIO3--* HC1 + SC103
Acknowledgements--We thank the Science Research Council for
(5)
HCI then substitutes readily into PBr3OH +, giving rise to mixed species. The initial spectrum of the POBr3-KF mixture in HSC103 showed a strong peak at 83ppm (PBr3OW), with a weak singlet at - 2 ppm, ascribed to P(OH)4 ÷ and a weak doublet at 8 ppm, ~JP-F 990 Hz, due to PF(OHh +. After 14 days, further signals were present at 23 (triplet, 'JP-v 1000 Hz) and 36 ppm (quartet, tJp_v 1060 Hz), assigned to PF2(OHh + and POF3 respectively, the latter remaining unprotonated[6]. Interestingly, no mixed bromochloro-derivatives were found, suggesting that substitution by HF (or F-) takes place in preference to exchange with HCI in this system. HF would be readily formed by an analogous reaction to that in eqn (5), while HCI would result from the establishment of the equilibrium in eqn (3). Since PBr3OH + is stable to exchange in these solvents in the absence of added halides (1), the results provide clear evidence for an indirect mechanism of halogen exchange, via species such as HF and HCI, as suggested previously[l, 2]. We therefore conclude that the phosphorus(V) halides and oxyhalides studied do not exchange in strongly acidic solvents, even HSFO3 and HSC103, unless halide ions or hydrogen halide molecules are either produced by a chemical reaction or added deliberately. The
JINC Vol 41, No, 9--B
the award of a maintenance grant (to M.P.N.) and Dr. A. Royston for assistance with the F.T. spectrometer.
REFERENCES
I. K. B. Dillon, M. P. Nisbet and T. C. Waddington. J. Chem. Soc. (Dalton Trans), 1455, (1978). 2. K. B. Dillon, M. P. Nisbet and T. C. Waddington, J. Chem. Soc. (Dalton Trans ) (1979). (in press). 3. K. B. Dillon,T. C. Waddingtonand D. Younger, lnorg. Nucl. Chem. Lett. 9, 63 (1973). 4. A. Schmidpeter and H. Brecht, Angew. Chem. 79, 535 ~1%7). 5, J. R. Van Wazer, private communicationquoted in Ref. [4]. 6. G. A. Olah and C. W. McFarland, Inorg. Chem. II, 845 (1972). 7. R. J. Gillespie and E. A. Robinson, Can. J. Chem. 40, 658 (1%2). 8. K. B. Dillon and T. C. Waddington,J. Chem. Soc. (A), 1146 (1970). 9. K. B. Dillon, M. G. Craveirinha Dillonand T. C. Waddington, manuscript in preparation. 10. L. C. D. Groenweghe and .1. H. Payne, J. Am. Chem. Soc. 81. 6357 (1959). l I. V. Mark, C. H. Dungan, M. M. Crutchfield and J. R. Van Wazer, Topics Phosphorus Chem, 5, 227 (1%7). 12. G. A. Olah and C. W. McFarland, J. Org. Chem. 36. 1374 (1971).
HALOGEN EXCHANGE REACTIONS OF PHOSPHORUS(V) COMPOUNDS IN STRONGLY ACIDIC SOLVENTS---A 31p NMR STUDY K. B. DILLON, M. P. NISBET and T. C. WADDINGTON Chemistry Department, University of Durham, South Road, Durham DHI 3I_,E,England (Received 23 October 1978; received.for publication 23 January 1979) Abstract--Halogen exchange reactions of phosphorus(V) halides and oxyhalides in strongly acidic solvents have been investigatedby means of 31p spectroscopy. Neither PCI5 and PBrs, nor POCI3 and POBr3, undergo mutual exchange in an "inert" (non-exchanging)solvent such as 25 oleum at, or just above, room temperature. Exchange is observed when a suitable alkali metal halide is added to a solution of POBr3 in HSC103or HSFO3,but does not take place in the absence of the halide. The results confirm that exchange in these solvents does not involvedirect reaction of the phosphorus(V)compound with the solvent, but occurs via an indirect mechanism.
INTRODUCTION
present after 42 days and the PCI3OH ÷ signal had increased in intensity relative to the PCI4÷ peak. Slow solvolysis of the tetrahalophosphonium ions thus occurs, again in agreement with the data for PC15 and PBr~ separately, but there is no halogen exchange. The appearance of PCI3OH ~ in the initial spectrum is due to the instability of the hexachlorophosphate ion in strongly acidic media[l]. Very similar results were obtained for a PCIs-PBr~ mixture in 65 oleum; the initial 31p solution spectrum contained resonances at -88 (PCI4+), -23 (PCI3OH÷), 75 (PBr3OH ÷) and 85 (PBr4+) ppm. Some solvolysis occurs because the instability of PCIt- causes a marked local reduction in acid strength, as shown by eqn (1) for reaction with disulphuric acid. (65 oleums contains disulphuric and higher polysulphuric acids)J7].
During an investigation of the behaviour of some phosphorus(V) halides and oxyhalides in strongly acidic solvents, halogen exchange was observed in the reactions of PCIs, Et4NPCI~, PCLBCI4, PBr5 and PBr4BBr4 with HSFO3, and of PBr5 with HSCIO3[1]. No exchange took place, however, between partially protonated POCI3 or POBr3 and halosulphuric acids [1]. Mixed species have also been found in the products of reaction between phosphorus(Ill) halides and HSFO3 or HSC10312]. The results were interpreted in terms of an indirect exchange mechanism, with HF or HC1, formed either as byproducts or in side reactions, acting as the source of the exchanging halogen in HSFO3 and HSC103 respectively, rather than the solvents themselves. We have extended this work to other phosphorus(V) systems with the potential for halogen exchange, firstly to see whether two different solutes would undergo mutual exchange in an "inert" (non-exchanging) acidic solvent such as 25 oleum, and secondly to try to confirm the mechanism for exchange proposed previously. 3tp NMR solution spectroscopy has been used to identify the reaction products.
PCIt- + 3H2S207 ~ PCI3OH + + HC1 + 2HS207 + 2HSCIO,.
EXPERIMENTAL
All manipulations, including NMR sample preparation, were carried out under an inert atmosphere of dry nitrogen. Chemicals of the best available commercial grade were used, generally without further purification except for the alkali metal halides which were re-dried before use. ~P NMR spectra were recorded at 307.2K as described previously[l,2], using the F.T. spectrometer and 5ram o.d. sample tubes. Chemical shifts were measured relative to external 85% H3PO4, and are expressed with the upfielddirection taken as positive. RESULTS AND DISCUSSION A mixture of PCI5 and PBr5 was prepared in 25 oleum and its 31p NMR spectrum recorded. Signals were found at -88 (PC14+)[1.3-51, -27 (PC13OH+)[1, 2] and 85 ppm(PBr, ÷)[1-3]. (Phosphoryl compounds except H3PO4 will be only partially protonated [1, 6], but are represented in the protonated form here and subsequently for clarity.) The results are in very good agreement with those for PC15 and PBr5 separately in 25 oleum [1]. The spectrum was unchanged after 7 days, but a new weak signal at 60ppm (PBr3OH+)[I,2] was
(1)
Both HC1 and HSCIO3 are weaker acids than H25207. Once the system is homogeneous, however, further solvolysis is very slow. The spectrum showed no change after 50 days, but the PX~OH + peaks had increased somewhat in relative intensity after 10 months. No halogen exchange was detected, however, even after this length of time. A mixture of POCI3 and POBr3 in 25 oleum was also prepared. The initial 3~p spectrum showed signals at --25 and 65 ppm, corresponding to the (partially) protonated species PC13OH+ and PBr3OH+[I]. No change was apparent after 42 days, but additional weak signals at 4 and 15 ppm were found after 10 months, indicating that slow solvolysis to P(OH)4 + and condensed phosphate species occurs, as found for inorganic phosphates in 25 oleum[8]. Intermediate halophosphates such as PBr2(OH)2÷ remained below the detection limit in this system. The results are again similar to those for the components separately in 25 oleum and indicate that no halogen exchange occurs. Although POBr3 is readily soluble in POC13, the compounds do not exchange at room temperature or just above, as shown by 3rp NMR spectroscopy[9]. Rapid exchange takes place at 573 K, however, and may be used to obtain the mixed species[10]. Exchange can be
1273
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K.B. DILLON et al.
brought about at 307.2 K by addition of a catalytic quantity of the lanthanide shift reagent Eu(fod)3, either to a POCI3-POBr3 mixture in an organic solvent such as 1,2-dichloroethane, or to a mixture of the pure components[9]. This property was utilised to obtain NMR data for the (partially) protonated mixed species in HSCIO3. A small amount of Eu(fod)3 was added to a solution of POBr3 in POC13, and the mixture allowed to stand for one day. A portion of the mixture was then placed in an NMR tube, while a similar portion was dissolved in HSCIO3 in a second tube. The spectroscopic results for both solutions are represented diagramatically in Fig. 1 and the chemical shifts for the protonated forms and their precursors are given in Table 1. (The signals derived from POC13 are more intense because it is present in relative excess). The values for the parent compounds are in excellent agreement with literature data[11] and protonation causes the expected downfield shift in all cases[l, 6, 12]. Assignments made previously [1, 2] for PCI2BrOH÷ and PCIBr2OH÷ are thus confirmed. One interesting point (Fig. 1) is that while slow exchange continues in the parent sample over a 1 month period to increase the concentration of the mixed halides at the expense of POBr3, no further exchange takes place in HSCIOa. It is quite probable that the Eu(fod)3 reagent is destroyed, or at least rendered inactive, in such a highly acidic medium, thus preventing further exchange. Protonation of the phosphoryl oxygen may also hinder the approach of an Eu(fod)3 molecule, inhibiting the formation of weak complexes in which the phosphorus-halogen bonds are more labile than in the uncomplexed compounds [9]. As mentioned in the introduction, partially protonated POBr3 and POC13 do not undergo halogen exchange in HSFO3 or HSC103, even over a 4-month period[l]. Exchange was observed when PX5 or PX3 molecules were dissolved in these solvents, and this difference in behaviour was ascribed to indirect exchange via HF or HCI, for which plausible mechanisms of formation could be suggested in each case[l, 2]. To test this hypothesis, solutions of POBr3 in HSFO3 and HSC103 were reacted with both NaCI and KF separately. The alkali halides dissolved with effervescence. The 3tp NMR spectrum of the POBr3-NaCI mixture in HSFO3 showed a strong signal at 83 ppm (PBr3OH+), together with a strong signal at -2 ppm (P(OH)4+)[8, 12] and a weak doublet at 8 ppm, ~J'P-F 990 Hz, due to PF(OH)3 + [2, 6]. After 14 days the same resonances were present, together with a new peak at
Table 1. 8 31p (ppm) for some mixed phosphorus(V)oxyhalides in POCI3and HSC103 Species Solvent
POCI3 POC12Br POC1Br2
POCI3 HSCIO3
-2 -22
66 41
C1- + HSFO3--, HCI + SFO3-.
HC! + HSFO3~HF + HSCIO3.
H F + H S F O 3 ~ H 2 F + + SFO3-.
(4)
Thus the formation of both HCI and HF can be rationalised and these are likely to be the active species in exchange reactions with PBr3OH +. The POBr3-KF mixture in HSFO3 initially showed a'strong peak at -2 ppm, due to P(OH)4+ and a weaker signal at 78ppm (PBr3OH+). After 14 days an additional weak doublet at 8 ppm, ~Jp-v 990 Hz, was present, assigned to PF(OHh +. The results show that KF can act as a source of fluorine for exchange in HSFO3, although substitution in this instance appears to be quite slow. The data from HSC103 were even more convincing. The POBr3-NaCI mixture gave a strong 3~p signal at 83 ppm (PBr3OH+), with weaker signals at - 2 (P(OH)4+), 15 (PC12BrOH+) and 36 (PC1Br2OH+) ppm. Mixed species were thus rapidly formed. After 14 days the peak at 36ppm had disappeared and a new weak resonance at -15ppm, due to either PCI3OH+ or PC12(OH)2+, was apparent. The chloride ion presumably reacts with the HSCt0 3 Solution
i
lapin.
(3)
ARhough this equilibrium is expected to lie over to the left, protonation of HF, which is a stronger base than HC1, eqn (4), will pull it further to the right (1).
,
I
I
i
1 month
t
(2)
This reaction will reduce the acid strength locally, accounting for the appearance of solvolysis products such as P(OH)4+. HCI can then equilibriate with the solvent, eqn (3).
,i
-30
103 79
36 ppm, assigned to PCIBr2OH+. NaCI can therefore act as a source of both fluorine and chlorine for exchange in HSFO3. The initial reaction of chloride ion with the solvent will produce undissociated HCI, eqn (2).
POCI3 Solution
Initially
30 10
POBr3
120
t
-30
i
p.pm.
120
Fig. 1. Relative peak heights in the 31p NMR spectra of POCIr-POBr3mixtures with Eu(fod)3present.
1275
Halogen exchange reactions of P(V) compounds solvent to form undissociated HC1, eqn (5), simultaneously causing local reductions in acid strength and some solvolysis of PBr3OH + to P(OH). +.
mechanism of exchange is indirect, and does not involve a primary reaction of the phosphorus compound with the solvent.
CI + HSCIO3--* HC1 + SC103
Acknowledgements--We thank the Science Research Council for
(5)
HCI then substitutes readily into PBr3OH +, giving rise to mixed species. The initial spectrum of the POBr3-KF mixture in HSC103 showed a strong peak at 83ppm (PBr3OW), with a weak singlet at - 2 ppm, ascribed to P(OH)4 ÷ and a weak doublet at 8 ppm, ~JP-F 990 Hz, due to PF(OHh +. After 14 days, further signals were present at 23 (triplet, 'JP-v 1000 Hz) and 36 ppm (quartet, tJp_v 1060 Hz), assigned to PF2(OHh + and POF3 respectively, the latter remaining unprotonated[6]. Interestingly, no mixed bromochloro-derivatives were found, suggesting that substitution by HF (or F-) takes place in preference to exchange with HCI in this system. HF would be readily formed by an analogous reaction to that in eqn (5), while HCI would result from the establishment of the equilibrium in eqn (3). Since PBr3OH + is stable to exchange in these solvents in the absence of added halides (1), the results provide clear evidence for an indirect mechanism of halogen exchange, via species such as HF and HCI, as suggested previously[l, 2]. We therefore conclude that the phosphorus(V) halides and oxyhalides studied do not exchange in strongly acidic solvents, even HSFO3 and HSC103, unless halide ions or hydrogen halide molecules are either produced by a chemical reaction or added deliberately. The
JINC Vol 41, No, 9--B
the award of a maintenance grant (to M.P.N.) and Dr. A. Royston for assistance with the F.T. spectrometer.
REFERENCES
I. K. B. Dillon, M. P. Nisbet and T. C. Waddington. J. Chem. Soc. (Dalton Trans), 1455, (1978). 2. K. B. Dillon, M. P. Nisbet and T. C. Waddington, J. Chem. Soc. (Dalton Trans ) (1979). (in press). 3. K. B. Dillon,T. C. Waddingtonand D. Younger, lnorg. Nucl. Chem. Lett. 9, 63 (1973). 4. A. Schmidpeter and H. Brecht, Angew. Chem. 79, 535 ~1%7). 5, J. R. Van Wazer, private communicationquoted in Ref. [4]. 6. G. A. Olah and C. W. McFarland, Inorg. Chem. II, 845 (1972). 7. R. J. Gillespie and E. A. Robinson, Can. J. Chem. 40, 658 (1%2). 8. K. B. Dillon and T. C. Waddington,J. Chem. Soc. (A), 1146 (1970). 9. K. B. Dillon, M. G. Craveirinha Dillonand T. C. Waddington, manuscript in preparation. 10. L. C. D. Groenweghe and .1. H. Payne, J. Am. Chem. Soc. 81. 6357 (1959). l I. V. Mark, C. H. Dungan, M. M. Crutchfield and J. R. Van Wazer, Topics Phosphorus Chem, 5, 227 (1%7). 12. G. A. Olah and C. W. McFarland, J. Org. Chem. 36. 1374 (1971).