The Oxidizing Properties of the Third Transition Series Hexafluorides and Related Compounds

Some of the transition metal hexafluorides demonstrate an astonishing oxidizing power. In particular may be mentioned PtF 6 , which is capable of oxidizing molecular oxygen or xenon, a process requiring an electron affinity, E(PtF 6 ), > −156 kcal·mole −1 . From a comparative study of all of the hexafluorides of the third transition series the electron affinity is seen to increase regularly in the sequence WF 6 < ReF 6 < OsF 6 < IrF 6 < PtF 6 . The increase in E with unit increase in atomic number of M appears to be ≈ −20 kcal·mole −1 . On the other hand the ability of the hexafluorides to accept F − decreases along this series. This effect enables IrF 6 to be more effective than PtF 6 in the generation of nitrogen oxide trifluoride, ONF 3 , by the oxidative fluorination of nitrosyl fluoride, ONF. Both PtF 6 and IrF 6 interact spontaneously with ONF or O 2 NF to generate fluorine, at or below room temperature. The decrease in F − ‐acceptor ability along the series, which stands in sharp contrast to the increase in electron affinity, suggests that ligand crowding increases sharply across the series from WF 6 to PtF 6 . This accords with the observed decrease in molecular volume along the series, both in the hexafluorides and in the MF 6 − salts. It is clear from this comparison that the species IrF 6 and PtF 6 are close to a minimum volume for this series. The oxide pentafluorides ReOF 5 and OsOF 5 are similar in oxidizing ability to their respective hexafluorides but are poorer F − acceptors. Evidently the ligand crowding in MOF 5 molecules is greater than in MF 6 .