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

Abstract

AbstractSome of the transition metal hexafluorides demonstrate an astonishing oxidizing power. In particular may be mentioned PtF6, which is capable of oxidizing molecular oxygen or xenon, a process requiring an electron affinity, E(PtF6), > −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 WF6 < ReF6 < OsF6 < IrF6 < PtF6. 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 IrF6 to be more effective than PtF6 in the generation of nitrogen oxide trifluoride, ONF3, by the oxidative fluorination of nitrosyl fluoride, ONF. Both PtF6 and IrF6 interact spontaneously with ONF or O2NF 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 WF6 to PtF6. This accords with the observed decrease in molecular volume along the series, both in the hexafluorides and in the MF6− salts. It is clear from this comparison that the species IrF6 and PtF6 are close to a minimum volume for this series. The oxide pentafluorides ReOF5 and OsOF5 are similar in oxidizing ability to their respective hexafluorides but are poorer F− acceptors. Evidently the ligand crowding in MOF5 molecules is greater than in MF6.