Acid properties of the oxytetrafluorides of molybdenum, tungsten, and uranium toward some inorganic fluoride ion donors

Article abstract of DOI:10.1021/ic50150a016

The behavior of MoOF₄, WOF₄, and UOF₄ has been studied in the presence of ion donors of various strength: FNO, ClOF₃, ClF₃, ClF₅, and HF. For both MoOF₄ and WOF₄ in HF solutions, evidence for a partial ionization into Mo₂O₂F₉^- and W₂O₂F₉^- has been shown. From the equilibrium constant values, WOF₄ is found to be a stronger Lewis acid than MoOF₄, whereas no direct comparison with the strength of UOF₄ is possible because of its quasiinsolubility in HF. Lewis acid properties of the oxytetrafluorides studied are also demonstrated by the ionic structures found for the adducts with the strong bases FNO and ClOF₃. The new adducts NOMo₂O₂F₉, NOW₂O₂F₉, ClOF₂Mo₂O₂F₉, ClOF₂MoOF₅, (NO)₂MoOF₆, and (NO)₂WOF₆ were prepared. Their vibrational spectra and, for some of them, X-ray powder diffraction patterns are given. The unit cell parameters of the adducts NOMoOF₅, NOWOF₅, and (NO)₂WOF₆ have been determined. A study of the HF solutions of the nitrosyl salts has been made by Raman and ¹⁹F NMR spectroscopy. This, together with data on propylene carbonate solutions of the same adducts, allowed the identification of the species in the oxytetrafluoride-FNO-HF system. Depending on the F^- concentration the anions M₂O₂F₉^-, MOF₅^-, and MOF₆^2- (M = Mo or W) are formed, which are in equilibrium with the solvent. The mechanisms of these equilibria are proposed. The results obtained with the F^- donor fluorinating agents indicate that ionic intermediary steps occur in the fluorination process of the oxytetrafluorides to the corresponding hexafluorides.