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

The behavior of MoOF4, WOF4, and UOF4 was 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 Mo2O2F/sub 9-/ and W2O2F/sub 9-/ was found. From the equilibrium constant values, WOF4 was 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 were determined. A study of the HF solutions of the nitrosyl salts was 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 M2O2F/sub 9-/, MOF/sub 5-/, and MOF62- (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. (U.S.)