and the spectroscopic characterization of fluoride-alkoxides of niobium and tantalum in the highest oxidation state are reported. Suspensions of MF5 (M = Nb, Ta) in a chlorinated solvent reacted with up to three equivalents of ROSiMe3 (R = Me, Et, Ph) to afford polynuclear derivatives and variable amounts of FSiMe3. Thus MF4(OR) (R = Et, Ph) and MF3(OR)2 were obtained by selective 1 : 1 and 1 : 2 reactions