A series of tris(.mu.-fluoro)divanadates(III, IV, V) containing the bioctahedral [V2(O,F)9]3- unit. Synthesis, characterization, and structures of (NMe4)3V2F9, (NMe4)3V2O2F7, (NMe4)3V2O4F5, and (NMe4)2KV2O4F5.H2O