Polyoxometalates from heteropoly "brown" precursors. A new structural class of mixed valence heteropolytungstates, [(XO4)W(IV)(3)W(VI)(17)O62Hx](n-)

Reduction of the alpha-Keggin anion [Xn+O4W12O36]([8-n]-)(X=H-2(2+), B3+, S i4+) by six electrons results in the known tungsten "brown" species [Xn+O4( H2O)(3)(W3W9O33)-W-IV-O-VI]([8-n]-) in which three W atoms have been reduce d from W-VI to W-IV, forming a metal-metal bonded triad. The W-IV atoms hav e terminal water coordinated in place of terminal oxo groups. Additional tu ngstate can condense onto these water molecules in aqueous solution between pH=4 and 6.5 to form the species reported here, [(XO4)(W3W17O62Hx)-W-IV-O- VI](y-). The boron derivative (X=B3+) is more stable than the metatungstate (X=H-2(2+)), both of which have been characterized by elemental analysis, W-183 NMR and X-ray crystal structure analysis. Eight additional tungstate groups condense in the form of a partial Keggin structure containing two tr iads and one dyad which is rotated 60 degrees relative to a hypothetical al pha-isomer.