Polyoxometalates from heteropoly "brown" precursors. A new structural class of mixed valence heteropolytungstates, [(XO4)W3IVW17VIO 62Hx]n-

Reduction of the α-Keggin anion [Xⁿ⁺O₄W₁₂O₃₆]^{[8 − n]−} (X = H₂²⁺, B³⁺, Si⁴⁺) by six electrons results in the known tungsten “brown” species [Xⁿ⁺O₄(H₂O)₃W^IV₃W^VI₉O₃₃]^{[8 − n]−} in which three W atoms have been reduced from W^VI to W^IV, forming a metal–metal bonded triad. The W^IV atoms have terminal water coordinated in place of terminal oxo groups. Additional tungstate can condense onto these water molecules in aqueous solution between pH = 4 and 6.5 to form the species reported here, [(XO₄)W^IV₃W^VI₁₇O₆₂H_x]^y−. The boron derivative (X = B³⁺) is more stable than the metatungstate (X = H₂²⁺), both of which have been characterized by elemental analysis, ¹⁸³W NMR and X-ray crystal structure analysis. Eight additional tungstate groups condense in the form of a partial Keggin structure containing two triads and one dyad which is rotated 60° relative to a hypothetical α-isomer.