Polyoxometalates from heteropoly "brown" precursors. A new structural class of mixed valence heteropolytungstates, [(XO4)W3IVW17VIO 62Hx]n-
Reduction of the α-Keggin anion [X n + O 4 W 12 O 36 ] [8 − n]− (X = H 2 2+ , B 3+ , Si 4+ ) by six electrons results in the known tungsten “brown” species [X n + O 4 (H 2 O) 3 W IV 3 W VI 9 O 33 ] [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, [(XO 4 )W IV 3 W VI 17 O 62 H x ] y − . The boron derivative (X = B 3+ ) is more stable than the metatungstate (X = H 2 2+ ), both of which have been characterized by elemental analysis, 183 W 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.