Icosaniobate [Nb20O54]8- was synthesized by reacting [H4Nb6O19]4- with NO in tetrahydrofuran or MeNO2. A single-crystal X-ray diffraction study of its n-tetrabutylammonium salt [a = 17.7284(18) A, b = 33.542(3) A, c = 34.316(2) A, Z = 4, and space group P22(1)2(1)] revealed a dimeric structure where two decaniobate ions are condensed sharing two terminal O atoms. Unlike that in [(NbW5O18)2O]4-, the Nb-O-Nb bridges in icosaniobate are bent. The nonlinear bridging reduces the maximum possible symmetry of the dimeric anion to mm2, which it closely approximates.

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A tetra-n-butylammonium (TBA) salt of [Ta10O28](6-) was synthesized by heating TBA6[H2Ta6O19] in toluene for a prolonged period. X-ray structural analysis of TBA6[Ta10O28]·6H2O revealed that the anion has the decametalate structure and is isostructural with the decavanadate and decaniobate anions [a = 15.8517(8) Å, b = 19.364(1) Å, c = 21.935(1) Å, β = 93.638(1)°, V = 6719.4(6) Å(3), Z = 2, and space group P2(1)/n at 292(2) K].

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