Icosaniobate: A New Member of the Isoniobate Family

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) Å, b = 33.542(3) Å, c = 34.316(2) Å, Z = 4, and space
group P22121] 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.