Color and Variable Valence in Niobium and Tantalum Halides

Abstract

The very stable polynuclear halide ions [M₆X₁₂]²⁺ (M = Nb or Ta, X = Cl or Br) are unique in that though the X-ray structure analyses of the complex (Fig. 1) demonstrate that the metal ions are all in equivalent environments, the net charge of +2 on the [M₆X₁₂]²⁺ ion implies that two of the M ions have formal charge +3 (nd²) and four have the formal charge +2 (nd³). Actually, quantum mechanics demands that these formal valences oscillate equally among all six metal ions with the result that states arise for the polynuclear ion which have only a distant relationship to those of either the M²⁺ or M³⁺ ions.