VIBRATIONAL ANALYSIS FOR NB60198- AND TA60198- AND RAMAN INTENSITY CRITERION FOR METAL-METAL INTERACTION

Raman and infrared spectra of the octahedral oxyanions Nb6O198- and Ta6O198- have been subjected to an approximate normal-coordinate analysis. Substantial interaction force constants are required for an adequate fit of the spectra, reflecting the complexity of the force field in these highly condensed complexes. The principal metal-oxygen force constants are quite satisfactory, however, for all three types of oxygen atoms: terminal, bridging, and central. The force constant ratios ˜6:3:1 are consistent with a simple bonding scheme in which each oxide ion shares four valence electrons with its neighboring metal ions. As with the structurally analogous Bi6(OH)126+ a set of cage angles can be replaced in the analysis by a kineticaily equivalent set of metal-metal interactions. This internal coordinate is the primary contributor to the lowest frequency A1g fundamental: the “breathing” mode of the metal ions. For Bi6(OH)126+ this mode produces the most intense band in the Raman spectrum, and metal-metal bonding has been invoked to account for the intensity. For the. M6O198- complexes, this mode produces one of the weakest Raman bands, consistent with the anticipated absence of metal-metal interaction for these d0 metal ions. However the “metal-metal” force constants are about the same as for Bi6(OH)126+. Such force constants evidently have little meaning in bridged polynuclear complexes, and Raman intensity would appear to offer a more reliable criterion for metal-metal interaction. © 1969, American Chemical Society. All rights reserved.