Electronic structure of Mo and W [M7O24]6− isopolyanions

Abstract

The structure and bonding in [M₇O₂₄]⁶⁻ isopolyanions of Mo and W have been investigated using density-functional methods. Good computational–experimental agreement for the geometrical parameters has been obtained. The electronic structure of the anions has been probed with molecular-orbital and Mulliken–Mayer methods. All M–O interactions have been found to be largely M d–O p in character. Multicentred σ and π bonding involving different metal and bridging-oxygen atoms has been observed, and some of the corresponding molecular orbitals can be described in terms of [M_nO_n] closed loops. Mayer indexes correspond to fractional multiple character for terminal bonds, and approximately single or low-order character for bridging bonds, and are thus in satisfactory agreement with bond-valence results. The valency analysis has yielded similar overall bonding capacity for the various oxygen atoms. A distribution of the negative charge over all types of oxygen sites, and metal charges considerably smaller than the formal oxidation states have been obtained from the Mulliken analysis.