BONDING ANALYSIS IN INORGANIC TRANSITION-METAL CUBIC CLUSTERS .1. NONCENTERED HEXACAPPED M8(MU-4-E)6LN (N-LESS-THAN-OR-EQUAL-TO-8) SPECIES

The bonding in cubic M8(mu4-E)6L(n) cluster compounds is analyzed by means of extended Huckel and self-consistent field multiple-scattering Xalpha calculations. The results indicate that an optimal number of metallic valence electrons (MVEs) of 120 is favored with electronegative metals and/or terminal pi-acceptor ligands. The rather strong M-M bonding is mainly due to through-space M-M interactions but also to through-bond M-E interactions. A delocalized bonding picture is therefore necessary to describe the electronic structure of these species. For the 120-electron count, the M-(mu4-E) bonding is maximized, whereas the M-M bonding is not. The latter is strengthened upon depopulation of the top of the d band. Open-shell electronic configurations are then expected for clusters bearing terminal pi-donor ligands. The slight antibonding/nonbonding nature of the top of the metallic d band allows a large range of electron counts (from 99 to 120 so far) without altering the cubic metallic core. When terminal ligands borne by the metal atoms are missing, the favored count for compounds of formula M8(mu4-E)6L(n) is 120 - 2(8 - n) MVEs. M-L bonds result primarily from the combination of a metallic sp hybrid and the sigma lone pair orbital of L. When n terminal ligands are lost, the n corresponding metallic hybrids become nonbonding but remain high in energy and are vacant, leading to an electron count diminished by 2n units compared to that of the M8(mu4-E)6L8 parent species.