BONDING ANALYSIS IN INORGANIC TRANSITION-METAL CUBIC CLUSTERS .2. METAL-CENTERED HEXACAPPED M(9)(MU(4)-E)(6)L(8) SPECIES

The bonding in metal-centered M(9)(mu(4)-E)(6)L(8) cubic clusters is analyzed by means of extended Huckel and self-consistent field-multiple scattering-X alpha calculations. Different electron counts are allowed depending on the magnitude of the interaction of the interstitial metal atom (M(c)) with its metallic cubic host (M(s)) and the nature of the capping E ligands (either bare or substituted). In all cases, a strong interaction is observed between the s and p AOs of the encapsulated atom and metallic MOs of the cube. Significant additional M(c)-M(s) bonding is obtained if strong interactions occur between the five d AOs and corresponding metallic MOs. This still hypothetical situation, which leads to a count of 120 metallic valence electrons (MVEs). is favored for long M(s)-M(s) and short M(c)-E contacts. Another closed-shell configuration, corresponding to 124 MVEs, is obtained if the interaction of the M(c) t(2g) d AOs with the metallic cube is large and the e(g) one weak. This is the case for Ni-9(mu(4)-GeEt)(6)(CO)(8). Electron counts corresponding to open-shell ground-state configurations can occur when the capping E ligands are strong donors and/or when the MVE count is larger than 120. In such cases, the levels which may be partly populated an of e(g), t(2g), and (for large electron counts) t(1g) symmetry. For example, the ground-state electron distribution of the 124-MVE clusters Pd-9(mu(4)-E)(6)(PPh(3))(8) (E = As, Sb) corresponds to (t(2g))(4)(e(g))(0)(t(1g))(0), while it is found to be (e(g))(4)(t(1g))(4)(t(2g))(2) for the 150-MVE cluster Ni-9(mu(4)-Te)(6)(PEt(3))(8). The various possibilities for the electron distribution in these levels are discussed for various MVE counts, in relation to the M-M and M-E bond distances and the nature of E. The possibility of incorporating main-group elements at the center of the metallic cube is also discussed.