Topological models of the chemical bonding in transition metal-silicon clusters

Models are proposed for the mixed transition metal-silicon bare clusters CuSi₁₀ and MSi₁₆ (M=Cr, Mo, W) in which the transition metals are located in the centers ofD _4d silicon polyhedra which are dual polyhedra of the square antiprism and 4,4-bicapped square antiprism (gyroelongated square bipyramid), respectively. In CuSi ₁₀ ⁺ and MSi₁₆ (M=Cr, Mo, W) the central transition metals have the favored 18-electron rare gas configurations receiving electrons from the silicon atoms through Si₂M three-center bonds analogous to the three-center C₂M bonds in metal-olefin complexes. Thus in CuSi₁₀ the silicon polyhedron functions as a chelating persila-tetraolefin whereas in MSi₁₆ (M=Cr, Mo, W) the silicon polyhedron functions as a persila-octaolefin in which six of the eight double bonds are complexed to the central metal atom leaving two uncomplexed double bonds.