THE NONOCTAHEDRAL STRUCTURES OF D(0), D(1), AND D(2) HEXAMETHYL COMPLEXES

The homoleptic d(0) hexamethyl complexes [M(CH3)(6)](2-) (M = Ti, Zr, Hf), [M(CH3)(6)](2-) (M = V, Nb, Ta), [M(CH3)(6)] (M = Cr, Mo, W). and [M(CH3)(6)](+) (M = Tc, Re) all prefer nonoctahedral structures deriv ed from a trigonal prism. This was shown by density functional calcula tions which employed quasirelativistic effective-core potentials. The nonconventional structures are due to both improved sigma bonding M - CH3 interactions in nonoctahedral versus octahedral structures, and to core polarization. While most of the anions feature regular trigonal- prismatic equilibrium structures B of D-3 symmetry, the neutral and ca tionic species are distorted towards a C-3 symmetrical structure A. as found previously for [W(CH3)(6)]. The computed trends may be understo od on the basis of energy denominators for second-order orbital intera ctions. and ligand-ligand repulsion. As both are increased by scalar r elativistic effects in the 5d row, the 4d complexes exhibit the larges t deviations from a regular prism. C-3v structures derived from an oct ahedron are generally less stable than the prismatic arrangements. Thi s is also valid for the d(1) complexes [Tc(CH3)(6)] and [Re(CH3)(6)], and for the d(2) systems [Ru(CH3)(6)] and [Os(CH3)(6)]. However, due t o the additional electrons present, these species are predicted to fav or regular rather than distorted prisms. NMR and IR spectroscopic para meters for the, as yet unknown, compound [Os(CH3)(6)] are predicted to facilitate experimental characterization. The molecular and electroni c structures of all the species are discussed with the help of natural population analyses.