THEORETICAL-STUDY OF THE GEOMETRIC AND ELECTRONIC-STRUCTURES AND SPECTRA OF TRANS-ME2(PH3)(4) COMPLEXES (M = MO, W, E = S, SE, TE)

The optimized geometries of ME2(PH3)(4) complexes (hi = Mo, W; E = S, Se, Te) have been calculated using nonlocal. quasi-relativistic densit y functional theory. In all cases the most stable structure was found to have C-4v symmetry. Comparison with crystallographic data (D-2d sym metry) for ME2(PMe3)(4) (M = Mo, E = S, Se, Te; M = W, E = Se, Te) rev eals excellent agreement between theory and experiment, The ground-sta te electronic structures Of all six title complexes are found to resem ble those obtained fr om previous local density functional (X alpha) c alculations and hence to differ from db initio molecular orbital schem es that place the metal d(xy)-localized level several electronvolts be low the chalcogen p(pi) lone pair highest occupied molecular orbital, Electronic transition energies an calculated using the transition stat e method, A consistent assignment of the electronic absorption spectra of WE2(PMe3)(4) and MoE2(Ph2PCH2CH2PPh2)(2) (E = S, Se, Te) is propos ed. This assignment is different from either the experimental or ab in itio conclusions, though on the key question of the origin of the lowe st energy band the present density functional data reinforce previous ab initio conclusions that it is due to a chalcogen p(pi) --> pi prom otion and not the anticipated ligand field transition. Thus the densit y functional and nb initio approaches agree when used to calculate phy sically observable electronic promotion energies, although their groun dstate molecular orbital orderings differ considerably.