Ab initio calculation of the electronic coupling element in bimetallic model compounds [M---L---M], M = Be, Mg, Zn; L = O, S, ---CH2---, ---CC---; electron correlation effects and dependence on the bridge nature

Ab initio calculations of the electron-transfer matrix element V_ab in bimetallic model systems [M---M]⁺ and [M---L---M]⁺ with M = Be, Zn and L = O, S, ---CH₂--- and ---CC---, are reported. Electron correlation effects are estimated through a second-order method in which only the differential space contributing to the transfer integral H_ab is considered. V_ab is found to be noticeably larger for bridged systems, showing that the through bond mechanism is pre-eminent in these systems. In most of the calculations, electron correlation contributions are found to increase V_ab, the corrections in some cases being as large as 35%. Analysis of the second-order contributions in these cases shows that intruder states involving ligand or metal ligand excitations strongly mix with the zeroth-order model wave functions.