A model for two-electron four-center mixed valence systems. Application to [W 4O 8Cl 8(H 2O) 4] 2−

Abstract

The energy levels of a two-electron four-center mixed-valence system are calculated as functions of a resonance integral (β′) and an electron repulsion parameter ( k′ o). Using configuration interaction, exact solutions are given for all intermediate cases between the valence-bond limit (small β′/ k′ o) and the molecular-orbital limit (large β′/ k′ o). The results are applied to the [W 4O 8Cl 8(H 2O) 4] 2− case. The main predictions are the occurrence of a diamagnetic ground state with a large singlet-triplet gap, and the existence of an intense intervalence band polarized in the molecular plane. These predictions are supported by experiment. From the analysis of the optical spectrum it is concluded that this system is nearer to the molecular-orbital limit than to the valence-bond limit.