Electroabsorption of dimers containing MM (M = Mo, W) quadruply bonded units: Insights into the electronic structure of neutral coupled redox centers and their relationship with mixed valence ions

The electroabsorption spectra for the metal-to-ligand charge transfer transition in complexes containing oxalate and terephthalate bridged MM quadruply bonded units, [(MM)(pivalate)₃]₂- μ₂-BR, where M = Mo or W and BR = oxalate or terephthalate, are reported. The measured magnitude of the change in dipole moment (|Δμ^→|) and the change in polarizability (Δ∝) that accompany this electronic transition are found to be small and not to follow the behavior expected on the basis of the two-state model. In addition, the trend in the value of Δ∝ for the neutral states is mirrored by the trend in the degree of electronic coupling (H_AB) for the strongly coupled mixed valence states formed by the same complexes in their singly oxidized states.