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)-mu(2)-BR, where M = Mo or W and BR = oxalate or terephthalate, are reported. The measured magnitude of the change in dipole moment (vertical bar Delta(mu) over right arrow vertical bar) and the change in polarizability (Delta alpha) 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 Delta alpha 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.