Oxalate bridged MM (MM = Mo₂, MoW, and W₂) quadruply bonded complexes as test beds for current mixed valence theory: Looking beyond the intervalence charge transfer transition

The spectroscopic features of a series of oxalate bridged complexes [( ^tBuCO₂)₃MM]₂-μ₂-O ₂CCO₂ (where MM = Mo₂, MoW, and W₂) in their neutral and singly oxidized (mixed valence) states are examined as a function of temperature and solvent. A large degree of electronic coupling between the two MM centers is evident, principally involving the MM δ orbitais mediated by the oxalate bridge π* orbital. In the oxidized states these mixed valence ions show solvent independent intervalence charge transfer (alternatively termed charge resonance) bands, consistent with assignment to Class III (or electronically delocalized) within the Robin-Day classification scheme. In both the neutral and oxidized states these complexes also show an intense metal-to-ligand charge-transfer (MLCT) transition, involving the lowest unoccupied molecular orbital (LUMO) of the bridge. The solvent and temperature dependence of this transition is also reported along with an inspection and simulation of the vibronic features, which are notably altered when switching between the neutral and the mixed valence states as well as with variation of the nature of the MM unit. Collectively, these observations allow us to comment on the validity and limitations of current theories dealing with mixed valence ions that have hitherto ignored the information that can be gained from MLCT transitions.