Electronic Spectroscopy of Metal-Metal σ-Symmetry Interactions

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The electronic spectra of d7-d7 metal-metal-bonded compounds are dominated by σ→σ* transitions, where the lowest energy transition is either metal-metal (dσ→dσ*) or axial-ligand charge transfer (L(σ)→dσ*); the relative energetics are determined mainly by metal-metal bond strength and axial-ligand reducing character. In some cases, both σ→σ* transitions are observed. Relatively weak electronic absorption bands are attributable to dπ→dσ*(M2) and dπ→dσ*(M-Leq), where the ligand-field strength of the equatorial ligand controls the relative excitation energies. The equatorial ligands also affect the strength of the metal-metal bond, and the L(σ)→dσ* transition is shown to be strongly responsive to this perturbation. If the lowest energy σ→σ* transition is L(σ)→dσ*, however, its energy need not correlate with the metal-metal bond length. The electronic spectra of mixed-valence (e.g., d7-d8) and longer-chain metal-metal-bonded compounds can be interpreted in terms of the molecular orbital levels developed for d7-d7 species.