Solution and solid state electronic absorption, magnetic circular dichroism, and resonance Raman spectroscopies have been used to probe in detail the excited state electronic structure of LMoO(bdt) and LMoO(tdt) (L ) hydrotris-(3,5-dimethyl-1-pyrazolyl)borate; bdt ) 1,2-benzenedithiolate; tdt ) 3,4-toluenedithiolate). The observed energies, intensities, and MCD band patterns are found to be characteristic of LMoO(S-S) compounds, where (S-S) is a dithiolate ligand which forms a five-membered chelate ring with Mo. Ab initio calculations on the 1,2-enedithiolate ligand fragment, -SCdCS -, show that the low-energy S f Mo charge transfer transitions result from one-electron promotions originating from an isolated set of four filled dithiolate orbitals that are primarily sulfur in character. Resonance Raman excitation profiles have allowed for the definitive assignment of the ene-dithiolate S in-plane f Mo d xy charge transfer transition. This is a bonding-to-antibonding transition, and its intensity directly probes sulfur covalency contributions to the redox orbital (Mo d xy ). Raman spectroscopy has identified three totally symmetric vibrational modes at 362 cm -1 (S-Mo-S bend), 393 cm -1 (S-Mo-S stretch), and 932 cm -1 (MotO stretch), in contrast to the large number low-frequency modes observed in the resonance Raman spectrum of Rhodobacter sphaeroides DMSO reductase. These results on LMoO(S-S) complexes are interpreted in the context of the mechanism of sulfite oxidase, the modulation of reduction potentials by a coordinated ene-dithiolate (dithiolene), and the orbital pathway for electron transfer regeneration of pyranopterin dithiolate Mo enzyme active sites.

The first ionization energy in the gas-phase photoelectron spectra (PES) of Tp*Mo(E)(tdt) complexes (where E = O, S, NO; Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate; tdt = 3,4-toluenedithiolate) is essentially independent of the nature of E, even though the formal oxidation state of the Mo center ranges from +2 to +5. The PES data for the tdt complexes contrast with the results for analogous complexes with alkoxide ligands, which show large variations in first ionization energy (Westcott, B. L.; Enemark, J. H. Inorg. Chem. 1997, 36, 5404−5405). For the tdt complexes the relative intensities of the two lowest energy ionizations do not substantially change as the excitation source is varied among Ne I, He I, and He II radiation, even though the atomic photoionization cross sections for Mo 4d and S 3p orbitals change dramatically over this energy region. These results all point to substantial covalency in the Mo−S bonds. It is proposed that the S atoms of the tdt ligand act as an “electronic buffer” to the effects of strongly bound axial ligands, and that this is an important role of ene-dithiolate (dithiolene) coordination in the molybdenum centers of enzymes.

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