Ligand K-Edge and Metal L-Edge X-ray Absorption Spectroscopy and Density Functional Calculations of Oxomolybdenum Complexes with Thiolate and Related Ligands: Implications for Sulfite Oxidase

X-ray absorption spectra have been measured at the S K-, Cl K-, and Mo L 3 - and L 2 -edges for the d 0 dioxomolybdenum(VI) complexes LMoO 2 (SCH 2 Ph), LMoO 2 Cl, and LMoO 2 (OPh) (L = hydrotris(3,5-dimethyl-1-pyrazolyl)borate) to investigate ligand−metal covalency and its effects on oxo transfer reactivity. Two dominant peaks are observed at the S K-edge (2470.5 and 2472.5 eV) for LMoO 2 (SCH 2 Ph) and at the Cl K-edge (2821.9 and 2824.2 eV) for LMoO 2 Cl, demonstrating two major covalent contributions from S and Cl to the Mo d orbitals. Density functional calculations were performed on models of the three Mo complexes, and the energies and characters of the Mo 4d orbitals were interpreted in terms of the effects of two strong cis -oxo bonds and additional perturbations due to the thiolate, chloride, or alkoxide ligand. The major perturbation effects are for thiolate and Cl - π mixed with the d xz orbital and σ mixed with the d z 2 orbital. The calculated 4d orbital energy splittings for models of these two major contributions to the bonding of thiolate and Cl ligands (2.47 and 2.71 eV, respectively) correspond to the splittings observed experimentally for the two dominant ligand K-edge peaks for LMoO 2 (SCH 2 Ph) and LMoO 2 Cl (2.0 and 2.3 eV, respectively) after consideration of final state electronic relaxation. Quantification of the S and Cl covalencies in the d orbital manifold from the pre-edge intensity yields, ∼42% and ∼17% for LMoO 2 (SCH 2 Ph) and LMoO 2 Cl, respectively. The Mo L 2 -edge spectra provide a direct probe of metal 4d character for the three Mo complexes. The spectra contain a strong, broad peak and two additional sharp peaks at higher energy, which are assigned to 2p transitions to the overlapping t 2g set and to the d z 2 and d xy levels, respectively. The total peak intensities of the Mo L 2 -edges for LMoO 2 (OPh) and LMoO 2 Cl are similar to and larger than those for LMoO 2 (SCH 2 Ph), which agrees with the calculated trend in ligand−metal covalency. The theoretical and experimental description of bonding developed from these studies provides insight into the relationship of electronic structure to the oxo transfer chemistry observed for the LMoO 2 X complexes. These results imply that anisotropic covalency of the Mo−S cys bond in sulfite oxidase may promote preferential transfer of one of the oxo groups during catalysis.