Ligand K-edge and metal L-edge X-ray absorption spectroscopy and density functional calculations of oxomolybdenum complexes with thiolate and relatedligands: 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 LMoO2(SCH2Ph), LM oO2Cl, and LMoO2(OPh) (L = hydrotris(3,5-dimethyl-1-pyrazolyl)borate) to in vestigate 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) fo r LMoO2(SCH2Ph) and at the Cl K-edge (2821.9 and 2824.2 eV) for LMoO2Cl, de monstrating two major covalent contributions from S and Cl to the Mo d orbi tals. Density functional calculations were performed on models of the three Mo complexes, and the energies and characters of the Mo 4d orbitals were i nterpreted in terms of the effects of two strong cis-oxo bonds and addition al perturbations due to the thiolate, chloride, or alkoxide ligand. The maj or perturbation effects are for thiolate and Cl- pi mixed with the d(xz) or bital and sigma mixed with the d(z)(2) orbital. The calculated 4d orbital e nergy 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 p eaks for LMoO2(SCH2Ph) and LMoO2Cl (2.0 and 2.3 eV, respectively) after con sideration of final state electronic relaxation. Quantification of the S an d Cl covalencies in the d orbital manifold from the pre-edge intensity yiel ds, similar to 42% and similar to 17% for LMoO2(SCH2Ph) and LMoO2Cl respect ively. 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 t wo additional sharp peaks at higher energy, which are assigned to 2p transi tions to the overlapping t(2g) set and to the d(z)(2) and d(xy) levels, res pectively. The total peak intensities of the Mo L-2-edges for LMoO2(OPh) an d LMoO2Cl are similar to and larger than those for LMoO2(SCH2Ph), which agr ees with the calculated trend in ligand-metal covalency. The theoretical an d experimental description of bonding developed from these studies provides insight into the relationship of electronic structure to the oxo transfer chemistry observed for the LMoO2X complexes. These results imply that aniso tropic covalency of the Mo-S-cys bond in sulfite oxidase may promote prefer ential transfer of one of the oxo groups during catalysis.