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{sub 3}- and L{sub 2}-edges for the d{sup 0} dioxomolybdenum(VI) complexes LMoO{sub 2}(SCH{sub 2}Ph), LMoO{sub 2}Cl, and LMoO{sub 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 (2,470.5 and 2,472.5 eV) for LMoO{sub 2}(SCH{sub 2}Ph) and at the Cl K-edge (2,821.9 and 2,824.2 eV) for LMoO{sub 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{sup {minus}} {pi} mixed with the d{sub xz} orbital and {sigma} mixed with the d{sub z}{sup 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{sub 2}(SCH{sub 2}Ph) and LMoO{sub 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, {approximately} 42% and {approximately} 17% for LMoO{sub 2}(SCH{sub 2}Ph) and LMoO{sub 2}Cl, respectively. The Mo L{sub 2}-edge spectra provide a direct probe of metal 4 d 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{sub 2g} set and to the d{sub z}{sup 2} and d{sub xy} levels, respectively. The total peak intensities of the Mo L{sub 2}-edges for LMoO{sub 2}(OPh) and LMoO{sub 2}Cl are similar to and larger than those for LMoO{sub 2}(SCH{sub 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{sub 2}X complexes. These results imply that anisotropic covalency of the Mo-S{sub cys} bond in sulfite oxidase may promote preferential transfer of one of the oxo groups during catalysis.
- Research Organization:
- Stanford Univ., CA (US)
- Sponsoring Organization:
- National Science Foundation (NSF); National Institutes of Health (NIH); USDOE
- OSTI ID:
- 20013681
- Journal Information:
- Journal of the American Chemical Society, Vol. 121, Issue 43; Other Information: PBD: 3 Nov 1999; ISSN 0002-7863
- Country of Publication:
- United States
- Language:
- English
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