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McNaughton RL, Helton ME, Cosper MM, Enemark JH, Kirk ML: Nature of the oxomolybdenum-thiolate pi-bond: implications for Mo-S bonding in sulfite oxidase and xanthine oxidase. Inorg Chem. 2004 Mar 8;43(5):1625-37.
The electronic structure of cis,trans-(L-N (2) S (2)) MoO (X) (where L-N (2) S (2) = N,N'-dimethyl-N,N'-bis (2-mercaptophenyl) ethylenediamine and X = Cl, SCH (2) C (6) H (5), SC (6) H (4)-OCH (3), or SC (6) H (4) CF (3)) has been probed by electronic absorption, magnetic circular dichroism, and resonance Raman spectroscopies to determine the nature of oxomolybdenum-thiolate bonding in complexes possessing three equatorial sulfur ligands. One of the phenyl mercaptide sulfur donors of the tetradentate L-N (2) S (2) chelating ligand, denoted S (180), coordinates to molybdenum in the equatorial plane such that the OMo-S (180)-C (phenyl) dihedral angle is approximately 180 degrees, resulting in a highly covalent pi-bonding interaction between an S (180) p orbital and the molybdenum d (xy) orbital. This highly covalent bonding scheme is the origin of an intense low-energy S --> Mo d (xy) bonding-to-antibonding LMCT transition (E (max) approximately 16000 cm (-)(1), epsilon approximately 4000 M (-)(1) cm (-)(1)). Spectroscopically calibrated bonding calculations performed at the DFT level of theory reveal that S (180) contributes approximately 22% to the HOMO, which is predominantly a pi antibonding molecular orbital between Mo d (xy) and the S (180) p orbital oriented in the same plane. The second sulfur donor of the L-N (2) S (2) ligand is essentially nonbonding with Mo d (xy) due to an OMo-S-C (phenyl) dihedral angle of approximately 90 degrees. Because the formal Mo d (xy) orbital is the electroactive or redox orbital, these Mo d (xy)-S 3p interactions are important with respect to defining key covalency contributions to the reduction potential in monooxomolybdenum thiolates, including the one- and two-electron reduced forms of sulfite oxidase. Interestingly, the highly covalent Mo-S (180) pi bonding interaction observed in these complexes is analogous to the well-known Cu-S (Cys) pi bond in type 1 blue copper proteins, which display electronic absorption and resonance Raman spectra that are remarkably similar to these monooxomolybdenum thiolate complexes. Finally, the presence of a covalent Mo-S pi interaction oriented orthogonal to the MOO bond is discussed with respect to electron-transfer regeneration in sulfite oxidase and Mo=S (sulfido) bonding in xanthine oxidase. |
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