| Abstract
| - The molybdenum hydroxylases catalyze the oxidation of numerous aromatic heterocycles andsimple organics and, unlike other hydroxylases, utilize water as the source of oxygen incorporated into theproduct. The electronic structures of the cis-MoOS units in CoCp2[TpiPrMoVOS(OPh)] and TpiPrMoVIOS(OPh) (TpiPr = hydrotris(3-isopropylpyrazol-1-yl)borate), new models for molybdenum hydroxylases, havebeen studied in detail using S K-edge X-ray absorption spectroscopy, vibrational spectroscopy, and detailedbonding calculations. The results show a highly delocalized MoS π* LUMO redox orbital that is formallyMo(dxy) with ∼35% sulfido ligand character. Vibrational spectroscopy has been used to quantitate Mo−Ssulfido bond order changes in the cis-MoOS units as a function of redox state. Results support a redoxactive molecular orbital that has a profound influence on MoOS bonding through changes to the relativeelectro/nucleophilicity of the terminal sulfido ligand accompanying oxidation state changes. The bondingdescription for these model cis-MoOS systems supports enzyme mechanisms that are under orbital controland dominantly influenced by the unique electronic structure of the cis-MoOS site. The electronic structureof the oxidized enzyme site is postulated to play a role in polarizing a substrate carbon center for nucleophilicattack by metal activated water and acting as an electron sink in the two-electron oxidation of substrates.
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