Sulfites bidentate

The stoichiometric reaction of S02 with coordinated 02 to form coordinated sulfate appears to be general for dioxygen complexes. lsO isotope substitution and IR spectroscopy have been employed to study the reaction betwen [Ir(X)(CO)(02)(PPh3)2] (X = Cl, Br, I) and S02 (reaction 91). IR spectral results are consistent with a bidentate sulfate, ligand. The mechanism proposed for sulfate formation involves a peroxosulfate intermediate, that is insertion of S02 into one of the Ir —O bonds and bond formation between S and the leaving O, with subsequent rearrangement of the peroxo-sulfite intermediate thus generated (see Scheme 15).330,331... 

The occurrence of Mo or W in Nature is, with the exeeption of nitrogenase, always accompanied by a unique pyranopterin dithiolene chelating ligand typically referred to as molybdopterin and this is always coordinated to the metal at the active site. The combination of Mo (or W) with either one or two bidentate molybdopterin ligands in addition to combinations of terminal oxido or sulfido ligands and amino acid side chains leads to an orderly classification of this enzyme family into three sub-groupsthe xanthine oxidase, sulfite oxidase and DMSO reductase families (Scheme 5.1). The eponymous enzyme in each case is historically the first one isolated and studied within its family. 

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