Sulfite coordination chemistry

The coordination chemistry of aluminum with oxyanion ligands is chiefly that of the more highly oxidized ions. Nitrites and phosphites are unknown. Arsenites, sulfites, selenites and tellurites are mentioned in the early literature but have received little recent attention. 

The coordination chemistry of NS (thionitrosyl), thiazate (NSO ), disulfidothionitrate (SsN"), sulfur monoxide (SO), disulfur monoxide (S2O), SO2, DMSO, thiosulfate, sulfite, and thiourea ligands is extensive. " Scheme 23 illustrates their coordination modes and some of the complexes that can be generated. 

AU -l,2,3,4-thiatriazolc-5-thiolate anions in a square-planar coordination geometry for the palladium atoms. The use of 1,2,3,4-thiatriazole complexes in analytical chemistry to determine the sulfite ion concentration was also reported <1996CHEC-II(4)691>. 

The chapter consists of nine sections. Sections II through VII deal with the pterin-containing molybdenum enzymes. Biochemical and model studies of molybdopterin, Mo-co, and related species are described in Section II. In Section III, we briefly survey physical and spectroscopic techniques employed in the study of the enzymes, and consider their impact upon the current understanding of the coordination about the molybdenum atom in sulfite oxidase and xanthine oxidase. Model studies are described in Sections IV and V. Section IV concentrates on structural and spectroscopic models, whereas Section V considers aspects of the reactivity of model and enzyme systems. The xanthine oxidase cycle (Section VI) and facets of intramolecular electron transfer in molybdenum enzymes (Section VII) are then treated. Section VIII describes the pterin-containing tungsten enzymes and the evolving model chemistry thereof Future directions are addressed in Section IX. 

These kinetics data are consistent with a preequilibrium dissociation of dmf from the molybdenum center to form a reactive five-coordinate species that rapidly reduces the Fe(III) center via an inner sphere (halogen transfer) reaction. Other one-electron atom transfer reactions are known in oxo-molybdenum chemistry (262). An innersphere (atom transfer) mechanism is not a viable model for intramolecular transfer in sulfite oxidase because in the enzyme the Mo and Fe centers are almost certainly held too far apart by the protein framework. Moreover, the 65-type heme center of sulfite oxidase is six-coordinate with axial histidine ligands from the protein and hence cannot participate in atom transfer reactions. 

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