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Coenzyme B12 model studies: Equilibrium constants for the pH-dependent axial ligation of benzyl(aquo)cobaloxime by various N- and S-donor ligands
Authors:D Sudarshan Reddy  N Ravi Kumar Reddy  V Sridhar and S Satyanarayana
Institution:(1) Department of Chemistry, Osmania University, 500 007 Hyderabad, India
Abstract:Equilibria of the axial ligation of benzyl(aquo)cobaloximes by imidazole, 1-methyl imidazole, histidine, histamine, glycine, ethyl glycine ester, thiourea and urea have been spectrophotometrically measured in aqueous solutions of ionic strength 1.0M (KCl) at 25°C as a function of pH. The equilibrium constants are in the order CN> 1-methyl imidazole > imidazole > histidine > histamine>glycine>ethyl glycine ester > thiourea > urea. The order of stability of benzyl(ligand)cobaloxime is explained based on the basicity of the ligand, Co(III) →>L dπ- pπback bonding and soft-soft and soft-hard interaction. Imidazole, substituted imidazoles, histidine and histamine form more stable complexes than glycine, ethyl glycine ester in contrast to the basicity of the ligands. Benzyl(ligand)cobaloximes were isolated and characterized by elemental analysis, IR and1H NMR spectra.
Keywords:Benzyl(aquo)cobaloxime  dimethylglyoxime  soft-soft interaction              -pπ   back bonding  imidazole  primary amine
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