Coenzyme B-12 model studies: An HSAB approach to the equilibria and kinetics of axial ligation of alkyl(aquo)cobaloximes by imidazole and cyanide

Kinetics and equilibria of the axial ligation of alkyl(aquo)cobaloximes by imidazole and cyanide have been measured spectrophotometrioally in aqueous solutions of ionic strength 1.0 M at 25 degreesC as a function of pH. Compa rison of K-IMD and K-CN- of CH3, C2H5 and BrCH2 cobaloximes indicates that their stability is in the order BrCH2 > CH3 > C2H5. As the electron-withdra wing capacity of the alkyl group trans to water increases, the electron den sity of the cobalt(lll) decreases and thus it becomes a stronger Lewis acid and binds more strongly to imidazole and cyanide. The association and diss ociation rate constants are better correlated to the relative softness of t he ligand showing that cyanide binds 30 times raster than imidazole. These complexes are isolated and are characterized by IR and H-1 NMR spectra.