V. Sridhar et S. Satyanarayana, Coenzyme B-12 model studies: An HSAB approach to the equilibria and kinetics of axial ligation of alkyl(aquo)cobaloximes by imidazole and cyanide, P I A S-CH, 112(6), 2000, pp. 579-591
Citations number
47
Categorie Soggetti
Chemistry
Journal title
PROCEEDINGS OF THE INDIAN ACADEMY OF SCIENCES-CHEMICAL SCIENCES
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.