Coenzyme B12 model studies: An HSAB approach to the equilibria and kinetics of axial ligation of alkyl(aquo)-cobaloximes by imidazole and cyanide

Abstract

Kinetics and equilibria of the axial ligation of alkyl(aquo)cobaloximes by imidazole and cyanide have been measured spectrophotometrically in aqueous solutions of ionic strength 1⋅0 M at 25°C as a function of pH. Comparison of K IMD and K CN-of CH 3 , C 2 H 5 and BrCH 2 cobaloximes indicates that their stability is in the order BrCH 2 > CH 3 > C 2 H 5. As the electron-withdrawing capacity of the alkyl group trans to water increases, the electron density of the cobalt(III) decreases and thus it becomes a stronger Lewis acid and binds more strongly to imidazole and cyanide. The association and dissociation rate constants are better correlated to the relative softness of the ligand showing that cyanide binds 30 times faster than imidazole. These complexes are isolated and are characterized by IR and 1 H NMR spectra.

Equilibria and kinetics forpH-dependent axial ligation of alkyl(aquo) cobaloximes with aromatic and aliphatic N-donor ligands

“…A soft or class b character has been assigned to cobaloximes (III) 36 and is consistent with the observed greater ligand affinity of cyanide, imidazole 37,38 , histidine or histamine than the hard glycine or ethyl glycine ester. Furthermore, softness appears to be related to the ability of a cobalt complex to stabilize a Co-C bond.…”

Equilibrium and kinetic studies on ligand substitution reactions of chloromethyl(aquo)cobaloxime with aromatic and aliphatic N-donor ligands

“…A soft or class b character has been assigned to Co(III) in cobaloximes 19 and is consistent with the observed greater ligand affinity of cyanide, imidazole, [20][21][22][23] histidine or histamine than that of the hard glycine or ethyl glycine ester. Furthermore, softness appears to be related to the ability of a cobalt complex to stabilize a Co-C bond.…”

Atrans influence study in propyl (aquo)cobaloxime by imidazoles and amino acids

“…"Soft" (or class b) character has been assigned to alkyl cobaloximes. [14][15][16] This is evident from the greater ligand affinity of imidazole [17][18][19][20][21] histidine or histamine compared to "hard" glycine or ethyl glycine ester. Further, softness may be directly related to the ability of a cobalt complex to stabilize a carbon-cobalt bond as seen in the cobaloximes.…”