Equilibria and kinetics for pH-dependent axial ligation of bromomethyl(aquo)cobaloxime by aliphatic amine ligands

Abstract

Kinetics and equilibria of axial ligation of bromomethyl(aquo) cobaloxime by a series of straight chain primary amines (methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine), cycloamines (cyclopentylamine, cyclohexylamine, cycloheptylamine) and secondary amines (N,N-dimethylamine, N,N-diethylamine) have been measured as functions of pH by spectrophotometric technique in aqueous solution, ionic strength 1 M (KCl) at 25°C. The rate of substitution of H 2 O varies with the pKa of incoming ligand, thus establishing nucleophilic participation of the ligand in the transition state. Binding and kinetic data are interpreted based on the basicity and steric influence of the entering ligand. To compare the rate constants of the entering ligands, pH independent second-order rate constants (k on) are calculated.

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.…”