Relative importance of the inductive and steric effects of alkyl groups (R) in understanding the nature of the CoIII–C bond in alkyl cobaloximes, and comments on related alkyl cobalamins

Abstract

We have shown that multiple regression analyses involving Taft's polar substituent constant, σ*, and Dubois' steric parameter, E′_s, can satisfactorily (r² 0.9) explain the properties of alkyl cobaloximes (R = alkyl) and alkyl cobalamins related to the effects of the R groups. The properties studied are the redox potential of the Co^III–Co^IV couple, Co→C charge transfer energy, trans influence and trans effect of R in alkyl cobaloximes, and trans influence of R in alkyl cobalamins. From these studies, it is concluded that the Co^III–R bond strength decreases with decrease in (σ*+λE′_s) where λ is the mixing coefficient. As indicated by the values obtained, the steric demand of the corrin moiety is found to be an order of magnitude higher than that of dimethylglyoximate. Such semi-quantitative estimation is the first of its kind. The trans influence of R is found to be just opposite to the general observation for this static phenomenon and is designated as a special feature of the Co^III–R bond.