The chemistry of vitamin B12. Part X. cis- and trans-Effects

Abstract

We have studied the effect of varying one axial ligand (X), usually an organo-ligand, on the stretching frequency of cyanide co-ordinated in the trans-position (Y) and on the spectrum of the equatorial corrin ring in these cyano-complexes, and have measured several additional formation constants for substitution of the axial ligands. The cyanide stretching frequency is very dependent on the nature of X, falling from 2132 cm.^–1 when X = benzimidazole to 2082 cm.^–1 when X = CH₃CH₂^–. All the evidence relating to cis- and trans-effects in corrinoids which contain only the light ligand atoms C, N, and O is compared. It is shown that there is a correlation between cis- and trans-effects and between ground-state and thermodynamic effects, and that, in general, as the ligand X becomes more polarisable, so the other metal–ligand bond lengths increase, cyanide co-ordinated in the trans-position becomes more ionic, and the formation constants for the substitution of H₂O fall to values more typical of ion-pairs. It is concluded that the main factor is the amount of negative charged donated to the cobalt atom through the σ-bond.