The chemistry of vitamin B12. Part V. The class (b) character of the cobaltic ion

Abstract

Formation constants have been determined for the substitution of co-ordinated H2O in aquocobalamin (vitamin B12a) in aqueous solution of ionic strength 0·5M(KNO3 or NaClO4) at room temperature by the following anions: F– just detectable, Cl– 1·3, Br– 1·9, I– 32, NCO– 530, NCS– 1200, NCSe– 8300, S2O32– 7300. The cobaltic ion in the cobalamins therefore has mild class (b) character. The cobalamins containing I–, NCSe–, and S2O32– show anomalous spectra similar to those of the alkylcobalamins. The pK for the protonation and displacement of the co-ordinated benziminazole by H2O varies with the nature of the trans-ligand in the order Cl– < Br–, I– or, in general, H2O, NH3, Cl– < CN–, carbanions, sulphur-containing ligands, Br–, I–. It is shown that the main effect of the axial ligand, both on the spectrum of the equatorial ring system (cis-effect) and on the equilibrium between H2O and benziminazole (trans-effect), is related to the position of the ligand in the nephelauxetic series.