The chemistry of vitamin B12. Part IX. Evidence for five-co-ordinate cobalt(III) complexes

Abstract

Solutions of organocobinamides show reversible changes in the optical spectrum and in the ¹H n.m.r. of the H-10 of the corrin ring on varying the temperature. Similar reversible changes in the optical spectrum are observed in the solid state on varying the water content. The simplest explanation is that the observed equilibria represent the reversible removal of co-ordinated water to form a five-co-ordinate complex: [Co^III·corrin·X·H₂O]=[Co^III·corrin·X]+ H₂O The magnitude of the equilibrium constant depends on the nature of the trans-ligand X, the ease of removing H₂O increasing in the order cyanide vinyl < methyl < ethyl < isopropyl. The following values were obtained for the enthalpy and entropy changes in methanol: X = vinyl, ΔH=+4·5 ± 2·0 kcal./mole, ΔS=+15·5 ± 7 e.u; X = methyl, ΔH= 4·4 ± 2·0 kcal./mole, ΔS= 16 ± 7 e.u. On varying the temperature of the solution or the water content of the solid, ethylcobalamin and 5′-deoxyadenosylcobalamin also show changes in spectrum, caused by the reversible co-ordination of the benzimidazole to the cobalt.