Electrochemistry of vitamin B12: Part IX. The Co(III)/Co(II) couple in non-aqueous solvents

The thermodynamic and kinetic characteristics of the Co(III)/Co(II) couple are investigated as a function of the solvent and thesixth axial ligand. With solvatocobalamins the thermodynamics of the Co(III)/Co(II) oxido-reduction reaction essentially reflect the strength of the Co(III)-solvent interaction. The order of the solvents in this connection appears to beH₂O>DMSO>ROH. The kinetics of the reaction parallels and amplifies the thermodynamic effects: the stronger the Co(III)-solvent interaction, the slower the reaction. While chloride and acetate ions appear as very weak ligands in water, they are much stronger in aliphatic alcohols and DMSO. The variations of the thermodynamic factors observed as a function of the solvents and the ligands are rationalized in terms of interaction of the solvent with Co(II) and with the anionic ligand and of the affinity of the ligand with Co(III). The kinetics of the Co(III)/Co(II) oxido-reduction reaction, involving both electron transfer and bond-breaking, again parallels and amplifies the thermodynamic effect. The stretching of the Co-ligand bond and the solvation of the anionicligands appear as the main kinetic factors.