The chemistry of vitamin B12. Part XI. Some further formation constants

Abstract

Formation constants have been determined for the substitution of co-ordinated H2O in aquocobalamin in aqueous solution (ionic strength usually 0·5) at room temperature by the following ligands: CH3·CO2–, 4·5 l./mole; N3–, 7·2 × 104; NO2–, 2·3 × 105; CH3·NC, 7 × 104; SO32–, 2·2 × 107; thiourea, 13. No complex was detected with the ligands CO, O2, NO, (CH3)2S, or Ph3P. Additional formation constants relating to the thermodynamic trans-effect have been determined (i) when the trans-ligand is SO32–, and (ii) for the substitution of H2O by N3–, CH3·NC, and SO32– with different trans-ligands. It is shown that SO32– is comparable with HCC– and CH2CH– in its cis- and trans-effects. The absence of CO complexes and the close parallel between formation constants involving CH3·NC and N3– are evidence for the absence of any significant π-bonding between the cobalt(iii) ion and the ligand.