The chemistry of vitamin B12. Part XII. The co-ordination of nitrogenous bases

Formation constants have been determined for the substitution of co-ordinated water in diaquocobinamide by ammonia and imidazole, in aquocobalamin by ammonia, and in cyanoaquocobinamide by ammonia, pyridine, imidazole, benzimidazole, and adenine; the last three bases can co-ordinate in the conjugate anion form. It is shown that the formation constants for the binding of ammonia are more sensitive to the nature of the trans-ligand than those for the binding of imidazole, and that the relative order of their formation constants can change as the trans-ligand is varied. Pseudo-first-order rate constants for the substitution of co-ordinated ammonia by water at 30° have been obtained for corrinoids with the following trans-ligands: H2O, 1·4 × 10-5 s–1; 5,6-dimethylbenzimidazole, 8·6 × 10-5 s–1; CN–, SO32–, and CH3–, 3 × 10–1 s–1. The order of ligands observed in the kinetic trans-effect parallels that for the thermodynamic trans-effect.