Factors affecting the rate of ligand substitution reactions of aquacobalamin (vitamin B_12a) ‡

Abstract

The ligand substitution reactions of co-ordinated H₂O in aquacobalamin (vitamin B₁₂) by OCN^–, SCN^–, SeCN^–, NO₂^–, S₂O₃^2– and N₃^– in a NaNO₃ medium, in contrast to an earlier report on some of these ligands in a KCl medium, proceeded too rapidly for saturation kinetics to be observed. The discrepancy, as suggested previously, is due to formation of a significant fraction of the more substitution-inert chloro complex in a KCl medium. Second-order rate constants, corrected for pH effects, for the reactions have been determined as a function of temperature, from which activation parameters were calculated. The identity of the donor atom of the ambidentate ligands in the rate-determining processes, k_II, is considered and correlations found between ΔH  ^‡_kII, the enthalpy of activation, and ΔS  ^‡_kII, the entropy of activation, respectively, the Mulliken population on the donor atom, and the energy of the highest occupied molecular orbital (HOMO) with σ symmetry. The parameters ΔH  ^‡_kII is strongly dependent on the HOMO energy, and more weakly dependent on the electron density on the donor atom, while the converse is true for ΔS  ^‡_kII.

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