Equilibrium, Kinetic and Leaving Group Effect Studies on Ligand Substitution Reactions of a Simple Model for Coenzyme B12

Abstract

The ligand substitution reactions of trans-[CoIII(en)2(Me)H2O]2+ and trans-[CoIII(en)2(Me)NH3]2+ were studied for CN−, SCN−, N3−, and NH3 as entering nucleophiles. These nucleophiles displace the coordinated H2O and NH3 molecules trans to the methyl group and form the six-coordinate complex trans-[Co(en)2(Me)L]. The complex-formation constant for the displacement of H2O by NH3 was found to be 17.4 ± 1.1 M−1 at 20 °C, and those for the displacement of NH3 by SCN− and N3− were found to be 4.4 ± 1.5 and 3.1 ± 0.4 at 10 °C, respectively. From temperature and pressure dependence studies, activation parameters (ΔH≠, ΔS≠ and ΔV≠) for the reaction of trans-[CoIII(en)2(Me)H2O]2+ with NH3 were found to be 62 ± 1 kJ mol−1, +3 ± 5 J K−1 mol−1 and +5.7 ± 0.3 cm3 mol−1, respectively, compared to 79 ± 2 kJ mol−1, +40 ± 6 J K−1 mol−1 and +9.0 ± 0.4 cm3 mol−1 for the reverse aquation reaction of trans-[CoIII(en)2(Me)NH3]2+. Based on the reported kinetic and activation parameters, the substitution of coordinated H2O by NH3 follows an Id mechanism in which the entering nucleophile participates in the transition state. The substitution of NH3 by SCN−, N3− and CN− proceeds via the intermediate aqua complex such that aquation of the amine complex becomes the rate-determining step at high entering ligand concentration. A detailed comparison with available data in the literature is made.