New Insights into the Mechanisms of Spontaneous and Base-Catalysed Substitution Reactions of the Inert Metal Amine Complexes

Abstract

For Co(III) amine complexes, there is a large body of evidence that points to spontaneous substitution reactions occurring via interchange mechanisms in which bond breaking substantially precedes bond making in the transition state. The mechanisms of substitution of the Rh(III), Ir(III), Ru(III), Os(III) and Cr(III) analogues depend very much on a number of factors, such as the nature of the leaving group and steric factors, as well as the electronic configuration of the metal ion. On the balance of evidence, substitution reactions of the d³ and d⁵ complexes can proceed via interchange mechanisms in which either bond breaking precedes bond making or vice versa. For the spontaneous substitution reactions of [M(NH₃)₅Cl]²+ complexes, the very large ionic solvation terms of the [M(NH₃)₅]³+ and Cl- components with respect to [M(NH₃)₅Cl]²+ strongly favor a transition state in which there is substantial charge separation, i.e., dissociative interchange mechanisms are energetically more accessible than associative interchange mechanisms. New crystallographic evidence indicates strongly that the different kinetic behavior observed for Co(III), Rh(III) and Cr(III) on the addition of steric bulk into [M(NH₂R)₅Cl]²+ complexes is not due to previously argued changes in mechanism with the nature of the metal ion. Rather, the kinetic differences reflect differences in the influence of π-bonding on the ground state and all of these complexes are believed to undergo substitution reactions via mechanisms in which bond breaking substantially precedes bond making. By contrast, the mechanisms of aquation reactions of the hexaammine complexes depend mainly on the electronic configuration and size of the central metal ion, since solvation terms in the transition state are similar to those in the ground state for either a dissociative or associative interchange mechanism. For these complexes, the preferred mechanism for substitution is an associative interchange for [M(NH₃)₆]³+ M = Cr, Ru and Os, and a dissociative interchange mechanism for M = Co. All of the mechanisms of base-catalysed aquations of metal amine complexes appear to involve five-coordinate intermediates in a dissociative conjugate-base mechanism (D_cb or S_N1_cb), independent of the metal ion or the leaving group. However, the geometries of municating the results contained in Ref. 23 prior to publication. Helpful discussions with James Beattie, Alan Sargeson and Henry Taube on a number of aspects are gratefully acknowledged. Some aspects of this work have been supported by grants from the Australian Research Council.