Attribution of Water-Exchange Mechanisms of Transition-Metal Hexaaqua Ions Using Quantum Chemical Methods

Abstract

The mechanism for the water-exchange reaction with the transition-metal aqua ions from Sc^III through Zn^II has been investigated. The exchange mechanisms are analyzed on a model that involves the metal ion with six or seven water molecules. The structures of the reactants/products, transition states, and penta- or heptacoordinated intermediates have been computed with Hartree-Fock or CAS-SCF methods. Each type of mechanism, associative, concerted, or dissociative, proceeds via a characteristic transition state. The calculated activation energies agree with the experimental ΔG^‡₂₉₈ or ΔH^‡₂₉₈ values, and the computed structural changes indicate whether an expansion or compression takes place during the transformation of the reactant into the transition state. These changes are in perfect agreement with the changes deduced from the experimental volumes of activation. The dissociative mechanism is always feasible, but it is the only possible pathway for high-spin d⁸, d⁹, and d¹⁰ systems. In contrast, the associative mechanism requires that the transition-metal ion does not have more than seven 3d electrons. Thus, Sc^III, Ti^III, and V^III react via the A, Ni^II, Cu^II, and Zn^II via the D (or I_d) mechanism, whereas all pathways are feasible for the elements in the middle of the periodic table.