Substitution reactions of solvated metal ions

Abstract

This chapter discusses the mechanisms of solvent exchange and ligand substitution on solvated metal ions of well-established stoichiometry. The most common ligand substitution on a solvated metal ion is the exchange of a water molecule in the first coordination sphere of a metal ion with a water molecule from the second coordination sphere. The metal ions may be conveniently considered in three categories, the first of which is the main group metal ions that, for a given ionic charge, exhibit an increase in kH2O with an increase in ionic radius, rM. The second category is the transition metal ions, all of which are six-coordinate with the exception of Pt2+ and Pd2+, which are square-planar four-coordinate. The third category is the heavy eight-coordinate trivalent lanthanides, whose lability decreases with the progressive filling of the 4f orbitals and the resulting lanthanide contraction, and that are very labile because of their large rM. An impressive armory of kinetic techniques has facilitated the study of ligand substitution processes in solution, ranging from those occurring at close to diffusion-controlled rates to those taking place over extended times. In the discussion on the classification of mechanisms, the simple transition-state theory is considered most convenient to describe ligand substitution processes, but it is acknowledged that in due course more sophisticated models may be adopted to describe the energetics of such rate processes. There are only five main group metal ions amenable to detailed mechanistic study of solvent exchange and simple ligand substitution processes: Be2+ (Beryllium), Mg2+ (Magnesium), A13+ (Aluminium), Ga3+ (Gallium), and In3+ (Indium). © 1995, Academic Press Inc.