Water and proton exchange processes on metalions

The simplest reaction on a metal ion in aqueous solution is the exchange of a water molecule between the first and second coordination shells. This reaction is fundamental in understanding not only the reactivity of metal ions in chemical and biological systems but also the metal-water interaction. The replacement of a water molecule from the first coordination shell represents an important step in complex-formation reactions of metal cations and in many redox processes (1).1MH2Onz-+ n H2O* ⇄ kexMH2O*nz++ n H2O. In solvent exchange reactions there is no net reaction and the Gibbs free energy change, ΔG0, of the reaction is zero because the reactant and the product are identical. The measured life times of a water molecule in the first coordination shell of a metal cation cover 19 orders of magnitude (Fig. 1). A water molecule stays at average nearly 300 years (9 × 109 s) in the first coordination shell of [Ir(H2O)6]3+ (2) before it is replaced by another one coming from the bulk solvent. The mean life time of H2O bound to [Eu(H2O)7]2+ is however only about 200 ps (2 × 10-10 s) (3,4). Astonishingly, both exchange processes follow an associative activation mode which means that for both hydration complexes the transition state or the intermediate encountered during the exchange reaction has an increased coordination number. The rates for water exchange depend strongly on the nature of the metal ion as observed in Fig. 1. The dependence on solvent is however much less pronounced as shown in different reviews (5,6). The discussion of solvent exchange and more specifically of water exchange is therefore, conveniently divided into categories of metal ions. A first group is represented by cations formed by main group elements. These ions have filled electron shells and they differ mainly in electric charge and ionic radius. The number of water molecules in the first shell around the ion, called coordination number CN, ranges from 4 up to 10 (7). A second category is formed by the d-transition metal ions, which are all hexa-hydrated, with the exception of Pd2+ and Pt2+, which are four-coordinated (square-planar), Sc3+ which is suggested to be hepta-coordinated (8) and Cu2+ where there is some evidence that it is five-coordinated (9). The water exchange rate constants of d-transition metal ions are strongly influenced by the occupancy of the d-orbitals. Considering the ionic radii, rM, alone, the first row transition metal ions should have kex of the same order of magnitude as Zn2+ for the divalent ions and as Ga3+ for the trivalent ions. The measured exchange rates vary however by 7 (divalent) and 15 (trivalent) orders of magnitude, depending largely on the electronic configuration. A third category involves the lanthanide ions which have eight or nine water molecules in the first coordination shell. Their kinetic behavior is mainly influenced by the decrease in ionic radius along the series and by the coordination equilibrium observed in the middle of the series. Replacing some of the first shell water molecules by one or more ligands, which are kinetically inert can have a strong influence on the rate and the mechanism of exchange of the remaining water molecule(s). Metal ion complexes still having one or more H2O directly bound are important in catalysis (10), as active centers in bio-molecules (11) and in the special case of gadolinium(III), as contrast agents in medical magnetic resonance imaging (12). In section II we will first summarize the concepts of solvent exchange reactions and how exchange reaction mechanisms can be assigned from variable pressure experiments. In section III, we will describe the oxygen-17 NMR techniques available to measure water exchange rates over 18 orders of magnitude. Water exchange rate constants and activation parameters measured directly by 17O NMR on aqua-ions, and on some metal complexes with an inert ligand and water molecules in the first shell, are discussed in the following sections. We would like to mention that solvent exchange reactions including other solvents have been recently reviewed in Advances in Inorganic Chemistry by Dunand, Helm and Merbach (6). © 2005 Elsevier Inc. All rights reserved.