| Abstract
| - The mechanism for the water-exchange reaction with the transitionmetal aqua ions from ScIII throughZnII has been investigated. The exchange mechanismswere analyzed on the previously reported model (Rotzinger,F. P. J. Am. Chem. Soc.1996, 118,6760) that involves the metal ion with six or seven water molecules.Thestructures of the reactants/products, transition states, and penta- orheptacoordinated intermediates have been computedwith Hartree−Fock or CAS-SCF methods. Each type of mechanism,associative, concerted or dissociative, proceedsvia a characteristic transition state. The calculated activationenergies agree with the experimentalΔG⧧298 orΔH⧧298values, and the computed structural changes indicate whether anexpansion or compression takes place during thetransformation of the reactant into the transition state. Thesechanges are in perfect agreement with the changesdeduced from the experimental volumes of activation(ΔV⧧298). The motions ofthe ligands involved in the exchangereaction are described by the imaginary vibrational mode. Allthese computed quantities allow the attribution of thewater-exchange reactions to the A, Ia, or D mechanisms withuse of the terminology of Merbach (Merbach, A. E.Pure Appl. Chem.1982, 54, 1479).Within the present model, no transition state has been found forthe Id mechanism.It remains to be verified, using an improved model, whether itreally does not exist. The dissociative mechanism isalways feasible, but it is the only possible pathway for high-spind8, d9, and d10 systems. Incontrast, the associativemechanism requires that the transition metal ion does not have morethan seven 3d electrons. Thus, ScIII,TiIII, andVIII react via the A, NiII, CuII,and ZnII via the D (or Id) mechanism, whereasfor the elements in the middle of theperiodic table, the high-spin 3d3−3d7systems, both associative (Ia/A) and dissociative (D)pathways are feasible.The present results suggest that for ScIII hexa- andheptacoordinated species could coexist in aqueoussolution.
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