Structure of the transition states and intermediates formed in the water-exchange of metal hexaaqua ions of the first transition series

The structures of the transition states and intermediates formed in the water-exchange of hexaaqua complexes of the first row transition elements have been computed with ab initio methods at the Hartree-Fock or CAS-SCF level. As an approximation, water molecules in the second coordination sphere except one, bulk water, and anions have been neglected. For each of the three types of activation, namely associative, concerted, and dissociative mechanism, a representative transition metal complex has been studied, viz. Ti(OH2)(6)(3+), V(OH2)(6)(2+), and Ni(OH2)(6)(2+). Each type of mechanism proceeds via a characteristic transition state. For the A and D mechanisms, respectively, hepta- or pentacoordinated intermediates are formed, and their lifetimes were estimated based on the energy difference between that of the transition state and the corresponding intermediate. The computed activation energies are in agreement with the experimental values and are independent of the mechanism or the charge on the metal center. The bond length changes occurring during the activation agree with the corresponding experimental Delta V-double dagger values. In a recent article, Akesson et al. (J. Am. Chem. Sac. 1994, 116, 8705) proposed an interpretation of the experimental Delta V-double dagger values that differs from that commonly applied (Merbach, A. E. Pure Appl. Chem. 1987, 59, 161). In particular, they claimed a dissociative activation for the water-exchange of the hexaaqua ions of V-II and Mn-II in spite of their negative volumes of activation. The present computational results on V-II are in perfect agreement with the I-a mechanism attributed on the basis of its Delta V-double dagger: value. It should be noted that in principle, the D mechanism is possible for al the hexaaqua ions of the first transition series, but in many cases, the associative or concerted pathway is preferred, For a given complex, all the possible mechanisms must be analyzed, before the most favorable pathway can be determined. The presently studied case of V-II, where equal energies of activation have been computed for the I-a and D mechanism, illustrates this point. The attribution of the mechanism was only possible by comparison with the experimental volume of activation. Computed energies of activation alone may not suffice to identify the mechanism; a safe attribution can only be made if the structural changes agree with the volume of activation.