THEORETICAL-STUDY OF WATER-EXCHANGE REACTIONS FOR THE DIVALENT IONS OF THE 1ST TRANSITION PERIOD

The binding energies of the sixth water ligand of the hexahydrated div alent first-row transition-metal ions from Ca2+ to Zn2+ have been obta ined by ab initio SCF calculations. A remarkably accurate linear corre lation is obtained between the calculated gas-phase dissociation energ ies and the logarithm of the experimentally determined reaction rate c onstants for water exchange in solution, excluding Ca2+ which has a hi gher hydration number. The result is consistent with a pentahydrated a ctivated complex (except for Ca2+), only weakly interacting with the e ntering and leaving water ligands in the transition state, i.e., an es sentially dissociative mechanism for all these ions. This is in confli ct with recent interpretations based on experimental activation volume s, which suggest an increasingly associative interchange mechanism to the left in the row. The reason for the discrepancy between the mechan isms for water exchange, proposed on the basis of these theoretical an d experimental results, is discussed and analyzed in molecular terms. In cases with weak or no ligand-field stabilization of the pentahydrat ed complexes, trigonal bipyramidal coordination gives the more stable structures, whereas for some of the ions with strong ligand-field or J ahn-Teller effects, Sc2+, V2+, Cr2+, Ni2+, and Cu2+, square pyramidal structures were favored. An accurate geometry description of the penta hydrated clusters using a large water basis set was found to be import ant in evaluating the binding energy. The energies of the d orbitals h ave been studied for an idealized gradual SQP --> TBP transition (Berr y pseudorotation) applied to [Mn(H2O)5]2+, in order to investigate the ir behavior as the geometry is changed.