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 divalent first-row transition-metal ions from Ca2+ to Zn2+ have been obtained by ab initio SCF calculations. A remarkably accurate linear correlation is obtained between the calculated gas-phase dissociation energies and the logarithm of the experimentally determined reaction rate constants for water exchange in solution, excluding Ca2+ which has a higher hydration number. The result is consistent with a pentahydrated activated complex (except for Ca2+), only weakly interacting with the entering and leaving water ligands in the transition state, i.e., an essentially dissociative mechanism for all these ions. This is in conflict with recent interpretations based on experimental activation volumes, which suggest an increasingly associative interchange mechanism to the left in the row. The reason for the discrepancy between the mechanisms for water exchange, proposed on the basis of these theoretical and experimental results, is discussed and analyzed in molecular terms. In cases with weak or no ligand-field stabilization of the pentahydrated complexes, trigonal bipyramidal coordination gives the more stable structures, whereas for some of the ions with strong ligand-field or Jahn-Teller effects, Sc2+, V2+, Cr2+, Ni2+, and Cu2+, square pyramidal structures were favored. An accurate geometry description of the pentahydrated clusters using a large water basis set was found to be important in evaluating the binding energy. The energies of the d orbitals have been studied for an idealized gradual SQP --> TBP transition (Berry pseudorotation) applied to [Mn(H2O)5]2+, in order to investigate their behavior as the geometry is changed.