THEORETICAL-STUDY ON WATER-EXCHANGE REACTIONS OF THE DIVALENT AND TRIVALENT METAL TONS OF THE FIRST TRANSITION PERIOD

Water-exchange reactions of the hexahydrated 3d ions have been studied using large basis set SCF computations on isolated penta- (in trigona l bipyramidal or square pyramidal conformation), hexa-, and heptahydra ted metal ion complexes, the latter with one single M-O distance and t wo angular coordinates optimized. A dissociative mechanism has been mo deled by using the dissociation energy of the sixth water ligand in a gas-phase process as the activation energy in an Arrhenius plot versus the logarithm of the experimental water-exchange rate in solution, un der the assumption that the energy contributions from solvent effects are similar within a series of ions. For the divalent 3d ions (V2+ to Zn2+), the remarkably good linear correlation found, in combination wi th the fairly constant solvation energies estimated in the preceding a rticle, strongly supports an essentially dissociative mechanism in sol ution. This is not compatible with the associative interchange mechani sms proposed for Mn2+ and V2+ on the basis of their negative experimen tal activation volumes, Delta V double dagger, for which an alternativ e explanation is proposed. For the trivalent ions Ti3+, V3+, Cr3+, Fe3 +, and Ga3+, the four first also with negative Delta V double dagger v alues, a corresponding plot gives a poor correlation. An associative m echanism has been investigated by using the binding energy of the seve nth water ligand in a similar Arrhenius plot. The divalent 3d ions sho wed no correlation, supporting the previous conclusion of a dissociati ve mechanism. The trivalent 3d ions with negative hr values gave an ac ceptable correlation with an associative interchange model, while the Ga3+ ion previously reported to react dissociatively, deviated as expe cted. Comparisons of calculated and experimental activation enthalpies supported an intermediate water interchange mechanism with increasing ly associative character in the order Cr3+, Fe3+, V3+, and Ti3+.