THEORETICAL-STUDY OF WATER-EXCHANGE REACTIONS OF HEXAHYDRATED DIVALENT-CATIONS IN THE FIRST TRANSITION SERIES - RELATIONSHIP BETWEEN REACTION-MECHANISM AND STABILITY OF HEPTACOORDINATED SPECIES

We studied the characteristics of model intermediary species on reacti on pathways using the ab initio molecular orbital method in order to e xplain a change in the mechanistic behavior of the water-exchange reac tion of hexahydrated divalent cations when the central elements were c hanged in the first transition series. Calculations show that the hexa coordinated species binding to the entering water in the second shell, the dissociative pentacoordinated species of scandium(II) and copper( II) ions, and the associative heptacoordinated species of scandium(II) ion are at local minima, whereas the heptacoordinated species of copp er(II) ion is at a second-order saddle point. We also considered calci um(II) and zinc(II) ions in order to examine the correlation between t he intermediary species and the reaction mechanisms. All of the interm ediary species far calcium(II) and zinc(II) ions resemble those of sca ndium(II) and copper(II) ions, respectively. The dissociative mechanis m can operate for both the calcium(II) and zinc(II) ions. The associat ive mechanism, however, can operate only for the calcium(II) ion, and the zinc(II) ion has no meaningful stationary point on the reaction pa th. We also considered the effect of the electronic structures on the reaction mechanisms. We conclude as the following: The dissociative me chanism is operative for a water exchange of all members of the first transition series. An associative mechanism is expected for the water- exchange reaction of the earlier members, which have stable heptacoord inated species. The possibility of the operation of the associative me chanism depends on the existence of appropriate associative heptacoord inated species.