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 reaction pathways using the ab initio molecular orbital method in order to explain a change in the mechanistic behavior of the water-exchange reaction of hexahydrated divalent cations when the central elements were changed in the first transition series. Calculations show that the hexacoordinated 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 copper(II) ion is at a second-order saddle point. We also considered calcium(II) and zinc(II) ions in order to examine the correlation between the intermediary species and the reaction mechanisms. All of the intermediary species for calcium(II) and zinc(II) ions resemble those of scandium(II) and copper(II) ions, respectively. The dissociative mechanism can operate for both the calcium(II) and zinc(II) ions. The associative mechanism, however, can operate only for the calcium(II) ion, and the zinc(II) ion has no meaningful stationary point on the reaction path. We also considered the effect of the electronic structures on the reaction mechanisms. We conclude as the following: The dissociative mechanism 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 heptacoordinated species. The possibility of the operation of the associative mechanism depends on the existence of appropriate associative heptacoordinated species.