The Stability of Seven-Coordinate Species of Earlier Members in the First Transition Series: Heptahydrated Ti(II), V(II), Cr(II), and Mn(II) Ions Relevant to Intermediate and Transition State in Water-Exchange Reaction

Abstract

Abstract The structure and stability of heptahydrated divalent cations of earlier members in the first transition series ([M(H2O)7]2+: M = Ti, V, Cr, and Mn) have been studied by ab initio molecular-orbital methods, because the heptahydrated cations are relevant to intermediates and transition states in their water-exchange reactions. The structure is pentagonal bipyramidal with a distorted equatorial plane. The heptahydrated divalent cations with d0, d1, d2, and d5 configurations are at the local minima, and those with d3 and d4 are at the saddle points, though there is an increase in hydration energy across the row of the periodic table from calcium to zinc. The trend of stability in a series of heptahydrated divalent cations strongly depends on the d-electron configuration, and differs significantly in the case of the penta- or hexahydrated cations that are at the local minima. It is concluded that an associative mechanism is possible for the water-exchange reaction on hexahydrated divalent cations of the earlier members in an aqueous solution.