THE STABILITY OF 7-COORDINATE SPECIES OF EARLIER MEMBERS IN THE FIRSTTRANSITION SERIES - HEPTAHYDRATED TI(II), V(II), CR(II), AND MN(II) IONS RELEVANT TO INTERMEDIATE AND TRANSITION-STATE IN WATER-EXCHANGE REACTION

The structure and stability of heptahydrated divalent cations of earli er members in the first transition series ([M(H2O)(7)](2+): M=Ti, V, C r, and Mn) have been studied by ab initio molecular-orbital methods, b ecause the heptahydrated cations are relevant to intermediates and tra nsition states in their water-exchange reactions. The structure is pen tagonal bipyramidal with a distorted equatorial plane. The heptahydrat ed divalent cations with d(0), d(1), d(2), and d(5) configurations are at the local minima, and those with d(3) and d(4) are at the saddle p oints, 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-e lectron configuration, and differs significantly in the case of the pe nta- or hexahydrated cations that are at the local minima. It is concl uded that an associative mechanism is possible for the water-exchange reaction on hexahydrated divalent cations of the earlier members in an aqueous solution.