The structural stability of seven-coordinate divalent cations in the first transition series relevant to the water-exchange reaction mechanism

We studied the structural stability of heptahydrated divalent cations in the first transition series as model intermediary species in associative reaction pathways for the water exchange of hexahydrated cations by ab initio molecular-orbital methods. All of the structures of heptacoordination are pentagonal bipyramidal with a distorted equatorial plane. The structural stabilities are strongly dependent on their d-electron configurations. An associative mechanism is possible for the water-exchange reactions of hexahydrated divalent cations having less than seven d electrons, because the seven-coordinate species are located at the local minima or at the saddle points on the potential-energy surface. The occupation of the antibonding b orbital induces a transition density corresponding to an antisymmetric distortion through an interaction with a low-lying 4s orbital. The large bonding interaction makes the heptahydrated manganese(II) ion to become located at a local minimum. The occupancy of antibonding a orbitals determines the pattern of the antibonding interaction. Hexahydrated divalent cations with d(3), d(6), and d(7) configurations (vanadium(II), iron(II), and cobalt(II)) prefer a cis attack, while chromium(II) with a d(4) configuration prefers a trans attack during the operative associative process of the water-exchange reaction.