THE STRUCTURAL STABILITY OF 7-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 associ ative reaction pathways for the water exchange of hexahydrated cations by ab initio molecular-orbital methods. All of the structures of hept acoordination are pentagonal bipyramidal with a distorted equatorial p lane. The structural stabilities are strongly dependent on their d-ele ctron configurations. An associative mechanism is possible for the wat er-exchange reactions of hexahydrated divalent cations having less tha n seven d electrons, because the seven-coordinate species are located at the local minima or at the saddle points on the potential-energy su rface. The occupation of the antibonding b orbital induces a transitio n density corresponding to an antisymmetric distortion through an inte raction with a low-lying 4s orbital. The large bonding interaction mak es the heptahydrated manganese(II) ion to become located at a local mi nimum. 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(I I)) 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.