Reaction mechanism of water exchange on di- and trivalent cations of the first transition series and structural stability of seven-coordinate species

Abstract

We studied the structural stability of heptahydrated trivalent cations of the first transition series from scandium to cobalt as model intermediary species in associative reaction pathways for the water exchange of the hexahydrated cations by ab initio molecular orbital methods. All the structures of heptacoordination are pentagonal bipyramidal with a distorted equatorial plane. The heptahydrated di- and trivalent cations with the same d-electron configuration are similar to each other for the structure and the structural stability which are strongly dependent on their d electron configurations. The associative mechanism is possible for the water-exchange reactions of the hexahydrated trivalent cations from scandium to cobalt, because the seven-coordinate species are located at the local minima or at the saddle points on the potential energy surface. The intrinsic reaction coordinates of the water-exchange reactions on the hexahydrated vanadium(II) and chromium(II) ions support the cis and trans attack of the associative mechanisms. The hexahydrated trivalent cations with d 3 and d 6 configurations [chromium(III) and cobalt(III)] prefer a cis attack, while manganese(III) with a d 4 configuration prefers trans attack during the operative associative process of the water-exchange reaction.