Kinetic Evidence for Five-Coordination in AlOH(aq) 2+ Ion

Abstract

Trivalent aluminum ions are important in natural bodies of water, but the structure of their coordination shell is a complex unsolved problem. In strong acid (pH < 3.0), Al III exists almost entirely as the octahedral Al(H 2 O) 6 3+ ion, whereas in basic conditions (pH > 7), a tetrahedral Al(OH) 4 – structure prevails. In the biochemically and geochemically critical pH range of 4.3 to 7.0, the ion structures are less clear. Other hydrolytic species, such as AlOH(aq) 2+ , exist and are traditionally assumed to be hexacoordinate. We show, however, that the kinetics of proton and water exchange on aqueous Al III , coupled with Car-Parrinello simulations, support a five-coordinate Al(H 2 O) 4 OH 2+ ion as the predominant form of AlOH(aq) 2+ under ambient conditions. This result contrasts Al III with other trivalent metal aqua ions, for which there is no evidence for stable pentacoordinate hydrolysis products.