title: 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(H2O)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(H2O)4OH2+ 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.
authors.affiliation: Department of Chemistry, University of Calgary, Calgary, AB T2N 1N4, Canada.
authors.forename: Thomas W
authors.initials: TW
authors.lastname: Swaddle
authors.forename: Jörgen
authors.initials: J
authors.lastname: Rosenqvist
authors.forename: Ping
authors.initials: P
authors.lastname: Yu
authors.forename: Eric
authors.initials: E
authors.lastname: Bylaska
authors.forename: Brian L
authors.initials: BL
authors.lastname: Phillips
authors.forename: William H
authors.initials: WH
authors.lastname: Casey