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

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[superscript III] exists almost entirely as the octahedral Al(H₂O)₆³⁺ 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[superscript 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[superscript III] with other trivalent metal aqua ions, for which there is no evidence for stable pentacoordinate hydrolysis products.