Kinetic evidence for five-coordination in AlOH[.sup.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.sup.III] exists almost entirely as the octahedra[ Al[([H.sub.2]O).sub.6.sup.3+] ion, whereas in basic conditions (pH > 7), a tetrahedral Al[(OH).sub.4.sup.-] 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).sup.2+], exist and are traditionally assumed to be hexacoordinate. We show, however, that the kinetics of proton and water exchange on aqueous [Al.sup.III], coupled with Car-Parrinello simulations, support a five-coordinate Al[([H.sub.2]O).sub.4]O[H.sup.2+] ion as the predominant form of AlOH[(aq).sup.2+] under ambient conditions. This result contrasts [Al.sup.III] with other trivalent metal aqua ions, for which there is no evidence for stable pentacoordinate hydrolysis products.