Kinetic Evidence for Five-Coordination in AlOH(aq)2+ Ion
Thomas W. Swaddle,1
Jörgen Rosenqvist,2
Ping Yu,3
Eric Bylaska,6
Brian L. Phillips,7
William H. Casey2,4,5*
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
2O)
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
III, coupled with Car-Parrinello
simulations, support a five-coordinate Al(H
2O)
4OH
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.
2 Department of Land, Air, and Water Resources, University of California, Davis, CA 95616, USA.
3 Nuclear Magnetic Resonance Facility, University of California, Davis, CA 95616, USA.
4 Department of Geology, University of California, Davis, CA 95616, USA.
5 Department of Chemistry, University of California, Davis, CA 95616, USA.
6 Fundamental Sciences, Pacific Northwest Laboratory, P.O. Box 999, Richland, WA 99352, USA.
7 Department of Geosciences, State University of New York, Stony Brook, NY 11794, USA.
* To whom correspondence should be addressed. E-mail: whcasey{at}ucdavis.edu
