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.

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observed in these cobalt(III) complexes. This is consistent with the fact that the 2p peaks in the X-ray photoelectron spectra of cobalt(III) complexes are rarely accompanied by shake-up satellite peaks.14Comparison of CoL6 Spectra. No common characteristic features of the XANES spectra are found for CoL6 complexes, except that the main absorption peaks are found around 7730 eV for all the complexes examined. According to the MO calculations, these absorption bands are assigned to the transition from Co Is orbital to an MO with a Co p orbital as a main component. This means that the ls-4p orbital energy difference remains almost the same for all the trivalent cobalt complexes examined.Absorption in the range from the Is-3d to the ls-4p transition significantly changes among these complexes. Several intense absorption bands near the edge are found for [Co(dtc)3] but not for K3[Co(CN)6], The dtc ligand has unoccupied MO's to be mixed with the Co 4p orbital. On the other hand, the CN" ligand has only one unoccupied orbital, 6 , which can strongly interact with the Co 4p orbital. The absorption near the edge is correlated with unoccupied orbitals of the ligand in cobalt complexes. These suggest that the nature of the ligands reflects on the absorptions before the main peaks.Isomer Effect. In order to study the relationship between the Contribution from the Department of Chemistry,