The structure of the hydrated gallium(III), indium(III), and chromium(III) ions has been determined in aqueous perchlorate and nitrate solutions by means of the large-angle X-ray scattering (LAXS) and extended X-ray absorption fine structure (EXAFS) techniques. The EXAFS studies have been performed over a wide concentration range, 0.005-1.0 mol.dm(-)(3) (2.6 mol.dm(-)(3) for chromium(III)), while the LAXS studies are restricted to concentrated solutions, ca. 1.5 mol.dm(-)(3). All three metal ions were found to coordinate six water molecules, each of which are hydrogen bonded to two water molecules in a second hydration sphere. The metal-oxygen bond distance in the first hydration sphere of the gallium(III), indium(III), and chromium(III) ions was determined by LAXS and EXAFS methods to be 1.959(6), 2.131(7), and 1.966(8) Å. The LAXS data gave mean second sphere M.O distances of 4.05(1), 4.13(1), and 4.08(2) Å for the gallium(III), indium(III), and chromium(III) ions, respectively. The perchlorate ion was found to be hydrogen bonded to 4.5(7) water molecules with the O.O distance 3.05(2) Å and Cl.O 3.68(3) Å. Analyses of the Ga, In, and Cr K-edge EXAFS data of the aqueous perchlorate and nitrate solutions showed no influence on the first shell M-O distance by a change of concentration or anion. The minor contribution from the second sphere M.O distance is obscured by multiple scattering within the tightly bonded first shell. EXAFS data for the alum salts CsM(SO(4))(2).12H(2)O, M = Ga or In, showed the M-O bond length of the hexahydrated gallium(III) and indium(III) ions to be 1.957(2) and 2.122(2) Å, respectively.

The long elusive structure of Cu͑II͒ hydrate in aqueous solutions, classically described as a Jahn-Teller distorted octahedron and recently proposed to be a fivefold coordination structure ͓Pasquarello et al., Science 291, 856 ͑2001͔͒, has been probed with x-ray-absorption spectroscopy by performing a combined theoretical and experimental analysis. Two absorption channels were needed to obtain a proper reproduction of the x-ray-absorption near-edge structure ͑XANES͒ region spectrum, as already observed in other Cu͑II͒ complexes ͓Chaboy et al., Phys. Rev. B 71, 134208 ͑2005͔͒. The extended x-ray-absorption fine-structure ͑EXAFS͒ spectrum was analyzed as well within this approach. Quite good reproductions of both XANES and EXAFS spectra were attained for several distorted and undistorted structures previously proposed. Nevertheless, there is not a clearly preferred structure among those including four-, five-, and sixfold coordinated Cu͑II͒ ions. Taking into account our results, as well as many more from several other authors using different techniques, the picture of a distorted octahedron for the Cu͑II͒ hexahydrate in aqueous solution, paradigm of the Jahn-Teller effect, is no longer supported. In solution a dynamical view where the different structures exchange among themselves is the picture that better suits the results presented here.

The geometric structures of CflNOsk and ZnjNChk aqueous solutions in a wide range of concentrations 2.7-0.005 m have been determined by means of the extended X-ray absorption fine structure (EXAFS) technique. X-ray absorption spectra at the K-edges of Zn and Cr have been measured in the transmission and fluorescence modes at the Synchrotron Radiation Source (U.K.). The analysis of all the experimental data (13 solutions) is compatible with a unique structural model, which basically agrees with the concentric shell model of Frank and Evans for ionic hydration. Therefore, it is shown that the EXFAS technique allows the determination of a second hydration shell in a wide range of concentrations, from almost saturated to highly dilute solutions. -O distances (Cr-= 2.00 Á, Zn-= 2.05 Á) and the coordination number (6 for both cations) for the first hydration shell are not affected by concentration. A second hydration shell is detected in both cases, although for chromium solutions, this contribution to the EXAFS spectra is more important than for zinc. The distance is around 4.0 Á, but in Cr3+ solutions a slight increase in the Cr-On distance is observed with dilution (4.02 Á for 0.01 m, and 3.95 Á for 2.6 m). The Zn-On distance shows no systematic trend, the average distance being 4.1 Á. The coordination number for this shell is 13.3 ± 1 for Cr3+ solutions and 11.6 ± 1.6 for Zn2+ solutions. The most concentrated Zn2+ solution (2.7 m) presents a singular behavior, since its coordination number decreases to 6.8 ± 1.5. The data analysis procedure is thoroughly described, and the possibilities of an alternative hypothesis for the second contribution to the EXAFS spectrum, such as multiple scattering effects, are carefully discussed.