The structure of the hydrated gallium(III), indium(III), and chromium(III)ions in aqueous solution. A large angle X-ray scattering and EXAFS study

The structure of the hydrated gallium(III), indium(III), and chromium(III) ions has been determined in aqueous perchlorate and nitrate solutions by me ans of the large-angle X-ray scattering (LAXS) and extended X-ray absorptio n 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 c hromium(III)), while the LAXS studies are restricted to concentrated soluti ons, 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 i n a second hydration sphere. The metal-oxygen bond distance in the first hy dration 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 ) Angstrom. The LAXS data gave mean second sphere M ... O distances of 4.05 (1), 4.13(1), and 4.08(2) Angstrom for the gallium(III), indium(III), and c hromium(III) ions, respectively. The perchlorate ion was found to be hydrog en bonded to 4.5(7) water molecules with the O ... O distance 3.05(2) Angst rom and Cl ... O 3.68(3) Angstrom. Analyses of the Ga, In, and Cr K-edge EX AFS data of the aqueous perchlorate and nitrate solutions showed no influen ce on the first shell M-O distance by a change of concentration or anion. T he minor contribution from the second sphere M ... O distance is obscured b y multiple scattering within the tightly bonded first shell. EXAFS data for the alum salts CsM(SO4)(2). 12H(2)O, M = Ga or In, showed the M-O bond len gth of the hexahydrated gallium(III) and indium(III) ions to be 1.957(2) an d 2.122(2) Angstrom, respectively.