Hydration of some large and highly charged metal ions.

EXAFS studies of metal ions with hydration numbers higher than six in aqueous solution, often show asymmetric distribution of the metal-oxygen bond distances. The hydration number can be determined from a correlation with the bond distance. The mean Ca-O distance 2.46(1) A shows the calcium(II) ion to be eight-hydrated in a wide asymmetric distribution. Theoretically calculated EXAFS oscillations for individual snapshots from an MD simulation show large variations. The scandium(III) ion is surrounded by two groups of about eight water molecules, with the mean Sc-O distance 2.185(6) A. The yttrium(III) ion coordinates eight waters in an asymmetric distribution at 2.368(5) A, and the lanthanum(III) ion 6 + 3 water molecules at 2.52(2) and 2.65(3) A, respectively. For the the uranium(IV) and thorium(IV) ions, the M-O distances 2.42(1) and 2.45(1) A, respectively, indicate hydration numbers close to 10.