In order to study the coordinative behavior of doubly charged metal ions in water, a few representative metals have been chosen for theoretical studies. These are the group 2 metal ions beryllium, magnesium, and calcium and the group 12 zinc ion. The density functional method B3LYP has been used with very large basis sets. It is found that the water dipole moment and polarizabilities, which are critical for the accuracy of the binding energies, are very well reproduced provided that the basis set on the metal is included in the calculations. One of the main points of the present investigation has been to study the boundary between the first and second hydration shells. Trends of binding energies and structures are also discussed.

The structures of the hydrated lanthanoid(III) ions including lanthanum(III) have been characterized in aqueous solution and in the solid trifluoromethanesulfonate salts by extended X-ray absorption fine structure (EXAFS) spectroscopy. At ambient temperature the water oxygen atoms appear as a tricapped trigonal prism around the lanthanoid(III) ions in the solid nonaaqualanthanoid(III) trifluoromethanesulfonates. Water deficiency in the capping positions for the smallest ions starts at Ho and increases with increasing atomic number in the [Ln(H(2)O)(9-x)](CF(3)SO(3))(3) compounds with x=0.8 at Lu. The crystal structures of [Ho(H(2)O)(8.91)](CF(3)SO(3))(3) and [Lu(H(2)O)(8.2)](CF(3)SO(3))(3) were re-determined by X-ray crystallography at room temperature, and the latter also at 100 K after a phase-transition at about 190 K. The very similar Ln K- and L(3)-edge EXAFS spectra of each solid compound and its aqueous solution indicate indistinguishable structures of the hydrated lanthanoid(III) ions in aqueous solution and in the hydrated trifluoromethanesulfonate salt. The mean Ln--O bond lengths obtained from the EXAFS spectra for the largest ions, La-Nd, agree with estimates from the tabulated ionic radii for ninefold coordination but become shorter than expected starting at samarium. The deviation increases gradually with increasing atomic number, reaches the mean Ln-O bond length expected for eightfold coordination at Ho, and increases further for the smallest lanthanoid(III) ions, Er-Lu, which have an increasing water deficit. The low-temperature crystal structure of [Lu(H(2)O)(8.2)](CF(3)SO(3))(3) shows one strongly bound capping water molecule (Lu-O 2.395(4) A) and two more distant capping sites corresponding to Lu-O at 2.56(1) A, with occupancy factors of 0.58(1) and 0.59(1). There is no indication of a sudden change in hydration number, as proposed in the "gadolinium break" hypothesis.

The structure of the hydrated calcium(II) ion in aqueous solution has been studied by means of extended X-ray absorption fine structure spectroscopy (EXAFS), large-angle X-ray scattering (LAXS), and molecular dynamics (MD) methods. The EXAFS data displayed a broad and asymmetric distribution of the Ca-O bond distances with the centroid at 2.46(2) A. LAXS studies on four aqueous calcium halide solutions (1.5-2 mol dm(-)(3)) gave a mean Ca-O bond distance of 2.46(1) A. This is consistent with a hydration number of 8 determined from correlations between mean distances and coordination numbers from crystal structures. The LAXS studies showed a second coordination sphere with a mean Ca.O(II) distance of 4.58(5) A, and for the hydrated halide ions the distances Cl.O 3.25(1) A, Br.O 3.36(1) A, and I.O 3.61(1) A were obtained. Molecular dynamics simulations of CaCl(2)(aq) were performed using three different Ca(2+)-OH(2) pair potentials. The potential from the GROMOS program gave results in agreement with experiments, i.e., a coordination number of 8 and an average Ca-O distance of 2.46 A, and was used for further comparisons. Theoretical EXAFS oscillations were computed for individual MD snapshots and showed very large variations, though the simulated average spectrum from 2000 snapshots gave satisfactory agreement with the experimental EXAFS spectra. The effect of thermal motions of the coordinated atoms is inherent in the MD simulation method. Thermal disorder parameters evaluated from simulated spatial atom distribution functions of the oxygen atoms coordinated to the calcium ion were in close agreement with those from the current LAXS and EXAFS analyses. The combined results are consistent with a root-mean-square displacement from the mean Ca-O distance of 0.09(2) A in aqueous solution at 300 K.

Regular octahedral coordination has been reported for some copper() complexes in the solid state on the basis of crystallographic studies, e.g. hexaaquacopper() bromate, [Cu(OH 2 ) 6 ](BrO 3 ) 2 , hexaaquacopper() hexafluorosilicate, [Cu(OH 2 ) 6 ]SiF 6 , and hexakis(pyridine-1-oxide)copper() perchlorate, [Cu(ONC 5 H 5 ) 6 ](ClO 4 ) 2 . These results are not consistent with the elongated octahedral configuration expected from the Jahn-Teller theorem for the d 9 copper() ion nor, in some cases, with results from electron spin resonance studies. The present lattice-independent EXAFS study confirms that the local structure in the copper() complexes mentioned above is, in all cases, consistent with a Jahn-Teller induced elongation. Mean equatorial and axial Cu-O bond distances of 1.96(1) and 2.32(2) Å, and 1.95(1) and 2.27(3) Å, were obtained for the hexaaquacopper() ions in the bromate and hexafluorosilicate salts, respectively. For the hexakis(pyridine-1-oxide)copper() perchlorate only the equatorial mean Cu-O bond distance of 1.96(1) Å could be observed. Evidently, there is orientational disorder of the tetragonally elongated octahedral complexes resulting in too high crystallographic space group symmetry and copper sites in apparently regular coordination geometry. For the hydrated copper() ion in aqueous solution, fiveand six-coordinated models with different geometries have been evaluated by means of EXAFS and large angle X-ray diffraction (LAXS) data. The combined results are consistent with a Jahn-Teller elongated octahedral configuration with Cu-O eq 1.95(1) Å, Cu-O ax 2.29(3) Å, and a distinct second hydration sphere with about eight water molecules and a mean Cu ؒ ؒ ؒ O II distance of 4.17(3) Å. In dimethylsulfoxide solution EXAFS and LAXS methods show the solvated copper() ions to have mean equatorial and axial Cu-O bond distances of 1.96(1) and 2.24(2) Å, respectively. As a model compound for the EXAFS studies, the crystal structure of hexakis(dimethylsulfoxide)copper() perchlorate dimethylsulfoxide (1/2), [Cu(OS(CH 3 ) 2 ) 6 ](ClO 4 ) 2 ؒ2(CH 3 ) 2 SO, was determined.

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.