Structure of solvated matal ions

Abstract: The structure of the solvated group 3 ions scandium(III),yttrium(III) and lanthanum(III), has been determined in aqueoussolution and in some oxygen donor solvents, and compared withthe hydrated group 13 ions gallium(III) and indium(III). Acombination of X-ray absorption fine structure (XAFS), largeangle X-ray scattering (LAXS), crystallography and vibrationspectroscopy has been used for the structure studies of thehydrated ions and their dimethylsulfoxide andN,N´-dimethylpropylene urea solvates in solution and inthe solid state. For the hexahydrated gallium(III) andindium(III) ions in solution, the metal-oxygen distances werefound to be 1.959(6) Å and 2.131(7) Å to the firsthydration shell, and 4.05(1) and4.13(1) Å, respectively,to fairly well-defined second hydration shells containing abouttwelve water molecules. The hydration structure of thecalcium(II) ion was studied by combining EXAFS and LAXS studiesand Molecular Dynamics simulations. The biochemically importantcalcium ion was found to be octahydrated in aqueous solutionwith abroad and asymmetric distribution of the Ca-O bonddistances around the mean value 2.46(1) Å. Also theyttrium(III) ion was found to be octahydrated in aqueoussolution with an asymmetric distribution of the Y-O bonddistances around a mean value of 2.366(5) Å. The mean Y-Odistance in the eight-coordinated compound([(H2O)4Y(µ-OSO2CH3)2]CH3SO3)nis 2.35 Å. The hydration of thelanthanum(III) ion was consistent with a coordination of ninewater molecules with the La-O distances 6´2.52(2) and3´2.65(3) Å, probably with the oxygen atoms forming atricapped trigonal prism. Also for the hydrated scandium(III)ion in aqueous solution, a capped trigonal prismaticcoordination figure, with the mean Sc-O bond distance 2.17(1)Å to the water molecules in the prism, gives the bestdescription of the structure. There seems to be 2 or 3 cappingwater molecules, but the Sc-O distances in the range 2.3-2.5Å are not well-defined. The crystal structure of [Sc(H2O)8](CF3SO3)3, which at ambient temperature is isomorphous withthe [M(H2O)9](CF3SO3)3compounds with a series of nonahydrated tricappedtrigonal prismatic lanthanide(III) ions, was studied attemperatures down to 100 K. One capping water molecule wasfound to be missing in the[Sc(H2O)8](CF3SO3)3structure, causing a displacement of the scandiumionabout 0.2 Å from the centre of the trigonal prism. Aredetermination of the isomorphous[Yb(H2O)n](CF3SO3)3structure was made, resulting in n » 8.6 forthe slightly larger Yb(III) ion. Octahedrally coordinatedscandium(III) ions were found in the crystal structures of[Sc(H2O)6][Sc(CH3SO3)6]and trans-[Sc(H2O)4Cl2]Cl×2H2O, with mean Sc-O distances of 2.08 and 2.11Å, respectively. The scandium(III) ion was found to beoctahedrally six-coordinated in dimethylsulfoxide solution,with the mean Sc-O distance 2.09 Å, and in the solid[Sc(dmso)6]I3 compound with Sc-O 2.075(3) Å. The largeryttrium(III) ion is eight-coordinated in solution with the meanY-O distance 2.36(1) Å. In the crystal structure of[Y(dmso)8]I3 the mean Y-O distance is 2.36 Å, while theY-O distance to the two dimethylsulfoxide ligands in [Y(H2O)6(dmso)2]Cl3is only 2.27 Å and the mean Y-O waterdistance 2.37 Å. Also lanthanum(III) in dimethylsulfoxidesolution was found to form an eight-coordinated complex withthe mean La-O distance 2.50(2) Å. TheN,N´-dimethylpropylene urea solvated scandium(III),yttrium(III) and lanthanum(III) ions were found to have thecoordination number six, six and seven in solution,respectively, showing that steric ligand-ligand repulsioneffects are controlling the solvation numbers in aproticsolvents for these ions with noble gas electronconfiguration.