STRUCTURE OF THE SOLVATED STRONTIUM AND BARIUM IONS IN AQUEOUS, DIMETHYL-SULFOXIDE AND PYRIDINE SOLUTION, AND CRYSTAL-STRUCTURE OF STRONTIUM AND BARIUM HYDROXIDE OCTAHYDRATE

Single crystal X-ray data, collected at 298 K, are used to redetermine the crystal structure of the non-isomorphic compounds [Sr(H2O)(8)](OH )(2) and [Ba(H2O)(8)](OH)(2). The eight water oxygen atoms form distor ted Archimedean antiprisms around the octahydrated metal ions with mea n metal ion-oxygen distances 2.62 and 2.79 Angstrom for strontium and barium, respectively. A second coordination shell of 24 hydrogen-bonde d oxygen atoms with mean metal ion-oxygen distances M...O-II 4.76 and 4.80 Angstrom, respectively, is observed. The hydroxide ions in both s tructures have an unusual hydrogen bond arrangement with 5 bonds accep ted and one donated. The structure of the solvated strontium and bariu m ions in aqueous, dimethyl sulfoxide and pyridine solutions has been studied by means of large angle X-ray scattering and extended X-ray ab sorption fine structure spectroscopy techniques. In aqueous solution i ndependent determinations of the first-sphere metal-oxygen coordinatio n by the two techniques resulted in the bond lengths Sr-O 2.63(2) and Ba-O 2.81(3) Angstrom, and for both metal ions a hydration number of 8 .1(3). The second coordination spheres are very diffuse with only abou t 13 water molecules at similar M...O-II distances as in the crystal s tructures and 2-3 water molecules closer to the metal ions. In dimethy l sulfoxide solution both ions were found to coordinate 6.0 (5) solven t molecules with the distances Sr-O 2.54(1), Sr...S 3.77(1) Angstrom, and Ba-O 2.76(1), Ba...S 3.99(1) Angstrom. For the solvated ions in py ridine solution EXAFS measurements yielded the distances Sr-N 2.57(2) and Ba-N 2.78(3) Angstrom, with a probable solvation number of 6. Corr elations of hydration enthalpies and partial molar volumes with experi mental M-O bond distances in aqueous solution are used to discuss hydr ation numbers and bond character for all of the alkaline earth metal i ons.