Steric effects control the structure of the solvated lanthanum(III) ion inaqueous, dimethyl sulfoxide, and N,N '-dimethylpropyleneurea solution. An EXAFS and large-angle X-ray scattering study

The structure of the solvated lanthanum(III) ion has been determined in aqu eous, dimethyl sulfoxide, and N,N'-dimethylpropyleneurea solution by means of the EXAFS and large-angle X-ray scattering (LAXS) techniques. The close agreement between the EXAFS spectra of solid nonaaqualanthanum(III) trifluo romethanesulfonate and of an aqueous lanthanum(III) perchlorate solution sh ows that the hydrated lanthanum(III) ion in aqueous solution most probably has the same structure as in the solid, i.e., nine water molecules coordina ted in a tricapped trigonal prismatic configuration. The data analysis from EXAFS and LAXS measurements of the aqueous solution resulted in the La-O b ond distances 2.52(2) and 2.65(3) Angstrom to the water molecules in the pr ism and the capping positions, respectively. The LAXS study shows a second hydration sphere consistent with approximately 18 water molecules at 4.63(2 ) Angstrom. The EXAFS spectra of solid octakis(dimethyl sulfoxide)lanthanum (III) trifluoromethanesulfonate and a dimethyl sulfoxide solution of this s alt are also similar. The data analysis of EXAFS and LAXS measurements assu ming eight-coordination around lanthanum yielded an La-O bond distance of 2 .50(2) Angstrom, and an La ... S distance of 3.70(3) Angstrom, giving an La -O-S angle of 133(2)degrees. The EXAFS data of an N,N'-dimethylpropyleneure a solution of lanthanum(III) trifluoromethanesulfonate gave the La-O bond d istance 2.438(4) Angstrom and the La ... C distance 3.41(2) Angstrom, which correspond to an La-O-C angle of 131(2)degrees. The La-O bond distance is consistent with seven-coordination around lanthanum, on the basis of the va riation of the ionic radii of the lanthanum(III) ion with different coordin ation numbers.