Structure of the aqua ions and fluoride complexes of uranium(IV) and thorium(IV) in aqueous solution an EXAFS study

The structures of aqueous M4+(aq) and MF3+(aq), where M is uranium(IV) or t horium(IV), have been determined by L-III edge EXAFS using data from soluti ons of 1.5 M HClO4 in which the M(IV) concentrations ranged from 0.03 to 0. 3 M. A least-squares refinement of the data for the aqua ions indicated 10. 8 +/- 0.5 water molecules in the first hydration sphere of both ions and M- O bond distances for U(IV) and Th(IV) of 2.42 +/- 0.01 and 2.45 +/- 0.01 An gstrom, respectively. By considering both previous structure information an d the EXAFS data, we selected N = 10 +/- 1 as the most likely coordination number of both M(IV) aqua ions. EXAFS measurements from acidic aqueous uran ium(IV) and thorium(IV) solutions containing fluoride show that large chang es in the first coordination sphere occur. The experimental data indicates an asymmetrical distribution of the distances, probably as a result of diff ering M-F and M-O bond lengths. These can be described by a model that cont ains two different bond distances,one M-F distance at 2.10 Angstrom and one M-O distance at 2.45 Angstrom for U(IV); for Th(IV), the corresponding dis tances are 2.14 and 2.48 Angstrom. The total coordination number in this mo del is unchanged from the aqua ions, i.e., 10 +/- 1.