Solvent effects on uranium(VI) fluoride and hydroxide complexes studied by EXAFS and quantum chemistry

The structures of the complexes UO(2)F(n)(H(2)O)(5-n)(2-n), n = 3-5, have been studied by EXAFS. All have pentagonal bipyramid geometry with U-F of and U-H(2)O distances equal to 2.26 and 2.48 A, respectively. On the other hand the complex UO(2)(OH)(4)(2-) has a square bipyramid geometry both in the solid state and in solution. The structures of hydroxide and fluoride complexes have also been investigated with wave function based and DFT methods in order to explore the possible reasons for the observed structural differences. These studies include models that describe the solvent by using a discrete second coordination sphere, a model with a spherical, or shape-adapted cavity in a conductor-like polarizable continuum medium (CPCM), or a combination of the two. Solvent effects were shown to give the main contribution to the observed structure variations between the uranium(VI) tetrahydroxide and the tetrafluoride complexes. Without a solvent model both UO(2)(OH)(4)(H(2)O)(2-) and UO(2)F(4)(H(2)O)(2-) have the same square bipyramid geometry, with the water molecule located at a distance of more than 4 A from uranium and with a charge distribution that is very near identical in the two complexes. Of the models tested, only the CPCM ones are able to describe the experimentally observed square and pentagonal bipyramid geometry in the tetrahydroxide and tetrafluoride complexes. The geometry and the relative energy of different isomers of UO(2)F(3)(H(2)O)(2-) are very similar, indicating that they are present in comparable amounts in solution. All calculated bond distances are in good agreement with the experimental observations, provided that a proper model of the solvent is used.