title: Uranyl-halide complexation in N,N-dimethylformamide: halide coordination trend manifests hardness of [UO2]2+.
abstract: Complexation of [UO2](2+) with Cl(-), Br(-), and I(-) in N,N-dimethylformamide (DMF) was studied by UV-vis absorption spectroscopy and extended X-ray absorption fine structure (EXAFS) to clearly differentiate halide coordination strengths to [UO2](2+). In the Cl(-) system, it was clarified that the Cl(-) coordination to [UO2](2+) in DMF proceeds almost quantitatively. The coordination number of Cl(-) almost quantitatively increases up to 4, i.e., the limiting complex is [UO2Cl4](2-). Logarithmic gross stability constants of [UO2Cl(x)](2-x) (x = 1-4) were evaluated as log β1 = 9.67, log β2 = 15.49, log β3 = 19.89, and log β4 = 24.63 from UV-vis titration experiments. The EXAFS results well demonstrated not only the Cl(-) coordination, but also the DMF solvation in the equatorial plane of [UO2](2+). The interaction of Br(-) and I(-) with [UO2](2+) in DMF was also investigated. As a result, the Br(-) coordination to [UO2](2+) stops at the second step, i.e., only [UO2Br](+) and UO2Br2 were observed. The molecular structure of each occurring species was confirmed by EXAFS. The evaluated log βx values of [UO2Br(x)](2-x) (x = 1, 2) are 3.45 and 5.42, respectively. The much smaller log βx than those of [UO2Cl(x)](2-x) indicates that Br(-) is a much weaker ligand to [UO2](2+) than Cl(-). The EXAFS experiments revealed that the presence of I(-) in the test solution does not modify any coordination structure around [UO2](2+). Thus, I(-) does not form any stable [UO2](2+) complexes in DMF. Consequently, the stability of the halido complexes of [UO2](2+) in DMF is exactly in line with the hardness order of halides.
authors.affiliation: Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, 152-8550 Tokyo, Japan. ktakao@nr.titech.ac.jp.
authors.forename: Koichiro
authors.initials: K
authors.lastname: Takao
authors.forename: Shinobu
authors.initials: S
authors.lastname: Takao
authors.forename: Yasuhisa
authors.initials: Y
authors.lastname: Ikeda
authors.forename: Gert
authors.initials: G
authors.lastname: Bernhard
authors.forename: Christoph
authors.initials: C
authors.lastname: Hennig