Vibrational spectroscopic studies of uranyl complexes in aqueous and non-aqueous solutions

Vibrational spectra have been obtained for aqueous solution of uranyl-perchlorates, -fluorides, -chlorides, -acetates and -sulphates over a range of solution composition with added anions. We have prepared Buⁿ₄N]UO₂Cl₄], Me₄N]UO₂Cl₄], Prⁿ₄N]UO₂(NO₃)₃], Buⁿ₄N]UO₂(NO₃)₃], with the expectation that the large cation would give a better approximation to vibrational frequencies of the free anion and would allow measurements in non-coordinating solvents. As the perchlorate is not coordinated to UO₂]²⁺ in aqueous solution the expected structure is a solvated cation UO₂(OH₂)₅]²⁺ with characteristic infrared 962.5, 253 and 160 cm^?1 and Raman 874 and 198 cm^?1 bands. The formation of weak, solvated UO₂X]⁺ complexes (X=F, Cl) has been established with frequencies at 908, 827, 254, 380 cm^?1 and 956, 871, 254 and 222 cm^?1 for UO₂F]⁺ and UO₂Cl]⁺, respectively. Bidentate NO₃ coordination has been established for solid and dissolved (in CH₂Cl₂) R₄N]UO₂(NO₃)₃] (R=Prⁿ, Buⁿ). Aqueous solutions of UO₂(NO₃)₂ and CsUO₂(NO₃)₃] show no clear evidence that bidentate or monodentate nitrate is present. Both unidentate and bidentate linkage of acetate-uranyl were established for acetate complexes in aqueous solutions. For the uranyl sulphate system, monodentate sulphate coordination is the major mode at low SO₄:U ratios, and even at a ratio of 3:1 there is very little free sulphate.