VIBRATIONAL SPECTROSCOPIC STUDIES OF URANYL COMPLEXES IN AQUEOUS AND NONAQUEOUS 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 [Bu4nN][UO2Cl4], [Me4N][UO2Cl4], [Pr4nN][[UO2(NO3)3], [Bu4nN][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 = Pr(n), Bu(n)). Aqueous solutions of UO2(NO3)2 and Cs[UO2(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.