AQUEOUS URANIUM COMPLEXES .2. RAMAN-SPECTROSCOPIC STUDY OF THE COMPLEX-FORMATION OF THE DIOXOURANIUM(VI) ION WITH A VARIETY OF INORGANIC AND ORGANIC-LIGANDS

The Raman spectra of the aqueous dioxouranium(VI) ion have been measured as a function of ligand and uranyl concentrations at various constant pH values. The ligands investigated were F-, Cl-, Br-, SO42-, HSO4-, NO3-, ClO4-, CH3CO2-, and C2O42-. A linear correlation between the frequency of the O=U=O symmetrical stretching vibration (nu1, cm-1) and the average number nBAR of ligands coordinated equatorially to the central uranium atom in the given species has been established for the above-mentioned ligands according to the following equation: nu1 (cm-1) = -AnBAR + 870, where A is a characteristic coefficient (cm-1) representing the shift in the nu1 band for the addition of each ligand to the uranyl(VI) center relative to 870 cm-1 for UO22+(aq). Except for the case Of ClO4- (A = -1 5 cm-1), the values of A are positive and vary in the range 0-15. The preceding equation was derived earlier for OH-anions with A = 21.5 +/- 1.0 cm-1. Results obtained elsewhere for CO32- also fit this equation with A = 19.0 +/- 1.0 cm-1. This linear correlation provides a convenient, nondestructive method of identifying the possible stoichiometry of uranyl(VI) complexes in solution and a means for estimating their stability fields with respect to ligand concentration and pH. These results suggest that complex formation with these ligands leads to the weakening of the uranium-axial oxygen bonds in the following order: OH- > CO32- > C2O42- > F- > SO42-, CH3CO2- > Cl- > Br-, NO3-, ClO4-, HSO4-. This order reflects the decrease in the overall stability constants (log beta) of these complexes. Three linear correlations between log beta(n) and the shift in the vibration energy (DELTAnu1) between UO22+ and the given species (UO2Ln; n = 1-3) have been established.