The data available in literature on the vibration spectra of complex uranyl compounds indicate that the stretching vibration frequencies of the UO2+2group depend not only on the composition of the coordination sphere, but also on the outer surrounding of the complex anion. The outer-sphere cation, in case of uranyl complexes, acts as a competitor of the uranyl group to form bonds with, ligands. At the same time, as a result of decrease in the electron density localised on the U-F bonds, the strength of these bonds becomes less.