Equatorial coordination of uranyl: Correlating ligand charge donation with the O_yl-U-O_yl asymmetric stretch frequency

In uranyl coordination complexes, UO₂(L)_n²⁺, uranium in the formally dipositive [O=U=O]²⁺ moiety is coordinated by n neutral organic electron donor ligands, L. The extent of ligand electron donation, which results in partial reduction of uranyl and weakening of the U=O bonds, is revealed by the magnitude of the red-shift of the uranyl asymmetric stretch frequency, ν₃. This phenomenon appears in gas-phase complexes in which uranyl is coordinated by electron donor ligands: the ν₃ red-shift increases as the number of ligands and their proton affinity (PA) increases. Because PA is a measure of the enthalpy change associated with a proton-ligand interaction, which is much stronger and of a different nature than metal ion-ligand bonding, it is not necessarily expected that ligand PAs should reliably predict uranyl-ligand bonding and the resulting ν₃ red-shift. Here, ν₃ was measured for uranyl coordinated by ligands with a relatively broad range of PAs, revealing a surprisingly good correlation between PA and ν₃ frequency From computed ν₃ frequencies for bare UO₂ cations and neutrals, it is inferred that the effective charge of uranyl in UO₂(L)_n²⁺ complexes can be reduced to near zero upon ligation by sufficiently strong charge-donor ligands. The basis for the correlation between ν₃ and ligand PAs, as well as limitations and deviations from it, are considered. It is demonstrated that the correlation evidently extends to a ligand that exhibits polydentate metal ion coordination.