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

In uranyl coordination complexes, UO2(L)(n)(2)+, 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, v(3). This phenomenon appears in gas-phase complexes in which uranyl is coordinated by electron donor ligands: the v(3) 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 v(3) red-shift. Here, v(3) was measured for uranyl coordinated by ligands with a relatively broad range of PAs, revealing a surprisingly good correlation between PA and v(3) frequency From computed v(3) frequencies for bare UO2 cations and neutrals, it is inferred that the effective charge of uranyl in UO2(L)(n)(2+) complexes can be reduced to near zero upon ligation by sufficiently strong charge-donor ligands. The basis for the correlation between v(3) 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. (C) 2017 Elsevier B.V. All rights reserved.