Aqueous equilibria involving multiple tetravalent uranium (U(IV)) complexes with oxalate (Ox) were investigated by using various spectroscopic techniques over a wide range of pH values (1-7), ionic strengths (0.02-2 M), and temperatures (0-35 °C), as well as quantum-chemical calculations based on density functional theory (DFT). Ultraviolet-visible (UV-vis) absorption spectroscopy coupled with a liquid waveguide capillary cell revealed distinct red shifts in the U(IV) absorption spectra upon binding with multiple Ox ligands. Based on an equilibrium model involving U(Ox)n4-2n complexes (n = 1-4), multiple sets of spectrophotometric data were analyzed to estimate their stepwise equilibrium constants (log Kn°): 10.49 ± 0.18, 7.09 ± 0.18, 5.34 ± 0.04, and 2.33 ± 0.14, for K1°, K2°, K3°, and K4°, respectively. The corresponding values of reaction enthalpy (ΔrHn), entropy (ΔrSn), and specific ion-interaction coefficients (εn) were further assessed for each U(IV)-Ox complex. Dissolution measurements of synthetic U(Ox)2·xH2O(cr) in a free oxalate (∼pH 5) solution using attenuated total reflectance FTIR spectroscopy along with DFT calculations support the proposed complexation-equilibrium model, showing that the stable binding geometry of all four U(IV)-Ox complexes adopts a side-on bidentate or 5-membered-ring coordination structure; in particular, the formation of a ternary complex, U(OH)(Ox)+, was not energetically favorable.
Cha et al. (Thu,) studied this question.