Dioxouranium(VI)-carboxylate complexes

Calorimetric measurements were carried out following similar procedures and in the same experimental conditions as employed for the potentiometric measurements at I = 1.0 mol L−1 in NaCl. The stability of UO22+-oxalate2− complexes is fairly high and their main contribution to stability is entropic in nature. Some linear empirical relationships were found which make it possible to calculate (i) the contribution of a single bond: ΔGsb°=−16.6±0.2kJmol−1 and TΔSsb°=16.2±0.5kJmol−1; (ii) chelate stabilisation per ring: ΔGr°=−1.30±0.04kJmol−1 and TΔSr°=1.27±0.23kJmol−1 and (iii) the mean stability of negatively charged Na+-ion pair complexes: logTK = (0.46 ± 0.02)·|z| (z = charge of complex species), ΔG° = −(2.60 ± 0.1)·|z| kJ mol−1 and TΔS° = 2.5 ± 0.5 kJ mol−1. Both potentiometric and calorimetric results provide evidence of the penta-coordination of the species UO2(ox)34−. SIT parameters were calculated from the data at I = 0 and I = 1.02 mol kg−1. Comparisons are made with literature data. An insoluble dioxouranium(VI) ternary complex was synthesised (at I = 1.0 mol L−1 in NaCl) and characterised by thermoanalysis and elemental analysis.