THERMODYNAMICS OF COMPLEXATION OF THE URANYL(VI) ION WITH BENZOATE - THE CRYSTAL-STRUCTURE OF SODIUM TRIS(BENZOATO) DIOXOURANATE DIHYDRATE, NA[(PHCO2)3UO2].2H2O

The stability constant and the enthalpy and entropy changes of formation of the 1:1 complex of uranyl(VI) with benzoate were determined by potentiometric and calorimetric titrations in 1.0 mol dm-3 aqueous solution of NaClO4 at 25-degrees-C. The values obtained for the 1:1 complex agree with the general relationship holding for aliphatic monocarboxylato uranyl(VI) complexes. The structure of Na[UO2L3].2H2O (L = PhCO2-) was determined by X-ray crystallography and refined to R = 0.029 based on 772 unique observed reflections. The compound is orthorhombic, space group Pnna, with a = 9.387(3), b = 9.576(3), c = 26.491(3) angstrom. Three ligands are equatorially chelated to the uranyl group forming UO2L3- anions with C2 crystallographic symmetry. The Na+ cations, which also lie on two-fold axes, form approximate coordination octahedrons with oxygen atoms of water molecules and neighbouring anions.