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 formati on of the 1:1 complex of uranyl(VI) with benzoate were determined by p otentiometric and calorimetric titrations in 1.0 mol dm-3 aqueous solu tion of NaClO4 at 25-degrees-C. The values obtained for the 1:1 comple x agree with the general relationship holding for aliphatic monocarbox ylato uranyl(VI) complexes. The structure of Na[UO2L3].2H2O (L = PhCO2 -) was determined by X-ray crystallography and refined to R = 0.029 ba sed 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) angst rom. Three ligands are equatorially chelated to the uranyl group formi ng UO2L3- anions with C2 crystallographic symmetry. The Na+ cations, w hich also lie on two-fold axes, form approximate coordination octahedr ons with oxygen atoms of water molecules and neighbouring anions.