Abstract
The complex anion [W2O5(cit)2]6– was obtained by crystallisation from an equimolar aqueous solution of sodium tungstate and citric acid (H4cit). It has been characterised by elemental analysis, IR and 1H and 13C NMR spectroscopy. The IR spectrum is consistent with a monooxo-bridged dinuclear structure as revealed by a single-crystal X-ray diffraction study of Na6[W2O5(cit)2]·10H2O. The crystals are monoclinic, space group P21/a, a= 17.708(3), b= 9.908(2) and c= 9.018(2)A, β= 91.57(2)°, R= 0.025 for 3101 observed [I/σ(I) 2.5] reflections. The complex anion contains a O2WOWO2 core with the bridging oxo group lying at a crystallographic centre of symmetry (W–Ob–W 180°). Each citrate ligand is three co-ordinated to one tungsten atom through the deprotonated hydroxy, α-carboxylate, and one β-carboxylate group. Principal dimensions are: W–Ob 1.893(1), (W = Ot)av 1.757(2), W–Ohydroxy 1.958(2), W–Oα-carboxy 2.195(3) and W–Oβ-carboxy 2.289(2)A. All methylene and β-carboxylate groups become magnetically equivalent upon redissolution in water as a result of the dissociation of the less strongly bonded β-carboxylate group.
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