M1−x[W2O2X6] with M=K+, Tl+, Ag+, Hg2+, Pb2+; X=Cl, Br—A class of mixed valence tungsten (IV,V) compounds with layered structures, W–W bonds and high conductivity

The crystal structure of WOCl3, determined on the basis of powder diffraction data (tetragonal, P42/mnm, image, image), is isotypic to WOI3 and contains one-dimensional strands of edge-sharing double-octahedral W2O4/2Cl6 groups connected via common corners in trans position. A W–W bond of 2.99 Å is present within the planar W2Cl6 groups. A series of non-stochiometric, mixed valence W(IV,V) compounds M1−x[W2O2Cl6] can be obtained from WOCl3 by reaction with metal halides (TlCl, KCl, PbCl2) or by reaction of elemental Hg with WOCl4. All were characterized by single crystal structure determinations and EDX measurements (Tl0.981(2)[W2O2Cl6]: monoclinic, C2/m, image, image, image, image; K0.84(2)[W2O2Cl6]: monoclinic, C2/m, image, image, image, image; Pb0.549(3)[W2O2Cl6]: orthorhombic, Immm, image, image, image; Hg0.554(6)[W2O2Cl6]: monoclinic, C2/m, image, image, image, image). Two representatives of this family of compounds have already been reported: Na[W2O2Br6] [Y.-Q. Zhang, K. Peters, H.G. von Schnering, Z. Anorg. Allg. Chem. 624 (1998) 1415–1418] and Ag0.74[W2O2Br6] [S. Imhaïne, C. Perrin, M. Sergent, Mat. Res. Bull. 33 (1998) 927–933]. The Ag containing compound can be obtained from elemental Ag and WOBr3. The crystal structure, originally reported in the triclinic system, was redetermined and shown to be monoclinic with space group C2/m (image, image, image, image). The crystal structures of these compounds are in close relationship to the structure of WOCl3 and all contain W2O4/2X6 (X=Cl, Br) double strands with the mono and divalent cations coordinated by the terminal halogen atoms of the W2X6 groups and a short W–W bond (2.85 Å for X=Cl). A cube-shaped coordination environment is present for M=Tl, K and a trigonal-prismatic coordination for M=Ag, Hg. Hg0.55[W2O2Cl6] is a semiconductor with a non-Arrhenius behaviour, high specific conductivity of 0.05 Ω–1 cm−1 and a very small activation energy of 0.03 eV. Hg0.55[W2O2Cl6] and Ag0.8[W2O2Br6] show a temperature independent paramagnetism with a magnetic moment around 300×10–6 cm3 mol–1.