The mixed valence tungsten(IV,V) compound Na{W2O2Br6]

The reaction of W6Br12, NaBr, and WO2Br2 in the presence of Br-2 in a sealed silica tube yields Na[W2O2Br6] together with WOBr4 and WO2Br2 in the low temperature zone (temperature gradient 1030/870 K). Na[W2O2Br6] crystallizes orthorhombically in the space group Immm (no. 71) with a = 3.775 Angstrom, b = 10.400 Angstrom, c = 13.005 Angstrom and Z = 2. Pairs of condensed trans-[WO2Br4] octahedra with a common Br-2 edge form along [100] double chains (1)(infinity)[W2O4/2Br6](1-) via the oxygen atoms. The mixed valent tungsten atoms are bonded to W-2 pairs with a 2c-3e bond (d(W-W) = 2.946 Angstrom, d(W-O) = 1.888 Angstrom, d(W-Br-b) = 2.537 Angstrom, d(W-Br-t) = 2.535 Angstrom, measured angle O-W-O = 177.4 degrees, measured angle Br-b-W-Br-b (endocyclic)= 109.0 degrees). The Na+ cations connect the anionic double chains to form two-dimensional layers parallel (001), which interact by van der Waals forces. The cations are eightfold coordinated by a cube of the terminal Br-t ligands of the polymeric anions ((d) over bar(Na-Br) = 3.138 Angstrom). Na[W2O2Br6] may be discussed as an intercalation compound of the oxide bromide WOBr3.