THE CRYSTAL-STRUCTURE OF TUNGSTEN(II) BROMIDE, W6BR12

The crystal structure of tungsten (IT) bromide, W6Br12, was refined on the basis of X-ray data of a twinned crystal (space group Bbcm (No. 6 4); a = 11.799 Angstrom; b = 11.813 Angstrom; c = 14.790 Angstrom; Z = 4). W6Br(12) forms the well-known Mo6Cl12 structure, with layers of t wo-dimensional infinite networks (2)(infinity)[(W6Br8i)(B2Br4/2a-a)-Br -a]. The cluster bond lengths and bond angles show only small deviatio ns from 4/mmm symmetry, which result from internal strain in the 2D ne twork. Mean distances are: (d) over bar(W-W) = 2.633 Angstrom; (d) ove r bar(W-Br-i) = 2.630 Angstrom towards the exo-Br-a; (d) over bar(W-Br -i) = 2.619 Angstrom towards the bridging Bra-a; d(W-Br-a) = 2.550 Ang strom; d(W-Bra-a) = 2.656 Angstrom. The bond angle at the bridging Bra -a atom is about 132 degrees. The structural details of some isotypic compounds are compared and analyzed. It is shown that size effects as well as electronic effects play an important role in the tuning of the metal-to-metal bonds.