2 MODIFICATIONS OF COPPER(I) OCTAHEDRO-HEXATUNGSTEN(II) TETRADECABROMIDE, CU-2[W6BR14]

The reaction of W6Br12 With CuBr sealed in an evacuated silica tube at the temperature gradient 925/915 K and annealing at 625/300 K yields a mixture of orthorhombic alpha-Cu-2[W6Br14] and cubic beta-Cu-2[W6Br1 4] in the low temperature zone. alpha-Cu-2[W6Br14] crystallizes in the space group Pbca (no. 61), a = 15.126 Angstrom, b = 9.887 Angstrom, c = 15.954 Angstrom, Z = 4, oP88, and beta-Cu-2[W6Br14] crystallizes in the space group Pn (3) over bar (no. 201), a = 13.391 Angstrom, Z = 4 , cP88. The crystal structures are built up by [(W6Br8i)Br-6(a)](2-) c luster anions and Cu+ cations. The cluster anions show only in the per ipheral shells small deviation from m (3) over bar m symmetry ((d) ove r bar(W-W) = 2.630 Angstrom; (d) over bar(W-Br-i) = 2.618 Angstrom; (d ) over bar(W-Br-a) = 2.614 Angstrom). The anions are arranged in a sli ghtly compressed bce pattern (alpha) and ccp (beta) pattern, respectiv ely. The Cu+ cations are trigonal-planar coordinated by Br-a ligands w ith (d) over bar(Cu-Br) = 2.377 Angstrom (alpha) and 2.378 Angstrom (b eta). The cubic beta-modification is diamagnetic with an unexpected la rge susceptibility (chi(mol) = -884 x 10(-6) cm(3) mol(-1)) and have a band gap of 2.8 eV. It decomposes under dynamic vacuum in two steps a t 795 K und 1040 K.