Syntheses, crystal structures, and triple twinning of the cluster trimers Bi2[PtBi6Br12]3 and Bi2[PtBi6I12]3

Melting reactions of Bi with Pt and BiX3 (X = Br, I) yield shiny black, air insensitive crystals of the subhalides Bi-2[PtBi6X12]. Bi-2[PtBi6Br12](3) crystallizes in the monoclinic space group C2/m with lattice parameters a = 1617.6(2) pm, b = 1488.5(1) pm, c = 1752.4(2) pm, and beta = 110.85(4)degrees. Bi-2[PtBi6I12](3) adopts the triclinic space group Cl with pseudo-monoclinic lattice parameters a = 1711.2(2) pm, b = 1585.1(1) pm, c = 1865.7(2) pm, and alpha = 90 degrees, beta= 111.15(4)degrees, gamma = 90 degrees. The two homoeotypic compounds consist of cuboctahedral [(Pt-11Bi61211X-1)](2-) clusters that are concatenated into linear trimers by Bi-III atoms. The ordered distribution of Bi-III atoms destroys the inherent threefold rotation axes in the packing of cluster anions. As a consequence of the pseudosymmetry the crystals are triple twinned along [201]. Due to different orientations of the cluster trimers there are two Bi-II center dot center dot center dot X inter-cluster bridges per Bin atom in Bi-2[PtBi6Br12](3) but only one bridge in Bi-2[PtBi6I12](3). The structure of the iodine compound can be deduced from the NaCl structure type, leaving 37 of 96 atomic positions unoccupied. The arrangement of the cuboctahedral clusters follows the motif of a body-centered cubic packing.