The title compounds were prepared through dephenylation of hexaphenyldisilane with acetyl bromide or acetyl iodide in the presence of the corresponding aluminium halide. Both substances were purified via sublimation and, for the first time, single crystals of hexabromodisilane, Si2Br6, and a new polymorph of hexaiododisilane, Si2I6, could be isolated. Molecules of Si2Br6 are located on a special position of site symmetry 2/m with a quarter of a molecule in the asymmetric unit. Molecules of Si2I6 are located on a special position of site symmetry 3̄ with a sixth of a molecule in the asymmetric unit. The bond lengths of Si2Br6 and Si2I6 are in the usual ranges and both molecules adopt a staggered conformation. It is interesting to note that Si2Br6 and Si2I6 do not form isomorphous structures. Moreover, an orthorhombic polymorph of the present structure of Si2I6 is already known [Jansen & Friede (1996). Acta Cryst. C52, 1333-1334]. Although the title compounds feature such small and simple molecules they show completely different crystal structures.
Keywords: crystal structure; hexabromodisilane; hexaiododisilane; polymorphism.