Tin halogen compounds. 2. Molecular and crystal structures of tin(IV) bromide and iodide

Single crystals of tin(IV) bromide and iodide were obtained by sublimation (SnBr4) and recrystallization (SnI4). The crystal structures of this compounds were examined by means of X-ray diffraction on a four circle diffractometer. SnBr4 crystallizes at -30 degreesC in the monoclinic space group P2(1)/c (No.14) with a=1037.1(2), b=700.6(1), c=1047.0(2)pm, beta =102.56(1)degrees, V= 742.5(2)10(6) pm(3), Z=4. SnI4 crystallizes at room temperature in the cubic space group Pa (3) over bar with a = 1226.81(8) pm, V= 1846.4(3) . 10(6) pm(3), Z = 8. Both structures consist of very slightly distorted tetrahedral SnX4-molecules. The mean Sn-X-distances are 242.3 pm (Sn-Br) and 265.1 pm (Sn-I), which are significantly smaller than the sum of the normal covalent radii. The crystal structures are based on a hexagonal (bromide) and cubic-close (iodide) packing of the halogen atoms, in which only 1/8 of the tetrahedral sites are occupied by tin atoms. An approach for the description of the crystal structure of tin(IV) iodide on the base of the cubic-close packing is given.