Inert Pair Effects in Tin and Lead Dihalides: Crystal Structure of Tin(II) Bromide

The crystal structure of stannous bromide, SnBr2, is compared with those of its analogs: SnCl2, PbBr2, and PbCl2. The structure of SnBr2 has been refined by Rietveld analysis of X-ray powder diffraction data in the orthorhombic space group Pbnm, with a=10.5251(7) ?, b=8.3936(5) ?, c=4.2369(2) ?, and Z=4. Refinement terminated with Rwp=11.90% and Rex=19.11%. Sn is in a trigonal-pyramidal coordination with three Br atoms, two of which are bridging and one terminal. The bridging units form polymeric chains, which run parallel to thec-axis. SnBr2 is related to, but is not isostructural with, analogs, PbCl2, PbBr2, and SnCl2. All the structures possess (MX2)n chains (M=Sn or Pb, X=Cl or Br), but those in SnBr2 are shifted with respect to each other, resulting in a difference in the secondary M–X coordination shell. Stereochemical distortions caused by the nonbonding valence electron pairs on the metal atoms are compared using unit cell parameters normalized to take into account differences in covalent radii between the structures.