Novel tin(II) sites in X-ray crystal structures of the tin(II) halide sulphates K3Sn2(SO4)3X (X = Br or Cl)

Abstract

The crystal structures of K3Sn2(SO4)3X (X = Br or Cl) have been determined by the heavy-atom method and refined by the full-matrix least-squares method to R= 0.0564 for the bromide and 0.0839 for the chloride. The crystals of both compounds are of the hexagonal space group P63(C66, no. 173) with a= 10.256(2), c= 7.582(4)A, and Z= 2 for K3Sn2(SO4)3Br, and a= 10.183(6), c= 7.540(2)A, and Z= 2 for K3Sn2(SO4)3Cl. The structures consist of three-dimensional networks of tin atoms and bridging sulphate groups with discrete potassium and halide ions in holes within the networks. The four tin atoms in the lattices are distributed together with two K+ ions in a six-fold general position and have three oxygen atoms at unusually long distances of 2.47, 2.57, and 2.63 A for the bromide and 2.45, 2.55, and 2.56 A for the chloride. The shortest tin–halogen contacts are 2.97 A for the chloride and 3.13 A for the bromide. The halide ion is surrounded in a distorted octahedron by tin(II) or potassium atoms. The arrangement of tin atoms, taken along with 119Sn Mossbauer data and the lengths of the Sn–O bonds in the structures, is consistent with cluster formation in which lone-pair electron density on the tin atoms is delocalised in cluster molecular orbitals.