Evidence for Cluster Orbital Formation in CsSn2X5 Compounds (X=Cl, Br)

The formation of cluster orbitals in CsSn2Br5 is discussed and related more generally to tetragonal compounds of the type AB2X5 (A=monovalent cation; B=Sn, Pb; X=Cl, Br, I). The crystal structures of CsSn2Cl5 and CsSn2Br5 have been solved by single-crystal X-ray diffraction. These compounds are isostructural with each other and a range of AB2X5 structural analogues. In many AB2X5 compounds where B is a subvalent main group metal a tetragonal cell is observed with space group I4/mcm. The structures of CsSn2Br5 and CsSn2Cl5 are layered with polymeric sheets of [Sn2X5]n-n separated by the Cs+ cations. Stereochemical considerations suggest that stabilization of this structural form, rather than the more ionic NH4Pb2Cl5 or NaSn2Cl5 structures, is through interaction of the "nonbonding" valence electron pairs on tin with low-lying empty d-orbitals on neighboring X atoms. Electronic structure calculations based on the structural data confirm the likelihood of cluster orbital formation. Crystal data: CsSn2Cl5, tetragonal, I4/mcm, a=8.153(1) ?, c=14.882(4) ?, Z=4, R1=0.0215, wR2=0.0503 [I>2SIGMA(I)], R1=0.0393, wR2=0.0536 (all data); CsSn2Br5, tetragonal, I4/mcm, a=8.483(6) ?, c=15.28(2) ?, Z=4, R1=0.0607, wR2=0.1411 [(I>2SIGMA (I)], R1=0.1579, wR2=0.1677 (all data).