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 AB(2)X(5) (A = monovalent ca tion; B = Sn, Ph; X= Cl, Br, I). The crystal structures of CsSn2Cl5 and CsS n2Br5 have been solved by single-crystal X-ray diffraction. These compounds are isostructural with each other and a range of AB(2)X(5) structural anal ogues. In many AB(2)X(5) compounds where B is a subvalent main group metal a tetragonal cell is observed with space group I4/mcm. The structures of Cs Sn2Br5, and CsSn2Cl5 are layered with polymeric sheets of [Sn2X5](n)(n-) se parated by the Cs+ cations. Stereochemical considerations suggest that stab ilization of this structural form, rather than the more ionic NH4Pb2Cl5 or NaSn2Cl5 structures, is through interaction of the "nonbonding" valence ele ctron 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) Angstrom, c = 14.882(4) Angstrom, Z = 4, R-1 = 0.021 5, wR(2) = 0.0503 [I > 2 sigma (1)], R-1 = 0.0393, wR(2) = 0.0536 (all data ); CsSn2Br5, tetragonal, I4/mcm, a = 8.483(6) Angstrom, c = 155.28(2) Angst rom, Z = 4, R-1 = 0.0607, wR(2) = 0.1411 [(I > 2 sigma (I)], R-1 = 0.1579, wR(2) = 0.1677 (all data). (C) 2001 Academic Press.