Evidence for Cluster Orbital Formation in CsSn_2X_5 Compounds (X=Cl,Br)

摘要

The formation of cluster orbitals in CsSn_2Br_5 is discussed and related more generally to tetragonal compounds of the type AB_2X_5(A=monovalent cation;B=Sn,Pb;X=Cl,Br,I).The crystal structures of CsSn_2Cl_5 and CsSn_2Br_5 have been solved by single-crystal X-ray diffraction.These compounds are isotructural with each other and a range of AB_2X_5 structural analogues.In many AB_2X_5 compounds where B is a subvalent main group metal a tetragonal cell is observed with space group I4/mcm.The structures of CsSn_2Br_5 and CsSn_2Cl_5 are layered with polymeric sheets of [Sn_2X_5]_n~(n-) separated by the Cs~+ cations.Stereochemical considerations suggest that stabilization of this structural form,rather than the more ionic NH_4Pb_2Cl_5 or NaSn_2Cl_5 structures,is through interaction of the "nonbonding"valence electron pairs on tin with low-lying emjpty d-orbitals on neighboring X atoms:Electronic structure calculations based on the structural data confirm the likelihood of cluster orbital formation.Crystal data:CsSn_2Cl_5,tetragonal,I4/mcm,a=8.153(1)A,c=14.882(4)A,Z=4,R_1=0.0215,wR_2=0.0503 [I>2 #sigma#(I)],R_1=0.0393,wR_2=0.0536 (all data);CsSn_2Br_5,tetragonal,I4/mcm,a=8.483(6)A,c=15.28(2)A,Z=4,R_1=0.0607,wR-2=0.1411[(I>2 #sigma#(1)],R_1=0.1579,wR_2=0.1677 (all data).