Tantalum Chlorides in Octahedral Cluster Chemistry: The Structures of Cs2PbTa6Cl18 and CsPbTa6Cl18

Abstract Two new series of Ta 6 chlorides, Cs 2 M ′ II Ta 6 Cl 18 and Cs M ′ II Ta 6 Cl 18 , have been isolated with M ′=Ba 2+ , Pb 2+ , Sr 2+ , Eu 2+ , Ca 2+ . From single-crystal structure refinements the compounds Cs 2 PbTa 6 Cl 18 [ R -3, Z =1, a =10.291(2) A, α =54.645(8)°] and CsPbTa 6 Cl 18 [ P -31 c , Z =2, a =9.2673(8) A, c =17.249(3) A] are isotypic with KLuNb 6 Cl 18 and CsLuNb 6 Cl 18 , respectively. In the former compound the structure consists of a fcc stacking of Ta 6 Cl 18 units, while a pseudohexagonal AA′A stacking of units is observed for the latter compound. The tetrahedral site of units, half-occupied by K + in KLuNb 6 Cl 18 is fully occupied by Cs + in Cs 2 PbTa 6 Cl 18 . The interatomic distances are consistent with valence electron concentrations per cluster (VEC) of 16 and 15 for Cs 2 PbTa 6 Cl 18 and CsPbTa 6 Cl 18 , respectively. It is the first time that such a VEC of 15 is observed for a quaternary Me 6 X 18 -unit based compound; this feature is discussed in relation to the density functional theory results previously reported on Me 6 X 18 ( Me =Nb, Ta; X =Cl, Br).