Tantalum chlorides in octahedral cluster chemistry: The structures of Cs2PbTa6Cl18 and CsPbTa6Cl18

Two new series of Tab chlorides, Cs2M'(Ta6Cl18)-Ta-II, and CsM'(Ta6Cl18)-Ta -II, have been isolated with M' = Ba2+, Pb2+, Sr2+, EU2+, Ca2+. From single -crystal structure refinements the compounds Cs2PbTa6Cl18 [R-3, Z = 1, a = 10.291(2) Angstrom, alpha = 54.645 (8)degrees] and CsPbTa6Cl18 [P-31c, Z = 2, a = 9.2673(8) Angstrom, c = 17.249(3) Angstrom] are isotypic with KLuNb6 Cl18, and CsLuNb6Cl18, respectively. In the former compound the structure c onsists of a fee stacking of Ta,CI,, units, while a pseudohexagonal AA;1 st acking of units is observed for the latter compound. The tetrahedral site o f units, half-occupied by K+ in KLuNb6Cl18 is fully occupied by Cs+ in Cs2P bTa6Cl18. The interatomic distances are consistent with valence electron co ncentrations per cluster (VEC) of 16 and 15 for Cs2PbTa6Cl18 and CsPbTa6Cl1 8,s, respectively. It is the first time that such a VEC of 15 is observed f or a quaternary Me6X18-unit based compound; this feature is discussed in re lation to the density functional theory results previously reported on Me6X 18 (Me = Nb, Ta; X= Cl, Br). (C) 1999 Academic Press.