The Zintl Chemistry of the Heavy Tetrel Elements

Abstract

Exploration of the alkali metal/alkaline-earth metal/heavy tetrel (Sn or Pb) systems has revealed a vast array of new chemistry and novel structure types. The structures and properties of these new materials have been studied in an attempt to understand the chemistry of these and other related systems. The first phase reported is Rb{sub 4}Pb{sub 9} (K{sub 4}Pb{sub 9} type). The compound contains two different types of Pb{sub 9}{sup 4-} deltahedra, a monocapped square pyramid and a distorted tricapped trigonal prism. Both cluster geometries correspond to a nido assignment even though the tricapped trigonal prism is not the classic Wade's rules nido deltahedron expected for a monocapped square antiprism. Also, a series of compounds that contain square pyramidal Tt{sub 5} polyanions of tin and lead has been obtained in alkaline-earth or rare-earth metal-tetrel systems by direct fusion of the elements to yield Sr{sub 3}Sn{sub 5}, Ba{sub 3}Pb{sub 5}, and La{sub 3}Sn{sub 5}. These phases contain square pyramidal clusters of the tetrel elements that are weakly interlinked into chains via two types of longer intercluster interactions that are mediated by bridging cations and substantially influenced by cation size and the free electron count. Attempts at incorporating another main-group element to form heteroatomicmore » clusters were also successful. In the case of A{sub 5}InPb{sub 8} (A = K, Rb), the compounds contain clusters composed of two Pb{sub 4} tetrahedra that are interbridged by a lone {micro}{sub 6}-In atom. The InPb{sub 8} units are weakly interlinked into sheets in the ab plane by long intercluster Pb-Pb interactions. Using As led to the formation of the compound K{sub 5}As{sub 3}Pb{sub 3} which is made up of As{sub 3}Pb{sub 3}{sup 5-} crown clusters that can be likened to a 6-atom hypho-cluster based on the tricapped trigonal parent as the closo structure. These crowns are connected via intercluster bonds to form infinite chains down the b axis. This work also reports on a disordered phase Na{sub 23}In{sub 40.4(1)}Pd{sub 2.51(2)}. The major structural features include a 12-bonded-closo-In{sub 12} icosahedron, a 15-bonded-closo-In{sub 15} cluster, and an 18-bonded-21-atom spacer.« less