Shapes and Ion Packing

2 Cluster Shapes and Ion Packing The structures of tetrahedral trigonal bipyramidal [Es] , and bicapped quadratic-antiprismatic [Eio] clusters strictly follow Wade s rules and form 12 = 4x2 + 4) skeletal electron (ske) nido-, 12 = 5x2 + 2) ske closo-, and 22 = 10 x 2 + 2) ske doso-dusters, respectively [34]. Each Group 14 atom has four valence electrons, two of which remain as a lone pair at each vertex equivalent to the covalent exo-BH bonds in boranes and are not involved in the framework bonding. 

The crystal structures of the ammoniates containing [E4 clusters show remarkable similarities to those of the corresponding intermetallic 1 1 phases with respect to cluster shape and coordination spheres [10]. Thus, strong contacts between the alkali metal cations and the tetrel atoms abound in all compounds. 

A deltahedral doso-cluster with nine atoms n=9) corresponds to a tricapped trigonal prism and Djh point group symmetry (Eigure 3.1c) if 2w + 2 = 20 ske are available for the cluster skeleton bonding. Hence, a cluster with a twofold negative charge, [Eg] , is appropriate for a closo-cluster, and [Eg] is on par with a nido-duster due to 2n + 4 = 22 ske, forming a C4v-symmetric monocapped square antiprism 

In solvate crystals obtained from liquid ammonia and ethylenediamine germanium, nido- Ge9] clusters were found together with naked K or Rb and Rb or Cs cations, respectively [37, 38], in which all cations were coordinated to the cluster anions in numerous contacts, resulting in a three-dimensional (3-D) network very similar to that in the binary intermetaDic phases. The solvent molecules in the lattice are attached to the alkaU metal atoms, but the compounds A+Geg(en) (A = Rb, Cs) lose their ethylenediamine easily upon heating, thereby leaving A+Geg [38]. 

In the crystal structure of [Na([2.2.2]crypt)]4Sn9 all cations are completely surrounded by cryptand molecules, and no interactions occur between the anionic nido-clusters and Na [23]. 

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