Square antiprismatic geometry electron counting

Symmetries given in Table 2 are idealized (and refer to the symmetry of the cluster without taking into account heteroatoms) and many of the clusters show distortions from these ideal symmetries. For example, the group 14 clusters Geg, Sng and Pbg have 2n + 4 cluster electrons and exist as the expected nido clusters with C4V monocapped square antiprismatic geometry. On the other hand, the bismuth cluster Big +, which also has a 2n + 4 electron count, instead exists in the closo Z>3h form with a tricapped trigonal prismatic structure. In fact, all these 9-atom clusters show some distortions from the idealised symmetry, and the energy differences between different geometries are small.The arachno clusters Sb4, Bi4 , Se4 + and Te4 + (square planar), and Sbg + and Big + (square antiprismatic) have... 

The influence of electron-count on cluster geometry has been very elegantly shown by a crystallographic study of the deep-red compound [K(ctypt)]g [Ge9]- [Ge9] .2.5en, prepared by the reaction of KGe with cryptand in ethylenediamine. [Ge9] has the C4, unicapped square-antiprismatic structure (10.10c) whereas [Ge9]- , with 2 less electrons, adopts a distorted Dit, structure which clearly derives from the tricapped trigonal prism (p. 153).The field is one of... 

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