Clusters and Cages of Main Group Elements

Most main group elements build solids that are either infinite arrangements or species of low dimensionality-chains or rings-packed in molecular solids. Exceptions to this behavior are carbon and phosphorus which lead to allotropic forms made up of cluster species. The best known of them is the elementary white phosphorus P4 which has a cluster structure itself and retains its shape in solution and in the gas phase. In the case of carbon a new allotropic form consisting of molecular clusters C, the fullerenes, has been discovered recently. These species are giant carbon cages with spherically shaped structures which lead to molecular solids and that are also stable in solution. 

The fundamental difference between transition metal elements and the Main Group Elements is the configuration of the valence electron shells. Main Group Elements have, if any, only closed d-shells. 

The chemistry of the electron-deficient boranes and carboranes which has indeed been employed as a useful model in the description of bonding in transition metal clusters appears to be a singular case among main group elements. From the point of view of the cluster chemistry, the properties of boron appear to meet with those of the transition metals. 

In this Chapter, some of the different possibilities that Main Group Elements offer for building atomic conglomerates will be analyzed. In such a context examples of electron-deficient cluster species such as the clusters formed by the alkali metals and some complementary aspects of the chemistry of boranes and 

Cubic body centered. A3 = Hexagonal close packed 

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