Naked Clusters of the Post-Transition Elements

The elements of group 16 in the Periodic Table have a comparatively large number of valence electrons to be distributed over the cluster framework in which they are incorporated. Their homoatomic clusters are therefore often electron-rich in the sense that they have an electron count higher than 5n. The more electron-poor character of the elements of group 15 results in a borderline situation where cationic clusters are found to be electron-poor (Sec. 1.29.4.4), whereas the anionic ones are electron-rich. The structurally characterized, electron-rich naked clusters of the post-transition elements are listed in Table 4. 

Whereas the transition-metal clusters are ligand-supported (in the typical case by carbonyls), the post-transition element clusters are most often unsupported and/ or destabilized by the interaction with ligands hence the use of the term ""naked cluster". 

The transition-metal clusters often display highly symmetrical metal frameworks, normally with symmetries derived from the platonic or archimedian solids or variations thereof. The post-transition element clusters, on the other hand, are not necessarily aflfected by such confinements. However, as will be explained in Sec. 1.29.4, relationships between the structure and the electron count do exist for the naked clusters, and regular, closed geometries are found for a rather large group of these species. For instance, the trigonal prismatic symmetry of the Rhf, framework in Fig. lA is also found in the naked tellurium cluster... 

The determination of the ec of naked post-transition element clusters is generally very easy (as compared to the transition-metal case) because of the lack of occupied valence d orbitals, interfering ligands and interstitial atoms.The ec of... 

Polyhedral clusters in which the electron count is less than Sn are electron-poor in the sense that the number of skeleton electrons is insufficient to form localized, 2c,2e bonds. Instead, models involving 3c,2e bonds or full global electron delocalization ( three-dimensional aromaticity) must be invoked to describe the bonding conditions. Electron-poor naked clusters are, as would be expected, formed by the early post-transition elements in groups 13 and 14. In addition, the cationic clusters of group 15 are electron-poor. Examples, derived from the compilation in Table 5, include the iso-electronic Pbg and Big " clusters and the 22-electron M5 species of thallium, tin, lead, antimony, and bismuth. 

The reaction chemistry of the naked post-transition element clusters is still a rather white spot on the map of cluster chemistry. To a large extent this is a consequence of experimental difficulties and the often extreme redox conditions. Thus, the reactions of anionic clusters are often trivial results of their strong reducing capac-ity[68] cationic counterparts are often exceedingly strong oxidizers. An... 

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