Polyhedral clusters, concentric

Concentric polyhedral clusters. Among the various contributions made by Corbett and co-workers to the different aspects of cluster science, special attention may be given to the investigations (preparation, structural description, electronic classification, etc.) carried out on phases containing multi-hedral, concentric polyhedral, clusters. 

Several excellent articles review homonuclear main-group clusters. In this article we will concentrate on homoatomic polyhedral clusters of the elements Ge, Sn, and Pb with special emphasis on the relationship between soluble and linked clusters, and on certain physical properties. For these elements, several soluble anions and polymeric solid structures with different valence concentrations are known. In the first part attention is turned to structures and properties of isolated molecular clusters synthesized by solution methods. In the second part, linked poly-hedra and the increased formation of lone pairs with increasing valence-electron concentration in solid-state compounds is discussed. The influence of lone-pair interactions on electronic structures and on the superconductivity found in some of the compounds will also be discussed. Related aspects of compounds containing elements adjacent to Ge, Sn, and Pb in the periodic table are mentioned. 

The largest molecular species containing metal-metal bonds that was characterized by X-ray diffraction is the AI77 cluster (23) as shown in Fig. 9. A central Al atom is surrounded by three concentric polyhedral shells con-... 

A kind of foam in which the gas bubbles have an unusually thick stabilizing film and exist clustered together as opposed to either separated, nearly spherical bubbles or the more concentrated, system-filling polyhedral bubbles. A microgas emulsion will cream to form a separate phase from water. Also termed aphrons or colloidal gas aphrons . 

The topological transition (Fig. 1) from polymeric solids in compounds containing linked polyhedral homoatomic clusters to discrete molecular (soluble) clusters can be conveniently studied by using as examples compounds rich in elements of the main-groups 13 to 15. It is possible for main-group elements in the middle of the periodic table to form homoatomic molecules or ions with localized homonuclear 2c-2e bonds. At higher valence-electron concentrations, fewer bonding and more free-electron pairs are formed. As electron deficiency increases, however, the formation of delocalized bonds becomes necessary, a situation typical of elements on the left of the periodic table. 

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