Homoatomic cluster compounds

Table 6 Homoatomic clusters of group 15 elements in solid state compounds...

A review of discrete and condensed homoatomic clusters of germanium, tin, and lead is presented in Section 5.7. These main group cluster compounds are usually called Zintl ions or Zintl phases in recognition of extensive studies of Zintl and coworkers in the 1930s." The science of nanoparticles with conducting and... 

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. 

A larger number of compounds with linked and discrete homoatomic clusters are known for the group-14 elements. The structures of the different modifieations of the elements themselves are indicative of a tendency to form clusters. For carbon, several crystalline modifications are known the polymeric diamond structure, the two-dimensional structure of graphite, and the isolated units of the fullerenes. There are two modifications of Si-Sn - the a or the diamond structure in which all atoms are four-connected and the metallic fi-iin structure with coordination number six (four shorter and two longer distances). A eoordination number greater than... 

Homoatomic clusters of group 14 are in many ways a remarkable class of compounds. The dismantling of the elemental structure and the formation of isolated units can lead to compounds with remarkable physical and chemical properties. 

Table 1. Effect of compound-type on the the charge on homoatomic clusters of groups 13 to 15 elements with n vertices.

Tetrahedral clusters in the solid state have long been known as units in intermetallic compounds (Table 3). The recent discovery that nine-atom clusters are present in binary solids is the first example of homoatomic clusters of the elements Ge, Sn, and Pb that ean exist in solution and in the solid state with very similar structures. In A4Ge9 (A = the occurence of discrete 9" clusters with C4V sym-... 

In the past decades, homoatomic polyanions in solid state compounds of group 14 elements have been a powerful starting material toward different kinds of chemical transformations. Recent developments, especially for nine-atom clusters, have been discussed elsewhere [5,125,126] and are not within the scope of this review. Here, we concentrate on neat solvates , which means that in addition to cation and anion... 

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. 

Clusters of group-14 elements usually form soluble ions in salts with large cations [e.g. [K-crypt]+ complexes).Of the many possible clusters only the five-, nine-, and ten-atom units have been structurally characterized (Table 2). Homoatomic tetrahedral units have been found only in Zintl phases. Discrete tetrahedral E4 clusters are isosteric with P4 these are listed in Table 3. A nine-atom representative has recently been characterized in the A4Ge9 (A = Rb, Cs) Zintl phases. In the A12E17 (E = Ge, Sn) phases four- and nine-atom clusters are present in the same compound (see below). 

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