Homoatomic clusters

We have attributed the origin of the HF symmetry breaking of homopolar simple and multiple bonds or in symmetric homoatomic clusters to umealistic constraints on the coefficients of the different VB components. Going to a valence CASSCF (ie an optimal valence Cl function) should restore the symmetry. 

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

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. 

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... 

Figure 1. Topological transition from a diamond network (left) to polyhedral homoatomic clusters (right). Some bonds between the tetravalent atoms break by adding electrons (middle). The formation of progressively more and more lone pairs eventually results in discrete cluster anions (right).

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-... 

This chapter reviews the synthesis, structures and physical properties of homoatomic Zintl ions and recent developments in the formation of mixed clusters consisting of the tetrel elements Ge, Sn, and Pb and various d-block elements. A brief summary is provided of the chemistry of homoatomic clusters (this has been reviewed earlier [1,2]), with the main focus being oriented towards the latest results in the field of intermetalloid clusters [3, 4]... 

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