Ligand-free metal clusters, studying

Negatively charged clusters of some of the p-metals and semimetals are also stable in solutions and can be crystallized from them as ionic molecular compounds. Known, at present, are a number of such ligand-free anionic clusters, called Zintl ions. Some examples are the clusters with general formulas PUy", Eg , Eg , aud Ej where Pn = P, As, Sb and E = Si, Ge, Sn, Pb. This chapter will limit its attention to the synthesis, structure, and functionalization of molecular anionic clusters of tin. Although both nine- and five-atom clusters are known (Figure 2.6.1), the focus will be predominantly on the nine-atom deltahedral clusters Sug and Sn which are the most readily prepared and most extensively studied species. 

NMR studies of solutions of inter-metallic phases led to recognition of the potential use of polyatomic Zintl ions as reagents for the generation of new cluster compounds (Eichhorn et al., 1988). Examples of transition-metal derivatives have been discussed in Chapter 5 and the structures of the ions themselves were used as examples of ligand-free main-group clusters in Chapter 2 (Corbett, 1985 Fassler, 2001). The following examples illustrate recent developments in the chemistry of these clusters derived from solid-state syntheses. 

The study of naked transition metal clusters forms the basis and reference for understanding the properties of the corresponding ligated and supported species. Most of the experimental evidence so far available deals with ligand-stabilized or supported clusters which are the species commonly encountered in colloidal solutions, catalytic materials, and cluster based nanostructured materials. Thus, systematic theoretical work on free unperturbed transition metal clusters is especially needed to provide fundamental information, since these clusters are not so easily accessible experimentally. 

Another set of fundamental properties of metal clusters involves their response to static external electric and magnetic fields. Transition metal clusters embedded in matrices have been extensively studied with these techniques. [143] Unfortunately, the size distribution of the particles is broad in these experiments and interactions with the matrix can introduce changes in the properties of the metal aggregates. This is also true for metal clusters stabilized by ligands, as will be discussed in Section 2.4.5.3. The study of clusters in molecular beams overcomes these difficulties. This powerful approach, when combined with mass spectrome-tric detection, allows the investigation of mass selected free clusters. The main disadvantage is that since the particle densities are quite low, most of the standard spectroscopic techniques cannot be used. 

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