Naked post-transition metal clusters

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... 

These heavy late- and post-transition metals in low positive oxidation states form clusters held together only by direct M—M bonds, with a relatively small number of terminal ligands. This cluster chemistry is distinctly different from that of all other aggregates (including the naked metal anions), and can be regarded as an expansion of the chemistry of Hg22+ it is most developed for... 

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... 

Configurations and bonding in naked clusters formed by the post-transition metals follow a pattern similar to that established for boranes and carboranes discussed in Sect 2.3 i.e. that in c/oso-deltahedra with n vertices there are n -h 1 low-lying skeletal orbitals so the relationship between polyhedral shape and electron count corresponds to that described in Table 2.11. 

Naked polyatomic clusters of post-transition metals can also be obtained as cations. They correspond to intermediate species between the metals and the compounds in conventional oxidation states. The well known dimercury cation stable in acidic conditions, may be considered a prototype of this kind of species. The best known examples of this class of compounds are the proper cluster species Bi " and Big" (vide infra). 

However, metal-carbonyl fragments can be used also to stabilize enigmatically elusive post-transition cluster species. A nice example is the Pb9 ion, which was unambiguously detected in solution as early as the 1930s (Sec. 1.29.2.1), but eluded isolation in the solid state for almost 60 years. In 1990, the Pb9" eore was finally isolated as the stable c/o o-cluster [CrPb9(CO)3]" , iso-structural with the previously described [CrSn9(CO)3j tetraanion. Stabilization by metal carbonyls has also produced the octahedral Sn6 c/oso-cluster core found in [Sn6 Cr(CO)s 6j, the first ever synthesized and characterized six-membered, electron-deficient naked cluster,... 

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