Homonuclear cluster ions

Bare group 13 metal vertices (e.g., Ga, In, Tl) provide, as noted above, only one skeletal electron each to polyhedral cluster structures. Thus it is not surprising that the bare metal cluster ions Enz (E = group 13 element) found in homonuclear alkali-metal/group 13 intermetallic phases [86-89] (mainly for In and Tl) have charges less negative than the — (n + 2) (i.e., z [Pg.21]

Figure 14.13 showed [Oeio] as a sole example of a 10-atom cluster. [Oeio] was reported in 1991, but the structure suffered from a crystallographic disorder (see Box 15.5). Homonuclear Zintl ions with more than nine cage atoms can be stabilized by the inclusion of an interstitial atom. Such endohedral Zintl ions (written as [M E ] where M is the interstitial atom) include ... 

The term Zintl phase is applied to solids formed between either an alkali- or alkaline-earth metal and a main group p-block element from group 14, 15, or 16 in the periodic table. These phases are characterized by a network of homonuclear or heteronuclear polyatomic clusters (the Zintl ions), which carry a net negative charge, and that are neutralized by cations. Broader definitions of the Zintl phase are sometimes used. Group 13 elements have been included with the Zintl anions and an electropositive rare-earth element or transition element with a filled d shell (e.g. Cu) or empty d shell (e.g. Ti) has replaced the alkali- or alkaline-earth element in some reports. Although the bonding between the Zintl ions and the cations in the Zintl phases is markedly polar, by our earlier definition those compounds formed between the alkali- or alkaline-earth metals with the heavier anions (i.e. Sn, Pb, Bi) can be considered intermetallic phases. 

There is a close relationship between the solution-based syntheses of the naked clusters, as described above, and pure solid state chemistry. This relationship stems from the fact that homonuclear bonding and discrete clusters in many cases also exist within alloys of the type used in the synthesis of Zintl ions in solution. Indeed, such alloys are often to be regarded as well-defined intermetallic compounds. ... 

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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