Bonding within Zintl ions

A.2.2 Nonpolar Binary Intermetallic Phases. Zintl phases are characterized by the presence of markedly heteropolar bonding between the Zintl ions (electronegative polyatomic clusters) and the more electropositive metal atoms. By contrast, the bonding between heteronuclear atoms within other intermetallic compounds is primarily covalent or metallic. A number of different structure types exist for any given... 

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. ... 

As more Zintl ions are isolated, challenges to the rationalization of the bonding within Wade s rules are eiKountered. 

It is generally presumed that removal of Na atoms into the vapor phase accompanied by reduction of the Na species initially contained within the ionic precursor causes formation of new Si-Si bonds that become templated around the remaining Na atoms or ions to form the clathrate cage structures. Because of the very large structural mismatch between the starting Zintl phase and the clathrate... 

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