Polyanionic clusters, Zintl phases

More often, polyhedral clusters with strong metal-metal (or metalloid-metalloid) bonding are the major structural motifs of classic Zintl phases. These are nominally salts composed of reduced p- (i.e., post-transition) elements that are usually inter-bonded into closed shell polyanions plus active metal cations, originally the alkali... 

K4Ge4, can be described as a polyanionic compound (as a Zintl phase also) containing the ion Ge44. This tetrahedral ion can be considered a naked (that is without any ligands bounded to the vertices) tetrahedral cluster formed by a main group element (that is Ee = 5 3 = 5X4 = 20). The electron count, on the basis of the Ge valence electrons and of the ion charge results in Ee = 4 X 4 + 4 = 20. 

Numerous investigations over the recent 10-15 years demonstrate a real flourishing in the field of Zintl phases and reveal that these semieonductor compounds can contain beautifiil homo- or heteroatomic polyanions [13, 17-22]. Zintl phases can mimic arrangements well known from the more classical insulator compounds but, in addition, they may occur with completely novel clusters and frameworks of the X components. 

Further examples of formally subvalent main group compounds that contain element-element bonds but not necessarily clusters are the Zintl phases. The bonding in these has been described as the octet rule for all atoms . The archetypal Zintl compound is NaTl, in which charges are assigned as Na+ and Tl, representing a formal transfer of electrons from the more to the less electropositive element. The Tl ion can be considered to be a group 14 pseudoelement, and in fact exists in NaTl as a three-dimensional polyanionic diamond framework (TN) stuffed with Na+ cations. The Zintl concept is extended more broadly to other binary and ternary solid-state compounds, whose structures show the formation of element-element bonds in one, two, or three dimensions. ... 

The starting point for this case as described in (a) was the work of Cui et al. [17,18] when a new class of Zintl ions, plumbaspherene, Pbj, and stannaspherene, Sn, were characterized by photoelectron spectroscopy as hosts for building up a new type of endohedral clusters. Gas-phase experiments and theoretical investigations predicted a remarkable stability for the icosahedral plumbaspherene Pbj2 and the stannaspherene Sn cluster anions of Ih point symmetry. These empty polyanions have not yet been synthesized in crystalline form. 

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