Bare Ionic Post-transition Metal Clusters The Zintl Phases

Bare Ionic Post-transition Metal Clusters The Zintl Phases 

The polyhedral boranes and carboranes discussed above may be regarded as boron clusters in which the single external orbital of each vertex atom helps to bind an external hydrogen or other monovalent atom or group. Post-transition main group elements are known to form clusters without external ligands bound to the vertex atoms. Such species are called bare metal clusters for convenience. Anionic bare metal clusters were first observed by Zintl and co-workers in the 1930s [2-5], The first evidence for anionic clusters of post-transition metals such as tin, lead, antimony, and bismuth was obtained by potentiometric titrations with alkali metals in liquid ammonia. Consequently, such anionic post-transition metal clusters are often called Zintl phases. 

As mentioned in the Introduction, no structural information on these species was available for more than 40 years after the discovery of the first Zintl metal cluster anions, since no pure crystalline phases could be isolated and characterized structurally. Nevertheless, early efforts to rationalize the observed formulas and chemical bonding of these intermetallics and related molecules utilized the Zintl-Klemm concept [75, 76] and the Mooser-Pearson [77] extended (8 — N) rule. In this rule N refers to the number of valence electrons of the more electronegative metal (and thus anionic metal) in the intermetallic phases. 

Such equations vaguely resemble Lipscomb s equations of balance for the polyhedral boranes [Eqs. (1) and (2)]. 

The limitations of the simple Zintl-Klemm concept can be illustrated by differences in the two [MT1] intermetallics (M = Na [79] and Cs [80]). Complete electron transfer from M to T1 leads to [ M TI, where the Tl anion with four valence electrons is isoelectronic with a neutral group 14 atom and four bonds and needed to attain the octet configuration. Hence, the Tl- anion should form structures similar to allotropes of carbon or heavier group 14 elements. Indeed, [NaTl] has a stuffed diamond structure [79] with internal Na and an anionic (Tl-) lattice similar to diamond. However, the Tl- anions in [CsTl] form tetragonally compressed octahedra [80] unlike any structures of the allotropes of carbon or its heavier congeners. 

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