Polyanionic Compounds, Zintl Phases

Polyanionic and Poly cationic Compounds. Zintl Phases... 

POLYANIONIC AND POLY CATIONIC COMPOUNDS. ZINTL PHASES... 

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. 

A summary of Zintl phases found in alkali-tin alloys has been reported by Fassler and Hoffmann (2003) together with the description of a new Na-Sn compound. It is Na7Sn12 which was synthesized by quenching stoichiometric amounts of the elements (700°C) in a sealed Nb ampoule and further thermal treatment at 270°C for 40 days. It was described as a Zintl phase suggesting that, on the basis of its structure, its formula can be rewritten as (Na )7[(2b)Sn2 ]1[(3b)Sn ]s [(4b)Sn°]6 (where 2b, 3b, etc. denote two-three-fold bonded atoms) resulting in a polyanion i(Sn]2). 

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 strong influence of Zintl on the description of chemical bonding in compounds at the border of salts and intermetallics led to the nomenclature Zintl ion f ° for soluble polyanions (as part of a polyanionic salt ) and Zintl phase f for compounds with anionic substructures obeying the (8 — N) rule. Further development and the perception that the salt-metal transition is not abrupt led to a continuous extension of these terms. Soluble polycations, discrete units, and low-dimensional substructures in Zintl phases are called Zintl ions. These ions commonly consist of metal- or semi metal-atoms, or of atoms of semiconducting elements. Clearly, they must be distinguished from classical ions as elucidated by a comparison of SnTe4 and the iso(valence)electronic ion S04 . 

Knowledge of the topochemistry of the Zintl phases of Ca with Si and Ge known is still rather limited. New approaches could include the use of different sources of chlorine or of protic solvents in the topochemical transformation of the layered Zintl phases. A first attempt in this direction has involved the use of aqueous solutions of C0CI2 for the removal of A different approach to broaden the topochemistry is the transformation of Zintl phases with different structures of the group IV polyanions, leading to Si or Ge compounds with other backbone structures.An example of this is the reaction of CaSi with HCl, which leads to the formation of linear polysilane chain polymers. However, the Zintl phases of Si and Ge have an even richer structural chemistry, containing polyanions in the form of tetrahedra or planar Si rings, which could lead to Si compounds of great interest from both a chemical and a physical point of view. ° ... 

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