Polyanionic valence compound

In this formula, which can only be applied if all bonds are two-electron bonds and additional electrons remain inactive in non-bonding orbitals (or, in other words, if the compound is semiconductor and has non-metallic properties), ecc is the average number of valence electrons per cation which remain with the cation either in nonbonding orbitals or (in polycationic valence compounds) in cation—cation bonds similarly cAA can be assumed to be the average number of anion—anion electron-pair bonds per anion (in polyanionic valence compounds). 

Calculation of VEC allows to classify a compound as a polyanionic, normal or polycationic valence compound. Polyanionic valence compounds are characterized by anion - anion bonds. In the normal valence compounds there are neither anion - anion nor cation - cation bonds and in the poiycationic valence compounds some valence electrons are used for cation - cation bonds and/or lone electron pairs on the cations. 

Examples for polyanionic valence compounds (VEC/ < 8) with normal and defect tetrahedral structures. We shall present red ZnP2 as an example for a normal and CU2P7 as an example for a defect tetrahedral structure. 

If VECa < 8 Polyanionic valence compound with /VA > 0 and C C = 0. The value of / A can be calculated using (7). Tetrahedra are linked by covalent A - A bonds (tetrahedron variant (II)) and, depending on the composition, also by sharing anions (tetrahedron variant (I)). The parameter of interest is the average number of A - A bonds per tetrahedron. Since there are n anions for every m central atoms, its value is given by... 

For the base tetrahedra in Figure 11 with tangling A - A bonds one calculates VECa < 8. These base tetrahedra can be used for the construction of the anionic tetrahedron complexes in polyanionic valence compounds. For all base tetrahedra where AA = C C = 0 one finds that VECa = 8- These are the base tetrahedra which are important for the anionic tetrahedron complexes in normal valence compounds. Finally, all base tetrahedra with C C>0 have VECa >8. These are the base tetrahedra which build up the anionic tetrahedron complexes in polycationic valence compounds. 

This concept was then extended to include compounds having anion—anion or cation—cation bonds, that is, the so-called polyanionic or polycationic valence compounds, assuming ... 

As above one can, depending on the VEC value, distinguish between polyanionic, normal and polycationic valence compounds. 

In addition to the simplest class of valence compounds, there are very large groups of nonvalence polycation and polyanion compounds. The relationship between the chemical composition and the semiconducting properties of these compounds is nmch more complex but some attempts to consider the crystallochemical groups of these materials from the point of view presented here and from other standpoints have produced interesting results [8,9]. 

Polyanionic compounds. Frequently, the M atoms lose all their valence electrons to the X atoms, i.e. no cation-cation bonds occur and no nonbonding electrons remain at the cations, fe(MM) = 0 and E = 0. Equation (13.5) then becomes ... 

Table 13.1 Examples of polyanionic compounds which have integral valence electron concentrations per anion atom...

Links between atoms serve to compensate for the lack of the electrons which are necessary to attain the electron configuration of the next noble gas in the periodic table. With a common electron pair between two atoms each of them gains one electron in its valence shell. As the two electrons link two centers , this is called a two-center two-electron bond or, for short, 2c2e bond. If, for an element, the number of available partner atoms of a different element is not sufficient to fill the valence shell, atoms of the same element combine with each other, as is the case for polyanionic compounds and for the numerous organic compounds. For the majority of polyanionic compounds a sufficient number of electrons is available to satisfy the demand for electrons with the aid of 2c2e bonds. Therefore, the generalized 8 —N rule is usually fulfilled for polyanionic compounds. 

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. 

Among the representatives of the ZrSe3 structure (93) only tetravalent cations are found, such as Ti, Zr, Hf, Th and U. In the case of the uranium compounds the quadrivalence of the cation is confirmed by magnetic measurements (94, 95). In all uranium trichalcogenides the behaviour of the susceptibility reveals two unpaired /-electrons. All ZrSe3-type compounds are non-metallic (33, 92, 96). Hence these phases must be polyanionic compounds, since two anion valence electrons per formula have to be saturated by anion-anion bonds. 

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 . 

We have already discussed the effect that the nature of the solvent and the presence of some cations, particularly quaternary ammonium cations, have on the reactivity of many systems. Silicates are one such system. It is also the case for polyoxometalates, and in particular polyoxovanadates. Furthermore, the formation of V(1V)/V(V) mixed-valence. polyanions also increases the number of structural varieties. Recent studies by A. MUller in Germany and W.G. Klemperer in the United States, have shown spectacular results in the chemistry of polyoxovanadates. In the following sections, the synthesis and structure of some of these compounds arc discussed. 

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