Polycationic 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). 

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 ... 

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. 

If VECa > 3 Polycationic valence compound with CC > 0, that is... 

Other examples (free of problems) for polycationic valence compounds are S 2Te3 and AlyTeio (Nesper Curda, 1987). 

Examples for non-cyclic molecular tetrahedral structures. Molecular tetrahedral structures are found with normal and polycationic valence compounds. Of the examples given in Figure 4 the compound Snl4 is a normal valence compound, all others are polycationic valence compounds. The values of Nnbo> I a/m calculated... 

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

If VECa > 8 Polycationic valence compound with C C > 0 and AA = 0. A tetrahedron variant (III) and/or (IV) occurs and, depending on the composition, also variant (I). The parameter of interest is the average number of C - C bonds per tetrahedron and/or the number of electrons on the central atom per tetrahedron used for lone electron pairs. Since each tetrahedron of the anionic tetrahedron complex has only one central atom, this value is identical to C C, that is... 

This equation is appiicable in the case that each anion is either unshared or shared with oniy one other tetrahedron. The x parameter refers to the electrons which remain with the central atom In the case of polycationic valence compounds. It expresses the ratio of the number of vaience eiectrons used for C - C bonds between the centrai atoms to the totai number of eiectrons which rest with the central atom to be used for C - C bonds (tetrahedron variant (III)) and lone electron pairs (tetrahedron variant (iV)). When X = 1 the eiectrons are used oniy for C - C bonds, and when x = 0 oniy for ione pairs. We shaii find out beiow that for the most common cases one does not need to consider the x vaiue. 

The C AC values calculated with (23) for the polycationic valence compounds with C C = 1 and 2 given in Figures 9 and 10 are iisted there in the second last row of the text blocks. These values correspond to the number of anion half circles in each drawing. 

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. 

From a geometrical point of view, a crystal structure may be considered as composed of coordination polyhedra. In inorganic compounds one usually discusses the cation coordination, i.e. the polyhedra formed around the cations by the bonded neighbors which are anions in normal valence compounds, but also include cations in polycationic compounds. The number of vertices of these polyhedra, the coordination number, depends largely on the cation-to-anion radius ratio. It is quite obvious that a minimum coordination number is necessary for the formation of a layer structure. With a coordination number 2, only chains or finite molecules are possible. A high coordination number, on the other hand, will lead to a three-dimensional or framework structure. 

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