Tetrahedral anion partial structures

Structures with Tetrahedral Anion Partial Structures... 

In analogy to (3) and (11) one can now write the tetrahedral structure equation as applied to the charged tetrahedral anion partial structure (Parth , 1989) as... 

EXAMPLES OF STRUCTURES WITH TETRAHEDRAL ANION PARTIAL STRUCTURES... 

For both compounds a tetrahedral structure including all atoms is not possible because VEC < 4. However, both can have tetrahedral anion partial structures, which in the particular case, since for both VEC > 6, should consist of non-cyclic molecules. For Sr3AS4 there exist two possibilities, that is an unbranched chain of 4 As atoms, as shown in the drawing and actually found in Sr3AS4, or a chain of 3 anions which is branched, the anion in the middle of the chain being linked to a forth anion. Note, that in both the crystal chemical formulae and the drawings the non-bonding orbitals have not been indicated, however, it should not be difficult for the reader to verify that the listed values of N i ibo are correct. 

Problem 2 The following binary compounds are characterized by tetrahedral anion partial structures. Find out what the probable features of these charged anion partial structures might be and denote them by means of crystal chemical formulae. 

There are known many iono-covalent compounds where only the anions adopt a tetrahedral structure. In a formalistic approach one may assume that the cations transfer all the valence electrons to the anions. The tetrahedral structure equation can then be applied to the charged anion partial structure. We shall use primed parameters, such as VEC, N nbo indicate that we refer to a charged anion partial... 

The drawings are complemented with text blocks detailing the numerical values of the different parameters which can be calculated from the valence electron equations discussed above. On top is given the total valence electron concentration, VEC. If VEC < 4 a tetrahedral structure involving all atoms is impossible. The parameters listed below VEC are derived from the valence electron concentration of the charged anion partial structure, VEC, and the next one from the partial valence electron concentration in respect to the anion, VECa- parameter C AC, to be discussed in the next paragraph, refers to the sharing of the anions and can be calculated from the composition of the compound. Finally, on the last row one finds a classification code for the base tetrahedron, also to be discussed later on. 

Silicate ceramics are well suited for structural applications because of their strength, which originates in the partially ionic, strong silicon-oxygen bonds in the tetrahedral orthosilicate anion. This structural unit appears in naturally occurring minerals and clays, which are fashioned into ceramic pieces through sintering and densification processes. 

Phosphorus can form five covalent bonds. The conventional representation of Pj (Pig. 10a), with three P—0 bonds and one P=0 bond, is not an accurate picture. In Pi four equivalent phosphorus-oxygen bonds share some double-bond character, and the anion has a tetrahedral structure (Fig. 10b). As oxygen is more electronegative than phosphorus, the sharing of electrons is unequal the central phosphorus bears a partial positive... 

The physical and chemical properties of the tetrahydroborates show more contrasts than the salts of nearly any other anion. The alkali metal salts are the most stable. In dry air, NaBH4 is stable at 300°C and in vacuo to 400°C with only partial decomposition. In contrast, several tetrahydroborates, including the titanium, thallium, gallium, copper, and silver salts, are unstable at or slightly above ambient temperatures. The chemical and physical properties of the tetrahydroborates are closely related to molecular structure. Sodium tetrahydroborate, which is typical of the alkali metal tetrahydroborates except for the lithium salt, has a face-centered cubic (fee) crystal lattice which is essentially ionic and contains the tetrahedral [BHJ- anion. The tetrahydroborates of the polyvalent metals are in many cases the most volatile derivatives of these metals known. Aluminum tris(tetrahydroborate)... 

Most inorganic chemistry texts list cut-off values for ther+/r ratios corresponding to the various geometries of interstitial sites (Table 2.3). However, it should also be pointed out that deviations in these predictions are found for many crystals due to covalent bonding character. An example for such a deviation is observed for zinc sulfide (ZnS). The ionic radius ratio for this structure is 0.52, which indicates that the cations should occupy octahedral interstitial sites. However, due to partial covalent bonding character, the anions are closer together than would occur from purely electrostatic attraction. This results in an effective radius ratio that is decreased, and a cation preference for tetrahedral sites rather than octahedral. 

Compounds of less electropositive metals also show structural effects that can be attributed to partial covalent bonding. CuCl and ZnO have structures with tetrahedral coordination although from radii the (octahedral) rocksalt structure would seem more likely. Partial covalent bonding involves some transfer of electrons back from the anions, into the empty 45 and 4p orbitals on Cu+ and Zn2+. Tetrahedral coordination is the normal bonding geometry when a complete set of 5 and p orbitals is used in this way (see Topics C2 and C6). Mercury forms an... 

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