Valence compounds tetrahedral structures

Parth6, E. (1980) Valence and Tetrahedral Structure Compounds, ed. Parth6, E. (Summer School on Inorganic Crystal Chemistry, Geneva). 

Tetrahedral structures . In a more limited field than that of the previously considered general octet rule, it may be useful to mention the tetrahedral structures which form a subset of the general valence compounds. According to Parthe (1963, 1964, 1991), if each atom in a structure is surrounded by four nearest neighbours at the corner of a tetrahedron, the structure is called normal tetrahedral structure . 

We have begun a study of the stabilization of seml-conductlng and metallic oxides with other metal cations that will font covalent metal-O-Cu bonds and a two level electronic band structure. These materials will be essentially semiconductors where the conductivity arises from doping to produce mixed-valence compounds. Ue chose to begin our study with cations that adopt tetrahedral coordination and focus on how to create structures that Incorporate distorted octahedral, square pyramidal and square planar coordination of copper compatible with still other electropositive (Ionic) cations. The mixed valency Introduced by doping can then be accommodated on the copper metal and adjacent oxygen atom sites by an accompanying bond polarization around the cation with tetrahedral coordination. 

VALENCE ELECTRON RULES FOR COMPOUNDS WITH TETRAHEDRAL STRUCTURES AND ANIONIC TETRAHEDRON COMPLEXES... 

ABSTRACT. For compounds with tetrahedral structure or anionic tetrahedron complex two valence electron concentration rules can be formulated which correlate the number of available valence electrons with particular features of the crystal structure. These two rules are known as the tetrahedral structure equation where the total valence electron concentration, VEC, is used as parameter and the generalized 8 - N rule where the parameter of interest is the partial valence electron concentration in respect to the anion, VEC. From the tetrahedral structure equation one can calculate the average number of non-bonding orbitals per atom and, in the case of non-cyclic molecular tetrahedral structures, the number of atoms In the molecule. An application of the generalized 8 - N rule allows the derivation of the average number of anion -anion bonds per anion or the number of valence electrons which remain with the cation to be used for cation - cation bonds and/or lone electron pairs. These rules have been used not only to predict probable structural features of unknown compounds but also to point out possible errors in composition or structure of known compounds. 

Tetrahedral structure compounds form a subset of the general valence compounds where each atom has at most four neighbours which are positioned at the corners of a surrounding tetrahedron. The tetrahedral structures are found with iono-covalent compounds which can be considered either as covalent or as ionic. For each hypothetical bonding state a particular valence electron concentration rule can be formulated which allows certain structural features to be predicted. 

Examples for normal valence compounds (VEC/ = 8) with normal and detect tetrahedral structures. There are known many nonnal valence compounds with normal eind defect tetrahedral structure. A subgroup of this family is formed by the adamantane... 

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. 

Examples for polycatlonic valence compounds (VEC/ > 8) with defect tetrahedral structures. Except for the high pressure form of B2O with diamond-like stmcture (Endo, Sato Shimada, 1987), no polycatlonic valence compounds with normal tetrahedral structure are known. As examples for compounds with defect tetrahedral structure we discuss here GaSe and "InsS/. The latter compound served as a test case for the validity of these valence electron rules. 

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... 

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... 

A problem might exist to recognize which atoms in a ternary compound participate on an anionic tetrahedron complex and which do not. Experimental evidence has shown that the alkali elements Na, K, Rb and Cs, the alkaline earth elements Ca, Sr and Ba and the rare earth elements never have a tetrahedral coordination. They function as cations C only which transfer their valence electrons to the remaining atoms forming the anionic tetrahedron complex. There are, however, other elements such as Al which in some compounds participate on the complex but in others not. To simplify matters will shall in the following examples consider only compounds where the recognition of the elements which function as cations C causes no problems. If for a compound VEC < 4 it is evident that all atoms can not participate on a tetrahedral stoicture. But also if VEC 4, where in principle a tetrahedral structure involving all elements would be possible, the above cited elements will never participate on the anionic tetrahedron complex. 

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. 

Valence compounds, like elements, satisfy the Hume-Rothery rule, as can be seen by calculating the average coordination number for these compounds. At the same time, they retain the tetrahedral distribution of the atoms, i.e., the tetrahedral bonds. The structure of defect diamond-like phases has been studied in some detail but corresponding data for excess phases are not available. The problem of the change in the structure of excess phases with varying valence electron concentration is more complicated since it is neither immediately apparent nor known how a sphalerite-type structure is transformed into a defect antifluorite-type structure. 

It has been shown that not only the elemental semiconductors of group IV and binary compounds which are their analogs crystallize in tetrahedral structures in fact, a whole series of ternary compounds of various types with an average of four valence electrons per atom have the same property. Thus, the formation of covalent bonds based on the sp hybrids is not peculiar to elemental semiconductors and binary semiconducting compounds, but is also found in ternary semiconducting compounds. 

Complexes of Copper(i).— In most of the copper(i) complexes whose structures have been described during the period of this survey the co-ordination at the metal atom is close to tetrahedral co-ordination numbers of two or three are much less common, while values greater than four are found exclusively in cluster compounds. The structures of mono- and bi-nuclear copper(i)-phosphine complexes have aroused a great deal of interest, and an attempt has been made to explain the observed variations in the lengths of Cu -P bonds. Attention has also been directed at polymeric copper compounds containing bridging halide or pseudohalide ions a number of such systems are described in the section on mixed-valence copper(i)-copper(n) compounds. 

It also forms compounds known as carbonyls with many metals. The best known is nickel tetracarbonyl, Ni(CO)4, a volatile liquid, clearly covalent. Here, donation of two electrons by each carbon atom brings the nickel valency shell up to that of krypton (28 -E 4 x 2) the structure may be written Ni( <- 0=0)4. (The actual structure is more accurately represented as a resonance hybrid of Ni( <- 0=0)4 and Ni(=C=0)4 with the valency shell of nickel further expanded.) Nickel tetracarbonyl has a tetrahedral configuration,... 

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