Normal valence compound rule

To an increasing weight of the chemical bond factor (ionic and/or covalent bonding) will correspond, as an extreme case, the formation of valence compounds. According to Parthe (1980), a compound CmAn can be called a normal valence compound if the number of valence electrons of cations (ec) and anions (eA) correspond to the relation 

If we consider only the s andp block elements without the noble gases, the number of valence electrons of the elements is included between 1 and 7. In this case, considering that no anions are formed from the elements of groups 1,2 and 3, the following formulae can be deduced for the normal valence compounds, formed in binary systems with large electronegativity difference between elements  

In these formulae each element is indicated by a number corresponding to its number of valence electrons for instance 17 indicates NaCl or KC1, KBr 3263 indicates A1203, etc. 

In the more general case where some electrons are also considered to be used for bonds between cations or between anions we have  

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). 

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FeS2, 3 2PO). The structure of CaC2 (Section 5.1.2) is similar to that of pyrite, but with elongation in one direction because of alignment of C ions. Intermetallic compounds with the CaC2 structure are listed in Table 9.3. Thus intermetallic compounds can follow the rules for "normal valence" compounds or those of the metallic state. The "normal valence" compounds have lower CN than found in cep or hep structures and are expected to be less "metallic" in terms of electrical conductance, etc. 

The chemistry of the compounds of the early transition metals in low oxidation states is full of examples of the occurrence of metal - metal bonds. These compounds show unusual compositions in terms of the traditional valence rules metal - rich compounds contain more metal atoms than one expects for a normal valence compound (example Nbelii instead of Nbis). The transition metal elements on the left In the periodic system, and of those mainly the 4d and 5d elements, are capable of using the excess valence electrons, not needed to complete the octets of the anions, to fbnn metal -metal bonds. 

Silicon (3), which resembles metals in its chemical behavior, generally has a valence of +4. In a few compounds it exhibits a +2 valence, and in silicides it exists as a negative ion and largely violates the normal valency rules. Silicon, carbon, germanium, tin, and lead comprise the Group 14 (IVA) elements. Silicon and carbon form the carbide, SiC (see Carbides). Silicon and germanium are isomorphous and thus mutually soluble in all proportions. Neither tin nor lead reacts with silicon. Molten silicon is immiscible in both molten tin and molten lead. 

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 . 

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. 

The short primary bonds that are responsible for the stereoactivity of the lone pairs on anions are normally described as covalent, but bmid valence theory makes no distinction between ionic and covalent bonds, since the bond valence is a variable that runs continuously across the whole spectrum of bond types. The ionic-covalent distinction does, however, reflect a marked difference between the structural chemistry of those compounds that obey the valence matching rule (9) in which the bonding is generally weak (less than about 0.8-1.0 vu), and those where the presence of anion lone pairs permits the formation of much stronger bonds by making the lone pairs stereoactive as described in Sect. 7.1. The bonds that obey the valence matching rule are those traditionally described as ionic, while bonds formed by atoms with stereoactive lone pairs are those traditionally described as covalent. Even though the bonds form a continuous series in which such a distinction is not necessary, the term covalent can be usefully applied to the primary (short) bonds formed by anions with stereoactive lone pairs. 

However, we do not need to abandon the bond valence model for those few inorganic compounds which contain homoionic bonds since there are a number of ways of adapting the model depending on the nature of the structure. If the two cations or two anions that form the bond are equivalent by symmetry, as the two Hg cations are, for example, in the tetragonal crystals of Hg2Cl2 (65441, Fig. 3.4), the normal rules still apply. In this compound the two Hg ... 

Compounds in which the valencies/oxidation states of the central element are greater than the normal ones, based upon the octet rule, are examples of hypervalency, and vary in steps of two because they are related to the unpairing and use of two extra electrons at each stage. 

The highly covalent nature of transition metal carbonyls and their derivatives leads to the 18-electron rule being closely followed. The mononuclear species Ni(CO)4, Fe(CO)5, Ru(CO)5, Os(CO)5, Cr(CO)6, Mo(CO)6 and W(CO)6 obey this well and, if the formalized rules of electron counting are applied, so do the metal—metal bonded and carbonyl bridged species. Such compounds are therefore coordinately saturated and the normal (but by no means unique) mode of substitution is dissociative (a 16-electron valence shell being less difficult to achieve than one with 20 electrons).94... 

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