More Complicated Covalent Compounds

As we progress from simple binary covalent compounds such as those discussed in Section 6.4 to other binary covalent compounds and to ternary compounds, we soon find the slot-filling procedure to be unworkable. In these examples, the atoms of interest (such as sulfur and chlorine) must have more 

To solve this problem, a set of steps is given in Table 6.1 to determine the correct Lewis electron-dot structure for these types of compounds. We now present some examples of the use of these steps. 

Draw the Lewis electron dot structure of carbon tetrachloride (CCI4). 

Step 1 A Carbon atom has four electrons in the outermost level, and each chlorine atom has seven electrons. The total count is 4 + 28 = 32. 

Step 4 The total count of electrons in step 3 is 4 x 8 = 32. This count matches the count in step 1.Therefore the drawing in 6.13b is the correct one. 

---

Intramolecular coordination cun occur when in the basic structure RXMgLn (connection between R and/or X and L, where I. may be a mono- or polydentate ligand. Several structural motifs are encountered in diis group, starting from simple monomers of type 3 via dimeric structures of type 4 to more complicated arrangements. Compounds with a covalent connection between R and L are presented in Scheme 9.1.4, those with a connection between X and L in Scheme 9.14. 

Boddington and Iqbal [727] have interpreted kinetic data for the slow thermal and photochemical decompositions of Hg, Ag, Na and T1 fulminates with due regard for the physical data available. The reactions are complex some rate studies were complicated by self-heating and the kinetic behaviour of the Na and T1 salts is not described in detail. It was concluded that electron transfer was involved in the decomposition of the ionic solids (i.e. Na+ and Tl+ salts), whereas the rate-controlling process during breakdown of the more covalent compounds (Hg and Ag salts) was probably bond rupture. 

It can be seen from Table 1 that the compounds LiF and NaF have almost pure ionic character. Magnesium oxide MgO, however, has a more complicated bonding character with considerable contribution of the covalent component. 

In order to fully appreciate and understand molecular structure, you will need to be able to construct representations of various molecules. One of the easiest ways to do this is using Lewis structures. The procedure is a bit more complicated than for ionic compounds because of the increasing complexity of covalent compounds. The basic procedure for constructing Lewis diagrams of molecules consists of 4 steps ... 

Since the Braggs first determination, thousands of structures, most of them far more complicated than that of sodium chloride, have been determined by x-ray diffraction. For covalently bonded low molecular weight species (such as benzene, iodine, or stannic chloride), it is often of interest to see just how the discrete molecules are packed together in the crystalline state, but the crystal structures affect the chemistry of such substances only to a minor degree. However, for most predominantly ionic compounds, for metals, and for a large number of substances in which atoms are covalently bound into chains, sheets, or three dimensional networks, their chemistry is very largely determined by the structure of the solid. 

As previously discussed, covalent compounds contain carbon chains, or infrastructures. These carbon chains are numbered so chemists are able to name them. Because the rules that govern the system of numbering can be tricky for beginners to learn, we will not go into to much depth. In the following illustration, butane is shown with correct numbering. Thereafter, another more complicated structure is shown with correct numbering, followed by an even more complicated structure. In each of these examples, the numbering demonstrates how compounds can be numbered and labeled for proper identification. 

Whereas ionization and dissociation are clearly defined processes on the left side of Scheme 35, the situation is more complicated for carbanionic systems (Scheme 35, right). Organic alkali metal compounds, for example, which often exist as aggregates, are often described as covalent species with a certain percentage of ionic character [140-142]. If the formal carbanion is a resonance-stabilized species (e.g, diphenylmethyl lithium or sodium), the species with the closest interaction between the organic fragment and the metal is usually called a contact ion pair. In... 

CoSii, and VaSi, and some have been known for more than 100 years (59). Other more complex types containing three or more components, such as MnsSiaC, are also known. They all form three-dimensional giant lattices, often of an unusual and complicated kind, and can best be considered as intermediate in nature between alloy systems and macromolecular covalent compounds. The study of silicides has considerably intensified in the past decade, since it has been found that some show promise as electronic materials, while at the same time they are strong and highly resistant to chemical attack. A number of books and general reviews may be noted (18, 257, 318, 340, 360, 423). 

For covalent solids, by far the major contribution to the dielectric constant results from electronic polarization. In ionic solids, the situation is more complicated, as discussed below. It should also be pointed out that the electronic polarizability of a compound can, to a very good approximation, be taken as the sum of the polarizabilities of the atoms or ions making up that compound. 

The valence concept described here is meaningful only for compounds made of two elements or groups. In Chapter 8, we shall define the covalency and the oxidation number of an atom—refinements of valence capable of describing and classifying more complicated cases. 

Because most chemical systems are molecules, it is important to understand how quantum mechanics is applied to molecules. When we use the word molecule, we are usually speaking of some chemically bonded system that exists as discrete collections of atoms bonded to each other in some specific way. This contrasts with ionic compounds, which are atoms (or groups of covalently bonded atoms, the so-called polyatomic ions) held together by their opposing charge that is, they are composed of cations and anions. As one might expect from the previous discussions about wavefimctions in multi-electron atoms, wavefimctions of molecules get even more complicated. In reality there are some useful simplifications, which we will get to in the next chapter, but a general consideration of a simple diatomic molecule is useful at this point... 

More complicated situation occurs in compounds, containing, like in the prototypical MgAgAs, both late main group and late transition elements, each of them showing tendency to occupy the Z site. Then the covalent and ionic factors are opposite and in MgAgAs the latter obviously dominates since As occupies the Z position, disregarding the somewhat larger sum of delocalization indices for the shortest interactions with Ag at the Z site. 

---