Covalent bonds Bond energies Lewis

When carbon forms four covalent bonds with halogen atoms the second quantum level on the carbon is completely filled with electrons. Most of the reactions of the Group IV tetrahalides require initial donation by a Lewis base (p. 91) (e.g. water, ammonia) which attaches initially to the tetrahalide by donation of its electron pair. Hence, although the calculated free energy of a reaction may indicate that the reaction is energetically favourable, the reaction may still not proceed. Thus we find that the tetrahalides of carbon... 

In 1923. Lewis published a classic book (later reprinted by Dover Publications) titled Valence and the Structure of Atoms and Molecules. Here, in Lewis s characteristically lucid style, we find many of the basic principles of covalent bonding discussed in this chapter. Included are electron-dot structures, the octet rule, and the concept of electronegativity. Here too is the Lewis definition of acids and bases (Chapter 15). That same year, Lewis published with Merle Randall a text called Thermodynamics and the Free Energy of Chemical Substances. Today, a revised edition of that text is still used in graduate courses in chemistry. 

The structures used to show the bonding in covalent molecules are called Lewis structures. When bonding, atoms tend to achieve a noble gas configuration. By sharing electrons, individual atoms can complete the outer energy level. In a covalent bond, an octet of electrons is formed around each atom (except hydrogen.)... 

In the adsorption of water molecules on metal electrodes in aqueous solutions, unpaired electrons in the frontier orbital of oi en atoms in water molecules form covalent bonds with surface metal atoms. Then, the adsorbate water molecules act as a Lewis base (covalent-electron providers) and the adsorbent surface metal atoms act as a Lewis acid (covalent-electron receivers). Since the bond energy (0.4 to 0.7 eV) of water molecules with the surface metal atoms is close to the energy of hydrogen bond (0.2 to 0.4 eV) between water molecules, the adsorbed water molecule is combined not only with the metallic surface atoms but also with the acijacent water molecules to form a bi-molecular layer rather than a monomer layer as shown in Fig. 5-31. 

What he does not seem to realize is that a perfectly good explanation existed for chemical bonding prior to the advent of the quantum mechanical explanation, namely Lewis s theory whereby pairs of electrons form the bonds between the various atoms in a covalently bonded molecule. Although the quantum mechanical theory provides a more fundamental explanation in terms of exchange energy and so on is undeniable but it also retains the notion of pairs of electrons even if this notion is now augmented by the view that electrons have anti-parallel spins within such pairs. 

If the nucleophilic site (HOMO) involves a nonbonded pair of electrons (path a), a stable covalently bonded complex will form. If the HOMO is a a bond, direct reaction is unlikely unless the bond is high in energy and sterically exposed, as in a three-membered ring, but if the bond is to H, hydride abstraction may occur (path b, steps 1 and 2) or a hydride bridge may form (path 6, step 1). The last two possibilities are discussed further in Chapter 10. If the HOMO is a n bond, a n complex may result (path c, step 1), or, more commonly, donation of the n electrons results in the formation of a a bond at the end where the n electron density was higher, the other end becoming Lewis acidic in the process (path c, steps 1 and 2). The effects of substituents on olefin reactivity were discussed in Chapter 6. 

Before 1927 there was no satisfactory theory of the covalent bond. The chemist had postulated the existence of the valence bond between atoms and had built up a body of empirical information about it, but his inquiries into its structure had been futile. The step taken by Lewis of associating two electrons with a bond can hardly be called the development of a theory, since it left unanswered the fundamental questions as to the nature of the interactions involved and the source of the energy of the bond. Only in 1927 was the development of the theory of the covalent bond initiated by the work o Condon28 and of Hertler and London27 on the hydrogen molecule, described in the following paragraphs. 

Association was first thought of as a reversible reaction between like molecules that distinguished it from polymerization, which is not reversible. Association is characterized by reversibility or ease of disassocialion, low energy of formation (usually about 5 and not more than 10 kcal per mole), and the coordinate covalent bond which Lewis called the acid-basc bond. Association takes place between like and unlike species. The most common type of this phenomenon is hydrogen bonding. Association of like species is demonstrable by one or more of the several molecular weight methods. Association between unlike species is demonstrable by deviation of the system from Raoult s law. 

If the system contains three electrons, the two occupying 4 will be stabilized, and the other one, localized in XV2, destabilized. Here, the stability of the molecule depends upon the relative energies of 4, Tf, and the AOs thus, HHe dissociates spontaneously, but the three-electron bond in He2+ is moderately robust. Note that, in contradiction with Lewis theory, a covalent bond may be formed with one or three electrons. Electron-deficient bonds (where there are fewer than two electrons per bond) are particularly prevalent amongst boron compounds. 

Table 10.4 summarizes the oxidation potentials for a variety of ligands (L) in acetronitrile (MeCN).7 Their relative Lewis basicity (nucleophilicity) increases as their oxidation potential becomes less positive. However, the potential at which L is oxidized (and L is reduced) within an MLX complex is shifted by the M—L covalent-bond energy (—AGBF). 

Therefore, instead of transferring electrons, the two atoms each share one electron with each other. In other words, each atom contributes one electron to a covalent bond. A covalent bond consists of a pair of shared electrons. Thus, each chlorine atom achieves a filled outer electron energy level, satisfying the octet rule. Examine Figure 3.15 to see how to represent a covalent bond with a Lewis structure. 

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