Valence structures classical

As previously described in Chapter 6, a bond separation reaction breaks down any molecule comprising three or more heavy (nonhydrogen) atoms, and which can be represented in terms of a classical valence structure, into the simplest set of two-heavy-atom molecules containing the same component bonds. For example, the bond separation reaction for methylhydrazine breaks the molecule into methylamine and hydrazine, the simplest molecules incorporating CN and NN single bonds, respectively. 

The idea of valence structures goes back to classical chemistry. And indeed it was even then at the heart of chemistry, primitive in many ways though the theory was in rationalizing molecular structure. There are both brief surveys [1,2,3] encompassing this history, as well as whole books on the subject [4,5], Indeed before 1925 the main achievements seem to have been primarily concerned with the structural possibilities, with considerations for individual molecules being of a quite qualitative nature. 

This mechanism was suggested by Zimmerman, who, however, formulated the initial excited state (12) as a simple biradical (16). At this point it should be emphasized that the use of conventional valence structures for excited states is incorrect in principle and can prove very misleading in practice. Indeed, much of the current confusion in organic photochemistry can be attributed to this error. The arguments on which the localized bond model is based (see Section 1.12) rest on the assumption that we are dealing with a system in which all the orbitals are either doubly occupied or empty. Since this condition is not met in the case of excited states, they cannot be represented by classical valence structures. 

This source of energy was discovered in quantum theory, and the principle, as it affects this discussion, can be stated roughly as follows if the electrons in a classical valency structure can be delocalized whilst obeying the Pauli exclusion principle, then the increased indeterminacy in their positions results in the molecule having a lower energy in the ground state than would be expected from the classical structure. 

In these latter structures, the valencies of N", N, and hT are 2, 3 and 4, respectively. We may note that each of the structures (1), (2) and (3) seems to have one more bond than have the corresponding Lewis structures, and therefore we might also designate structures (1), (2) and (3) as increased-valence structures. Alternatively, we may say that the quinquevalent nitrogen atom has increased its valence relative to the maximum of four which is allowed in the Lewis theory. Sometimes, the valence-bond structures such as (1), (2) and (3) are designated as classical valence structures , and we shall refer to them as such here. 

Although the use of octet structures such as (5)-(8) is extremely widespread, it is by no means universal Sometimes, the classical valence structures are used to account for certain empirical information, and the quantum mechanical basis for them is not discussed, i.e. it is not suggested how the nitrogen atom forms five covalent bonds. However, there have been three major attempts to explain how a nitrogen atom (or other first-row atoms - in particular, a carbon atom) may acquire an apparent valence of five, and we shall describe them briefly here. 

Because increased-valence structures such as (13) make clearer the nature of the spatial distributions of the electrons than do the classical valence structures such as (1), it would seem to be preferable to use the former types of valence-bond structures. They also have the advantage that they do not conceal the (spin-paired) diradical character, which is sometimes important for discussions of chemical reactivity. For example, O3 reacts with univalent radicals such as hydrogen and chlorine atoms, and NO, to form Oj + HO, CIO or NOj. In Chapter 22, we shall find that the eleetronic reorganization that may oeeur as the reactions proceed is easily followed through by using increased-valenee structure (14)... 

Trivalent ( classical carbenium ions contain an sp -hybridized electron-deficient carbon atom, which tends to be planar in the absence of constraining skeletal rigidity or steric interference. The carbenium carbon contains six valence electrons thus it is highly electron deficient. The structure of trivalent carbocations can always be adequately described by using only two-electron two-center bonds (Lewis valence bond structures). CH3 is the parent for trivalent ions. 

A novel and far-reaching type of isomerism concerns the possibility of valence isomerism between nonclassical (electron-deficient) clusters and classical" organoboron structures. Thus, n-vertexed /do-boranes. have cluster structures... 

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. 

Resonance theory [15] contains essentially three assumptions beyond those of the valence bond method. Perhaps the most serious assumption is the contention that only unexcited canonical forms, non-polar valence bond structures or classical structures need be considered. Less serious, but no more than intuitive, is the proposition that the molecular geometry will take on that expected for the average of the classical structures. This is extended to the measurement of stability being greater the greater the number of classical structures. These concepts are still widely used in chemistry in very qualitative ways. 

Hume-Rothery phases (brass phases, electron compounds ) are certain alloys with the structures of the different types of brass (brass = Cu-Zn alloys). They are classical examples of the structure-determining influence of the valence electron concentration (VEC) in metals. VEC = (number of valence electrons)/(number of atoms). A survey is given in Table 15.1. 

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