Among several valence bond structures

In this book the discussion has been restricted to the structure of the normal states of molecules, with little reference to the great part of chemistry dealing with the mechanisms and rates of chemical reactions. It seems probable that the concept of resonance can be applied very effectively in this field. The activated complexes which represent intermediate stages in chemical reactions are, almost without exception, unstable molecules which resonate among several valence-bond structures. Thus, according to the theory of Lewis, Olson, and Polanyi, Walden inversion occurs in the hydrolysis of an alkyl halide by the following mechanism ... 

Tho author has repeatedly [6j pointed out that the valency conception of non-localized bonds (resonance among several valence-bond structures, many centre molecular orbitals) lias no reality and only arises from a neglect of electronic repulsions. It is noteworthy that on this basis the possibility of a contribution of structures IIIc and d (IVc %od d) to Ufa and b (IVa and b) and vice versa would not enter and, thus, this objection against the discussed interpretation of the hydrogen bond would be removed ... 

The Resonance of Molecules among Several Valence-Bond Structures... 

The discussion in Section 1-3 about the element of arbitrariness in the concept of resonance may be recalled at this point with reference to the nitrous oxide molecule and the other molecules that are described in this chapter as resonating among several valence-bond structures. It is not necessary that the structures A, B, and C be used as the basis of discussion of the nitrous oxide molecule. We might say instead that the molecule cannot be satisfactorily represented by any single valence-bond structure, and abandon the effort to correlate its structure and properties with those of other molecules. By using valence-bond structures as the basis for discussion, howrever, with the aid of the concept of resonance, we are able to account for the properties of the molecule in terms of those of other molecules in a straightforward and simple way. It is for this practical reason that we find it convenient to speak of the resonance of molecules among several electronic structures. 

Extensive approximate calculations of resonance energies foi molecules, especially the aromatic carbon compounds, have been made, and explanations of several previously puzzling phenomena have been developed.1 Empirical evidence has also been advanced to showr the existence of resonance among several valence-bond structures in many simple and complex molecules.2... 

At the present time there is no satisfactory theory of these particles. It seems likely, however, that some of them may be classed with the fundamental particles (see the following section), and that others may be described as complexes of two or more fundamental particles, possibly with resonance among several structures, roughly analogous to the resonance of molecules among several valence-bond structures. 

Heterocyclic compounds are cyclic compounds in which one or more atoms other than carbon (usually nitrogen, oxygen, or sulfur) are present in the ring. An example is pyridine, C5H5N, a colorless liquid with an unpleasant odor, which is among the products of distilling coal. The electronic structure of pyridine can be described as a hybrid of several valence-bond structures ... 

It is mentioned above that in potassium metal each atom, with one valence electron, can form one covalent bond, and that this bond is not between the atom and a single neighboring atom, but instead resonates among several positions. For four potassium atoms in a square, we might write two valence-bond structures ... 

If we regard each electronic formula as dynamic rather than static, there is little to be gained by emphasizing the concept of resonance hybrids. Furthermore, the detailed theory of resonance has come under heavy fire. It is claimed by A. Burawoy that .. . the interpretation of polyatomic structures does not require the hypothesis of resonance among several idealized valence-bond structures, a speculative application of the quantum-mechanical conception of resonance, which is in disagreement with numerous facts. In view of this controversy, we shall avoid detailed application of the theory of resonance. The theory of molecular orbitals, an alternative treatment, is too complex for the present discussion. 

Let — fe be the charge of the anion located at a corner shared among several polyhedra. We now postulate the following electrostatic valence principle II. In a stable coordination structure the electric charge of each anion tends to compensate the strength of the electrostatic valence bonds reaching to it from the cations at the centers of the polyhedra of which it forms a corner that is, for each anion... 

---