On inferences of bond character from bond length; the important role of nonbonded interactions

Although experimental molecular structure is one of the oldest and most frequently applied diagnostic tools in studies of the nature of chemical bonds, it is concluded that its application to all but the grossest details of bonds such as carbon-carbon bonds remains entirely speculative. The concept of bond length is discussed to emphasize the need for careful consideration of ambiguities associated with the natural indeterminacy of atomic positions. A theoretical basis is given to the empirical Schomaker-Stevenson rule relating bond length to electronegativity. The divergent views of Pauling and Walsh on hybridization, ionic character, and bond strength are reconciled to some extent with the aid of a simple model. It is shown that structural effects commonly attributed to conjugation, hyperconjugation, hybridization, and partial ionic character can be rationalized to a remarkable extent in terms of nonbonded interactions. It is suggested that these factors are not fundamentally as distinct from each other as they are often assumed to be, and that much more serious attention should be given to the role of nonbonded interactions.