Chemistry is about energy and its changes: A critique of bond-length/bond-strength correlations

The current usage of bond-length/bond-strength (BLBS) correlations, namely, that a shorter bond must be associated with larger dissociation energy and/or force constant, is appraised. The numerous exceptions to these rules are noted. The originators of these rules considered them as useful empirical correlations, but in the course of time these relationships have often been painted as laws. As shall be seen, each exception to these rules can be explained by some effects, like strain, steric effects, dispersion stabilization, hybridization defects, bond ionicity, orbital shrinkage, and so on. As such, when the number of special reasons that can be invoked for failures of the BLBS rules, is close to the number of the exceptions to these rules, one must conclude that such correlations cannot be considered as anything even close to physical laws. Indeed, it is often the exceptions to the rules that point to interesting bonding aspects and/or reorganization processes. We argue against disregarding bond dissociation energies or related energy quantities in this context. While the various reorganization processes involved in determining these energy quantities may complicate the BLBS correlations appreciably, compared to the properties that probe structures only close to equilibrium, their consideration cannot be avoided if we want to extract chemical sense from the notion of a bond strength.