All the ways to have a bond

The concept of a chemical bond, so essential to chemistry, and with a venerable history, has life, generating controversy and incredible interest. Even if (or maybe because) we can’t reduce it to physics. I will discuss some of the common experimental criteria for judging the presence and strength of a bond: length, energy, force constants, magnetism, energy splittings and other spectroscopic criteria. On the theoretical side, I will look at bond orders, population analyses, bond critical points, and electron localization functions. And will give a personal opinion on the utility of the various measures. My advice at the end is likely to be: Any rigorous definition of a chemical bond is bound to be impoverishing, leaving one with the comfortable feeling “yes (no), I have (do not have) a bond,” but little else. Push this fuzzy concept to its limits, accept that (at the limits) a bond will be a bond by some criteria, maybe not others, respect chemical tradition, and have fun with the chemical richness of something that perhaps cannot be defined clearly.