The interaction between electrophiles (electron-accepting species) and nucleophiles (electrondonating species) is a fundamental concept in chemistry for describing the reaction of molecules. The concept introduced by Ingold in 1933 is based on earlier theories of valency and acid-base chemistry, and it provides an important language for explaining chemical reactivity and selectivity. For example, Mayr and co-workers have shown that the reactivity of nucleophiles and electrophiles can be quantified on scales that describe their relative reactivity. Specifically, the Mayr-Patz equation links the bimolecular rate constant of various organic reactions to experimentally derived parameters such as nucleophilicity (N ), electrophilicity (E), and a nucleophile-specific sensitivity factor (s N ):