This is a tutorial on use of external-electric-fields (EEFs) as effectors of chemical change. The tutorial instructs readers how to conceptualize and design electric-field effects on bonds, structures, and reactions. Most effects can be comprehended as the field-induced stabilization of ionic structures. Thus, orienting the field along the “bond axis” will facilitate bond breaking. Similarly, orienting the field along the “reaction axis”, the direction in which “electron pairs transform” from reactants- to products-like, will catalyse the reaction. Flipping the field's orientation along the reaction-axis will cause inhibition. Orienting the field off-reaction-axis will control stereo-selectivity and remove forbidden-orbital mixing. Two-directional fields may control both reactivity and selectivity. Increasing the field strength for concerted reactions (e.g., Diels–Alder's) will cause mechanistic-switchover to stepwise mechanisms with ionic intermediates. Examples of bond breaking and control of reactivity/selectivity and mechanisms are presented and analysed from the “ionic perspective”. The tutorial projects the unity of EEF effects, “giving insight and numbers”.

The reactivities of mononuclear nonheme iron(IV)–oxo complexes bearing different axial ligands, [Fe ^IV (O)(TMC)(X)] ⁿ⁺ [where TMC is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane and X is NCCH ₃ (1-NCCH ₃ ), CF ₃ COO ⁻ (1-OOCCF ₃ ), or N ₃ ⁻ (1-N ₃ )], and [Fe ^IV (O)(TMCS)] ⁺ (1′-SR) (where TMCS is 1-mercaptoethyl-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane), have been investigated with respect to oxo-transfer to PPh ₃ and hydrogen atom abstraction from phenol O H and alkylaromatic C H bonds. These reactivities were significantly affected by the identity of the axial ligands, but the reactivity trends differed markedly. In the oxidation of PPh ₃ , the reactivity order of 1-NCCH ₃ > 1-OOCCF ₃ > 1-N ₃ > 1′-SR was observed, reflecting a decrease in the electrophilicity of iron(IV)–oxo unit upon replacement of CH ₃ CN with an anionic axial ligand. Surprisingly, the reactivity order was inverted in the oxidation of alkylaromatic C H and phenol O H bonds, i.e., 1′-SR > 1-N ₃ > 1-OOCCF ₃ > 1-NCCH ₃ . Furthermore, a good correlation was observed between the reactivities of iron(IV)–oxo species in H atom abstraction reactions and their reduction potentials, E _p,c , with the most reactive 1′-SR complex exhibiting the lowest potential. In other words, the more electron-donating the axial ligand is, the more reactive the iron(IV)–oxo species becomes in H atom abstraction. Quantum mechanical calculations show that a two-state reactivity model applies to this series of complexes, in which a triplet ground state and a nearby quintet excited-state both contribute to the reactivity of the complexes. The inverted reactivity order in H atom abstraction can be rationalized by a decreased triplet-quintet gap with the more electron-donating axial ligand, which increases the contribution of the much more reactive quintet state and enhances the overall reactivity.