Ligand Field Effects and the High Spin–High Reactivity Correlation in the H Abstraction by Non-Heme Iron(IV)–Oxo Complexes: A DFT Frontier Orbital Perspective

The electronic structure explanation of H abstraction from aliphatic CH bonds by the ferryl ion, Fe^IVO²⁺, has received a great deal of attention. We review the insights that have been gained, in particular into the effect of the spin state. However, we emphasize that the spin state is dictated by the field of the ligands coordinated to the Fe ion and is but one of the effects of the ligand field. Using the model systems [FeO­(H₂O)₅]²⁺, representative of the weak field situation, and [FeO­(H₂O)_ax(NH₃)₄]²⁺, representative of a strong (equatorial) field, we distinguish the effect of spin state (high spin (quintet) versus low spin (triplet)) from other effects, notably the orbital interaction (pushing up) effect of the ligand donor orbitals and the electron-donating ability of the ligands, directly affecting the charge on the FeO group. We describe the changes in electronic structure during the reaction with the help of elementary orbital interaction diagrams involving the frontier orbitals. These give a straightforward electronic structure picture of the reaction but do not provide support for the description of the reactivity of FeO²⁺ as starting with oxyl radical formation.