FeIV

FeIV, or iron(IV), refers to iron in the +4 oxidation state. In coordination chemistry and biochemistry, FeIV species are high-valent oxidants, frequently encountered as oxo iron(IV) complexes, written Fe(IV)=O, and often described as ferryl species. In heme enzymes such as cytochrome P450, reactive intermediates include Fe(IV)=O species; the formal Compound I is commonly described as Fe(IV)=O porphyrin π-cation radical, while Compound II corresponds to Fe(IV)=O without the porphyrin radical.

Non-heme iron enzymes also generate Fe(IV)=O intermediates during substrate oxidation, including both natural and artificial oxygenases. In inorganic chemistry, a variety of iron(IV) oxo complexes are known, stabilized by porphyrin, corrole, or other strong-field ligands, enabling studies of C–H bond activation and related oxidations.

Fe(IV) species are typically short-lived and highly reactive, requiring carefully designed ligands or enzyme environments to stabilize them long enough for study. They can be generated by oxidation of Fe(III) precursors with strong oxidants (such as peracids), by photochemical methods, or during catalytic cycles of oxygen-activating enzymes. The electronic structure is influenced by ligands and spin state; commonly Fe(IV)=O centers exhibit high oxidation potential and can carry S=1 or S=2 depending on ligand field.

Studying Fe(IV) species informs understanding of biological oxidation mechanisms, aids the design of biomimetic oxidation catalysts, and guides development of industrial oxidation processes. Ongoing research seeks to characterize their reactivity, lifetimes, and spectroscopic signatures as part of broader efforts to map high-valent iron chemistry.