The High-Valent Compound of Cytochrome P450: The Nature of the Fe−S Bond and the Role of the Thiolate Ligand as an Internal Electron Donor

The FeIV−S bond of P450 iron oxene (1) has a small dissociation energy (6.3 kcal mol−1) and can vary its length with only a low energy cost, as a result of charge transfer from the ligand sulfur atom to porphyrin. P450 iron oxene is a redox mesomerism hybrid modulated by the Fe−S distance and the polarity of the environment.