For more than 40 years, chemists and biologists have used the radical rebound mechanism to describe what happens when enzymes with transition metals, such as cytochrome P450, hydroxylate C–H bonds. Such enzymes first remove a hydrogen atom from a C–H bond using a high-valent metal-oxo species. This produces a carbon radical and a metal-OH intermediate that then rebounds—the hydroxyl group moves onto the carbon radical and the metal is reduced by one electron. Although plenty of circumstantial evidence suggests this is what happens, observing the rebound step in this reaction has been impossible because it is fast compared with the hydrogen atom removal step. Now, chemists led by David P. Goldberg of Johns Hopkins University have created the first model system in which the rebound reaction can be observed (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b07979). The system features Fe–OH ensconced in a tris(triphenyl)phenyl corrole ligand, which stabilizes the high redox level of the Fe complex and has sufficient steric bulk to prevent dimerization—a problem that often plagues FeOH systems. “In our case, the rebound reaction is best described as a concerted process as opposed to a stepwise process,” Goldberg says. Such mechanistic insight, he adds, could be used to engineer an enzyme or synthetic catalyst to facilitate or inhibit the rebound step and thereby influence the efficiency and selectivity of the hydroxylation.