Mechanistic dichotomies in redox reactions of mononuclear metal–oxygen intermediates

There are mechanistic dichotomies with regard to the formation, electronic structures and reaction mechanisms of metal–oxygen intermediates, since these metal–oxygen species could be composed of different resonance structures or canonical structures of the oxidation states of metals and ligands, which may undergo different reaction pathways. Even the same metal–oxygen intermediates, such as metal–oxo species, may undergo an electron-transfer pathway or a direct hydrogen or oxygen atom transfer pathway depending on the one-electron redox potentials of metal–oxo species and substrates. Electron-transfer pathways are also classified into two mechanisms, such as outer-sphere and inner-sphere pathways. The one-electron redox potentials of metal–oxygen species and substrates are also shifted because of the binding of acids, which can result from either hydrogen bonding or protonation. There are a rebound pathway and a non-rebound pathway following the initial electron transfer or hydrogen atom transfer step to produce hydroxylated products, depending on the one-electron redox potentials of metal–oxo species and substrates. Nucleophilic reactions can be switched to electrophilic pathways, depending on reaction conditions such as reaction temperature. Spin states of metal–oxygen intermediates are also an important factor that controls the redox reactivity of oxidants in oxidation reactions. Here, we review such various mechanistic dichotomies in redox reactions of metal–oxygen intermediates with the emphasis on understanding and controlling the redox reactivity of metal–oxygen intermediates from experimental and theoretical points of view.