Diradical Nature of the Dioxygen Molecule

As rationalized by Sheldon and Kochi, metal catalyzed oxidations can be conveniently divided into two types homolytic and heterolytic. 

In homolytic reactions the hydrocarbon to be oxidized is generally not coordinated to the metal and is oxidized outside the coordination sphere via a radical chain. These processes are common and constitute the basis for several very important industrial applications (Sections 2.2 and 2.3). However radical chains are difficult to control, they do not often preserve the configuration of the substrate and typically lead to the formation of a wide variety of products. Consequently, reactions involving one-electron processes with dioxygen as the oxidant generally show only moderate to low selectivities towards the desired product. 

By contrast heterolytic oxidations generally require activation of the substrate by coordination to the metal and are often highly selective and stereospecific. They do not involve free radical intermediates and use second- and third-row transition metals which conserve their oxidation state or change it by a two electron redox step. 

---