Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry.
Formation of a trigonal bipyramidal dihydrogen intermediate is followed by cleavage of the H–H bond, due to electron back donation into the H–H σ*-orbital, i.e. a sigma complex.
[8] This system is also in chemical equilibrium, with the reverse reaction proceeding by the elimination of hydrogen gas with simultaneous reduction of the metal center.
Nucleophilic attack by the metal center at the less electronegative atom in the substrate leads to cleavage of the R–X bond, to form an [M–R]+ species.
For example, reaction of a square planar complex with methyl iodide: This mechanism is often assumed in the addition of polar and electrophilic substrates, such as alkyl halides and halogens.
The key difference being that ionic mechanisms involve substrates which are dissociated in solution prior to any interactions with the metal center.
[2] In addition to undergoing SN2-type reactions, alkyl halides and similar substrates can add to a metal center via a radical mechanism, although some details remain controversial.
[10] Oxidative addition and reductive elimination are invoked in many catalytic processes in homogeneous catalysis, e.g., hydrogenations, hydroformylations, hydrosilylations, etc.