Oxidative Addition

General Information

As the name implies, oxidative addition is the addition of a substrate molecule to a transition metal complex. In this process, the metal center is oxidized by two electrons. In the generic mononuclear examples below, the metal goes from the x to the x+2 oxidation state. In most cases, two new ligands are generated, but this need not always be the case. Binuclear processes are also possible as illustrated by these general examples:

Key Facts

Oxidative addition is formally the microscopic reverse of reductive elimination, and it is not surprising that a series of reactions involving an oxidative addition, a rearrangement and then a reductive elimination form the basis for a variety of industrially important catalytic cycles.

Important principles to remember about oxidative addition are:

1. Oxidative addition reactions are most facile when there is a good two-electron redox couple. In other words, both the starting and final oxidation states are relatively stable. For example, oxidative addition from Ir(I) to Ir(III) is common but an oxidative addition from Fe(III) to Fe(V), while possible, is generally unlikely.

   Note: Oxidative addition reactions can not occur on metal centers that are already in their highest oxidation state. For example, Ta⁵⁺ can not undergo oxidative addition to give Ta⁷⁺. In these cases, a sigma bond metathesis reaction is a likely alternative.

2. The more reduced a metal center is, the greater the reactivity towards oxidative addition.
3. The likelihood of oxidative addition of A-B to a metal, M, depends on the relative strengths of the A-B, M-A and M-B bonds. For example, oxidative addition of an alkane is much less common than oxidative addition of an alkyl halide. For the alkane case, the C-H bond is fairly strong compared to the M-H and M-R (R = alkyl) bonds (see the example under reductive elimination).

There are a number of mechanisms that one might postulate for the process of oxidative addition, but a detailed analysis is presently beyond the scope of this work.

Self-Test

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