The Organometallic Reader

Dedicated to the teaching and learning of modern organometallic chemistry.

Oxidative Addition: General Ideas

with 5 comments

A critical difference between the transition metals and the organic elements is the ability of the former to exist in multiple oxidation states. In fact, the redox flexibility of the transition metals and the redox obstinacy of the organic elements work wonderfully together. Why? Imagine the transition metal as a kind of matchmaker for the organic elements. Transition metals can take on additional covalent bonds (oxidation), switch out ligands (substitution), then release new covalent bonds (reduction). The resulting organic products remain unfazed by the metal’s redox insanity. Talk about a match made in heaven!

The following series of posts will deal with the first step of this process, oxidation. More specifically, we’ll discuss the oxidation of transition metals via formal insertion into covalent bonds, also known as oxidative addition (OA). Although we often think of oxidative addition as an elementary reaction of organometallic chemistry, it is not an elementary mechanistic step. In fact, oxidative addition can proceed through a variety of mechanisms. Furthermore, any old change in oxidation state does not an oxidative addition make (that almost rhymes…). Formally, the attachment of an electrophile to a metal center (e.g., protonation) represents oxidation, but we shouldn’t call this oxidative addition, since two ligands aren’t entering the fray. Instead, we call this oxidative ligation (OL).

Oxidative addition involves formal bond insertion and the introduction of two new ligands to the metal. Oxidative ligation, OTOH, involves the coordination of only one new ligand, an electrophile.

Oxidative addition involves formal bond insertion and the introduction of two new ligands to the metal. Oxidative ligation, OTOH, involves the coordination of only one new ligand, an electrophile.

Protonation is (formally) a kind of oxidation. Who knew?! SN2 reactions with the metal as nucleophile are also oxidative ligations. Of course, if the leaving group comes back and forms a new bond to the metal, we’re back to oxidative addition. Both reactions lead to an increase in the oxidation state of the metal by two units and a decrease in the d electron count of the metal by two electrons. However, note how the total electron count changes in each case. The total electron count does not change during an oxidative ligation. Think of it this way: the new ligand brings no electrons with it to the complex. On the other hand, the total electron count of the complex actually increases by two electrons during oxidative addition. As a result, eighteen-electron complexes do not undergo oxidative addition. Carve that sucker on a stone tablet. Seventeen-electron complexes can undergo oxidative addition via bimolecular OA reactions, which leave X on one metal center and Y on another.

What are the mechanisms of oxidative addition, anyway? Let’s begin with the “concerted” mechanism, which can be thought of as σ-complex formation followed by insertion. The metal first sidles up to the X–Y bond and a σ complex forms (ligand dissociation may be required first). As we’ve seen, σ complexes M(X2) are tautomeric with their M(X)2 forms. When back donation from the metal is strong enough, the σ complex disappears and M(X)2 is all that remains. The metal has been formally oxidized: oxidative addition!

Concerted oxidative addition: sigma complex formation followed by insertion.

Concerted oxidative addition: σ complex formation followed by insertion.

There are several variations on this theme. When X and Y are different, the σ complex is skewed and approach to the metal “asynchronous.” When the metal isn’t a great nucleophile, the reaction may stop at the σ-complex stage.

Other mechanisms of oxidative addition require multiple steps and the formation of polar or radical intermediates. An important two-step, polar mechanism involves SN2 attack of a nucleophilic metal on an electrophile, followed by coordination of the leaving group to the metal center. What we might call SN1-type mechanisms, involving dissociation of the electrophile before nucleophilic attack by the metal, also occur (HCl and other strong acids operate like this). Finally, both non-chain and chain radical mechanisms are possible in reactions of metal complexes with alkyl halides. We’ll dive into these mechanisms in more detail in upcoming posts.

Hopefully from this general discussion, you’ve gleaned a few trends. The metal must have a stable oxidation state two units higher than its current OS for oxidative addition to occur. For the reaction to work well, the metal typically needs to be electron rich (and in a relatively low oxidation state) and the organic compound needs to be electron poor. To see why, consider that during oxidative addition, the metal formally loses two d electrons. Furthermore, the main-group atoms X and Y gain electron density, since the new M–X and M–Y bonds are likely polarized toward X and Y. The metal needs to bear two open coordination sites (not necessarily at the same time) for oxidative addition to occur, because two new ligands enter the metal’s coordination sphere.  Since the new ligands need space, steric hindrance tends to discourage oxidative addition. Oftentimes ligand dissociation is required before oxidative addition can occur; in many of these cases, the rate of dissociation influences the overall rate of the reaction.

Factors affecting the kinetics and thermodynamics of oxidative addition.

Factors affecting the kinetics and thermodynamics of oxidative addition.

Lastly, it’s worth commenting that oxidative addition is the microscopic reverse of reductive elimination, the formal deinsertion of M–X and M–Y to form M and X–Y. This fact means that the mechanism of a reductive elimination reaction is the same as that of the corresponding oxidative addition run “backwards in time.” Factors that encourage oxidative addition discourage reductive elimination, and vice versa.

5 Responses

Subscribe to comments with RSS.

  1. Never thought of protonation as a formal oxidation before!

    James

    June 22, 2012 at 6:51 am

  2. azmanam

    June 26, 2012 at 7:32 am

  3. great picture, Dr. Azman🙂

    A Far

    June 29, 2012 at 1:54 pm

  4. […] Reductive elimination is the microscopic reverse of oxidative addition. It is literally oxidative addition run in reverse—oxidative addition backwards in time! My favorite analogy for microscopic reversibility is the video game Braid, in which “resurrection is the microscopic reverse of death.” The player can reverse time to “undo” death; viewed from the forward direction, “undoing death” is better called “resurrection.” Chemically, reductive elimination and oxidative addition share the same reaction coordinate. The only difference between their reaction coordinate diagrams relates to what we call “reactants” and “products.” Thus, their mechanisms depend on one another, and trends in the speed and extent of oxidative additions correspond to opposite trends in reductive eliminations. In this post, we’ll address reductive elimination in a general sense, as we did for oxidative addition in a previous post. […]

  5. Was a step skipped in the oxidative ligation mechanism? That type of Sn2 attack (where the metal behaves as a nucleophile) does increase the metal center by +2 and yes, there is one new anionic ligand present, but doesn’t the other part of the electrophile (e.g. a Br- in a CH3Br) then coordinate to the metal, leading to two new coordinated anionic ligands?

    For example, Ir(PPh3)2(CO)(Cl) is 16e and Ir(I). Nucleophilic attack produces a 16e cationic complex [Ir(PPh3)2(CO)(Cl)(CH3)+] and Ir is now in the +3 oxidation state. Given that we now have a 16e complex, does Br- not then coordinate to yield an 18e Ir(III) complex, ending up with an 18e complex with two new anionic ligands in addition to an increase of +2 in formal oxidation state?

    Cp2Fe

    May 4, 2015 at 5:54 am


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: