Mechanistic Steps in Organometallic Chemistry

Robert Grossman mentions in his book "The Art of Writing Reasonable Organic Reaction Mechanisms" that if you cannot classify an alleged pericyclic step in your mechanism, then probably that step is a fictional one. Like many other things on the book, I have found this advise very helpful and often saved me from committing a grave mistakes on paper. I figured this 'rule' must also extend to other cases of reaction mechanisms and not only to pericyclic reactions. 

I have noticed it is easy to identify things if we name them. So I am gonna call this the rule of classified mechanisms. Don't judge, no one said I am an expert on naming things. (I originally thought of calling it the "Grossman's Rule" but that is already exists in the book)

As organometallic chemistry is synthetically versatile and widely used in research, I planned to have a small page consisting all the valid reaction mechanism steps at one place. So any step you write while doing a organometallic mechanistic problem must fall into one of the categories listed below. 

But, a point of caution here, the theory of pericyclic reactions were first established in 1960s (Woodward-Hoffmann rules) and later have undergone detailed computational as well as experimental studies. These reactions are thus have a firm mechanistic ground and therefore the rule of classified mechanisms apply very firmly to them. However, organometallics can still sometimes have new and non-trivial steps (very special cases) and steps that are mechanistically very different from what they seems, so although the rule still applies to them broadly, one must be cautious so as to consider all possibilities. 

1. Oxidative Addition (OA)

• Concerted 
• SN2 (metal as electrophile, metal as nucleophile)
• Radical

2. Reductive Elimination (RE)

3. Sigma-bond Metathesis

• special case: Transmetallation (TM)

4. Migratory Insertions (MI)

• α-migratory insertion (intramolecular)
• 1,1-insertion (CO insertion)
• 1,2-insertion (e.g.,olefins)

5. β-Eliminations

• β-hydride eliminations (β-HE)
• β-halogen eliminations
• β-carbon eliminations

         etc.

6. Abstractions

• α-abstractions
• β-abstractions

7. Reductive Coupling

8. Halogen-Metal Exchange

9. Concerted Metallation-Deprotonation (CMD)

• example: Lithiation by n-BuLi at acidic (relatively) C-H bond, C-H activation of arenes.
• special note: . It is also sometimes referred to as Internal Electrophilic Substitution (IES),  and Ambiphilic Metal Ligand Activation (AMLA)

10. Ligand to Ligand Hydrogen Transfer (LLHT)

Some steps are not listed above but often seen in mechanisms such as alcoholysis of acyl palladium complexes.  I have not put it separately as they do not classify as an established mechanistic step and somewhat superficially resembles sigma-bond metathesis. However, they occur quite often especially in carbonylation reactions. 

The list is not complete, of course. I will add new steps as they appear in literature and continue updating it. If I have missed any step, let me know in the comments.