Inorganic — organometallic
Migratory insertion: CO insertion at a metal center
How the Migratory insertion works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: LₙM–CH₃ (e.g. Mn(CO)₅CH₃) + CO. Conditions: CO atmosphere, 25–100 °C. Bonds formed: C–C bond (acyl); M–C(acyl); bonds broken: M–CH₃. Stereochemistry: Alkyl migrates; stereochemistry at carbon retained. Rate and selectivity: Alkyl must be cis to CO; ligand then fills the vacant site.
At a glance
Step 1: Alkyl and CO cis on one metal
A metal alkyl complex has a CO ligand bound cis to the alkyl group — both share the same metal, positioned close together.
Step 2: Alkyl migrates onto CO
The alkyl group migrates directly onto the carbon of the coordinated CO, forming a new acyl ligand — the metal's coordination number drops by one, opening a site.
Step 3: New ligand fills the vacant site
A new ligand (like another CO or a solvent molecule) fills that open site, regenerating an 18-electron complex — this step is central to catalytic carbonylation processes.
Abbreviations and terms
- L
- Ligand. A Lewis-base group (often a phosphine) bound to a metal centre.
- M
- Metal. A generic metal centre.
- CO
- Carbon monoxide (carbonyl ligand). σ-donor / strong π-acceptor ligand; also a C1 building block.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
How it compares with related reactions
| Migratory insertion | Cross metathesis between two different alkenes | Olefin metathesis via metallacyclobutane | |
|---|---|---|---|
| Reagents | LₙM–CH₃ (e.g. Mn(CO)₅CH₃) + CO | Ru carbene (Grubbs II), two different alkenes | Metal alkylidene (Grubbs Ru=CHR) + alkene |
| Conditions | CO atmosphere, 25–100 °C | CH₂Cl₂, 25–40 °C; excess partner or ethylene removal | CH₂Cl₂ or toluene, 25–40 °C |
| Stereochemistry | Alkyl migrates; stereochemistry at carbon retained | Usually E-selective (thermodynamic) | Mostly E (thermodynamic) for internal alkenes |
| Rate / selectivity | Alkyl must be cis to CO; ligand then fills the vacant site | Initiation/phosphine-or-ether release often limits; [2+2] steps fast | Driven by release of volatile ethylene (RCM, ROMP strain) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Migratory insertion?
LₙM–CH₃ (e.g. Mn(CO)₅CH₃) + CO. Conditions: CO atmosphere, 25–100 °C.
Which bonds are formed and which are broken?
Formed: C–C bond (acyl); M–C(acyl). Broken: M–CH₃.
What is the stereochemical outcome?
Alkyl migrates; stereochemistry at carbon retained
What controls the rate or the selectivity?
Alkyl must be cis to CO; ligand then fills the vacant site
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam