Migratory Insertion

How it works: the cis-migration mechanism

Migratory insertion couples two ligands that are already bound to the same metal. For it to happen, the migrating group (an alkyl, aryl, or hydride) and the unsaturated ligand (CO or an alkene) must occupy mutually cis coordination sites, because the migrating group swings through the coordination sphere onto the neighboring ligand.

Take the classic case, [Mn(CO)₅(CH₃)]. The methyl group migrates onto a cis carbonyl, forming an acyl group, M-C(=O)CH₃. In doing so:

• A new C-C bond forms between the methyl carbon and the carbonyl carbon.
• The metal-carbon bond count drops from two (M-CH₃ plus M-CO) to one (M-acyl), so a vacant coordination site appears where the methyl used to be.
• The complex falls from 18 electrons to 16 electrons; the metal's formal oxidation state does not change.

That empty site is the whole point: it is immediately filled by an incoming ligand (another CO, a phosphine, or a solvent molecule), which traps the acyl product and lets the cycle turn over. Because no bond to the metal is broken heterolytically and no new element is oxidized or reduced, migratory insertion is redox-neutral, distinguishing it sharply from oxidative addition.

Why 'migratory,' not 'insertion': the labeling evidence

The name encodes a mechanistic subtlety proven in the early 1960s. If CO simply inserted into the M-CH₃ bond, an externally added, isotopically labeled ¹³CO should end up in the new acyl carbonyl. Calderazzo, Cotton, and coworkers found the opposite: when [Mn(¹²CO)₅(CH₃)] reacts with ¹³CO, the labeled carbon is found in a terminal CO ligand cis to the acyl, not in the acyl group itself.

This shows the methyl migrates onto a pre-existing coordinated CO, and the incoming ¹³CO merely fills the vacancy left behind. The reverse reaction, de-insertion (α-elimination or decarbonylation), sends the alkyl back to the metal and regenerates a coordinated CO; this microscopic reversibility is exploited in Rh- and Pd-catalyzed decarbonylation of aldehydes. The migration is stereospecific: chiral migrating carbons retain their configuration, and alkene insertions occur with strict syn (suprafacial) geometry.

1,1- versus 1,2-insertion

The mode depends on the unsaturated ligand.

• 1,1-insertion (CO and isocyanides): the metal and the migrating group end up on the same carbon, giving an acyl (M-C(=O)R) or iminoacyl. This is carbonylation, and it is the productive step in the Monsanto and Cativa acetic acid processes and in Pd-catalyzed carbonylative couplings.
• 1,2-insertion (alkenes and alkynes): the metal and migrating group add across adjacent carbons of the double bond, generating a new, longer metal-alkyl (for R = alkyl) or a metal-alkyl from a hydride (for R = H). Repeated 1,2-insertion of ethylene into a growing M-alkyl chain is polyolefin chain growth. When R = H (M-H + alkene), the insertion is the key step of catalytic hydrogenation and of the Heck reaction's initial carbometalation.

Regiochemistry matters enormously here: 1,2- versus 2,1-insertion of a terminal alkene decides whether hydroformylation gives a linear or branched aldehyde, a selectivity ligand designers spend great effort tuning.

Conditions, thermodynamics, and what accelerates it

Migratory insertion is usually mildly endothermic or thermoneutral for CO, and driven forward by the exothermic trapping of the open site by an incoming donor. Several factors speed it up:

• Lewis acids and electrophiles: AlCl₃ or other Lewis acids bind the carbonyl oxygen, polarizing the C=O and dramatically accelerating CO insertion, sometimes by orders of magnitude.
• Oxidation of the metal: removing an electron (e.g., anodically or with a mild oxidant) makes the alkyl more electrophilic and speeds migration, the basis of oxidatively induced insertion.
• Trapping ligands: a good incoming donor (phosphine, CO, or the alkene itself in a chain-growth cycle) pulls the equilibrium toward the inserted product by filling the vacancy.
• Sterics and the cis requirement: the two coupling ligands must be cis; bulky ancillary ligands that enforce or block a cis relationship gate the rate. Higher temperatures (often 60-150 °C in industry) overcome the modest barrier while pressure of CO or ethylene shifts the equilibrium.

Because the alkene case requires the alkene to stay bound before it inserts, high substrate concentration or pressure and a metal-alkyl (or metal-hydride) that does not undergo faster β-hydride elimination are both prerequisites.

Applications: from acetic acid to polyethylene

Migratory insertion sits at the center of some of the largest catalytic processes on Earth.

• Hydroformylation (oxo process): a Rh- or Co-hydride adds across an alkene by 1,2-insertion, then CO undergoes 1,1-insertion to give an acyl, which is cleaved by H₂ to an aldehyde. Ligand-controlled linear/branched selectivity governs plasticizer and detergent alcohol production.
• Ziegler-Natta and metallocene polymerization: chain growth is nothing but repeated 1,2-insertion of an alkene into the growing metal-alkyl bond. The stereochemistry of each insertion, dictated by the catalyst's chiral pocket, sets whether polypropylene is isotactic, syndiotactic, or atactic.
• Carbonylation to acetic acid: in the Monsanto (Rh) and Cativa (Ir) processes, a metal-methyl formed by oxidative addition of CH₃I undergoes CO migratory insertion to a metal-acetyl, ultimately releasing acetic acid, roughly 10 million tonnes annually.
• Cross-coupling and Heck chemistry: the carbopalladation (alkene 1,2-insertion into Pd-Ar) that defines the Heck reaction, and carbonylative Suzuki/Sonogashira variants, all rely on a migratory insertion step.

History and place in the catalytic cycle

Although the reactivity was implicit in Otto Roelen's 1938 discovery of hydroformylation and in Karl Ziegler's and Giulio Natta's mid-1950s polyolefin catalysts (Nobel Prize 1963), the mechanism was pinned down in the early 1960s. The ¹³CO-labeling experiments on manganese acyls by Fausto Calderazzo and F. Albert Cotton (around 1962) established that the alkyl migrates to a coordinated CO and that the process is reversible and stereospecific.

In a modern catalytic cycle, migratory insertion is one of the four canonical elementary steps: it typically follows oxidative addition and coordination of the unsaturated substrate, and precedes reductive elimination or β-hydride elimination that releases product. Understanding it lets chemists rationally tune ligand bite angle, electronics, and sterics to control regioselectivity and stereoselectivity in everything from drug-molecule carbonylations to commodity polymers.

The two common insertion modes

Feature	1,1-Insertion (CO)	1,2-Insertion (alkene)
Ligand inserted	Carbon monoxide	Alkene / alkyne
Atoms bridging M and R	Same carbon (1,1)	Adjacent carbons (1,2)
Product	Acyl, M-C(=O)R	New M-alkyl, longer chain
Stereochemistry at C	Retention at migrating C	Syn (cis) addition across C=C
Drives	Carbonylation, hydroformylation	Polymerization, hydrogenation, Heck