Migratory Insertion

Migratory insertion is the literal bond-forming heartbeat of the oxo process (hydroformylation), in which Rh or Co catalysts insert alkenes into M–H bonds and then insert CO into the resulting M–alkyl bond, ultimately delivering linear or branched aldehydes used to make plasticizer alcohols, detergents, and solvents on a multi-million-tonne annual scale — a transformation covered in depth wherever Hydroformylation (Oxo Process) is treated in full catalytic-cycle detail. The Monsanto and Cativa acetic acid processes run on exactly the same elementary step: CO migratory insertion into a Rh– or Ir–methyl bond is the C–C bond-forming event that converts methanol (via methyl iodide oxidative addition) into acetic acid, one of the largest-volume organic chemicals manufactured industrially. In polymer manufacturing, every strand of polyethylene or isotactic polypropylene made by Ziegler–Natta or single-site metallocene/post-metallocene catalysts grows by iterative 1,2-alkene insertion into a metal–alkyl bond (the Cossee–Arlman mechanism), with catalyst and ligand design devoted almost entirely to accelerating insertion relative to the competing reverse process, β-hydride elimination, so that chains grow long before termination. Modern "living" olefin polymerization catalysts are explicitly engineered around suppressing the reverse reaction discussed above. In homogeneous hydrogenation (Wilkinson's catalyst and its descendants, covered under Homogeneous Hydrogenation Mechanisms), alkene insertion into an Rh–H bond installs the first new C–H bond of the substrate, setting the stage for the subsequent oxidative-addition/reductive-elimination steps that deliver the second hydrogen. Asymmetric hydrogenation catalysts (Knowles, Noyori-type systems) achieve enantioselectivity in large part by controlling which face of the alkene inserts, directly exploiting the suprafacial, stereospecific nature of the insertion step derived above. In biological and bio-inspired catalysis, related migratory-insertion-like steps have been proposed in some organometallic enzyme mimetics and in CO/CO₂-utilizing catalytic systems being developed for carbon capture and utilization, where CO₂ insertion into M–H or M–alkyl bonds (a close cousin of CO insertion) is an active area of catalyst design for converting CO₂ into formate or carboxylate products.

Ask the tutor: How does migratory insertion combine with oxidative addition and reductive elimination to complete a full catalytic cycle in hydroformylation?

Start with a square-planar or octahedral metal center bearing two cis ligands: an alkyl (or hydride) R and either CO or an alkene. Both are drawn correctly as ligands already introduced elsewhere — the alkyl is a two-electron σ-donor from an sp³ carbon; CO is a σ-donor/π-acceptor through its carbon lone pair and empty π orbitals; an alkene is a σ-donor/π-acceptor through its filled π and empty π orbitals. The key structural fact is that R and the unsaturated ligand must be cis to each other — insertion is strictly a same-molecule, adjacent-ligand event, never an attack by a ligand on the far side of the metal. In CO insertion, the alkyl group does not truly "insert into" the M–C(O) bond as its old name (carbonyl insertion) implies. Isotope-labeling and kinetic studies (classically on CH₃Mn(CO)₅) show unambiguously that it is the alkyl group that migrates onto the carbon of an already-bound carbonyl, while the metal-CO framework stays essentially fixed. This is why the modern and mechanistically correct name is alkyl migration, even though "CO insertion" remains the common shorthand. The migrating methyl carbon attacks the carbonyl carbon using its M–C bonding electron pair, forming a new C–C bond and leaving an $\eta^1$-acyl group, $\text{M–C(=O)CH}_3$, bound through the acyl carbon. Because the alkyl group vacates its coordination site, and no new ligand has arrived from outside, the metal is left with one fewer ligand than before — an open coordination site appears cis to the new acyl group. That site must promptly be filled (usually by the very same CO that triggered the reaction in a substitution-assisted pathway, or by external L) or the complex will not proceed productively; if nothing fills it, the reaction is easily reversed. In alkene insertion, the same logic applies with a $\eta^2$-bound alkene instead of CO. The migrating group (R or H) attacks one alkene carbon using its M–R bonding pair; the alkene's other carbon becomes newly bonded to the metal. This is a 1,2-insertion: the metal and the migrating group end up on adjacent (1,2) carbons of what was the alkene, i.e., the metal has walked from being bonded to R to being bonded to a longer alkyl chain, with the alkene's terminal carbon now the new alkyl carbon attached to R. This is exactly the chain-growth step of Ziegler–Natta and metallocene polymerization, and the first C–C bond-forming step of alkene hydroformylation and hydrogenation cycles. The transition state for both processes is a four-membered (CO case) or three-membered-to-four-membered (alkene case) ring in which the migrating group's bonding electron pair swings from the metal onto the new partner atom while the metal retains a dative interaction with the "front" atom of the unsaturated ligand. No external reagent participates; the whole rearrangement is intramolecular, which is why it is fast and why it is stereospecifically suprafacial (the alkene inserts with retention of its face, giving syn-addition of M and R across the double bond).

The governing statement is a bookkeeping identity for oxidation state and electron count, together with a bond-reorganization criterion for geometry.

where: the metal's formal oxidation state and $d^n$ configuration are unchanged by migratory insertion — only the coordination number changes (decreases by one for the migrating-group departure from its own site, with the vacated site appearing cis to the new organic ligand).

where: CN is coordination number; the "$-1$" reflects that two ligands (R and CO, or R/H and alkene) have merged into one (acyl, or longer alkyl), while no new ligand has yet arrived to occupy the site vacated by the migrating group.

where: $\Delta G^\ddagger_0$ is an intrinsic barrier for a given metal/ligand combination; the barrier is lowered by good π-acceptor ability of the inserting ligand (CO ≫ most alkenes) because that stabilizes the developing negative charge buildup on the departing carbon in the TS, and raised by steric clash in the tight cyclic TS (bulky migrating groups or 1,1-disubstituted alkenes insert more slowly).

where: $K_{eq}$ governs the position of the insertion/deinsertion equilibrium; it is usually close to unity or modestly favorable for CO insertion (a coordinatively unsaturated acyl complex can easily decarbonylate back), and it is driven strongly forward in catalytic cycles only when the open site generated is trapped irreversibly by an incoming ligand (CO, phosphine, or another alkene), which is precisely the role of the "L" that fills the vacant site in the industrial cycles.

Reading: none of these relationships involve a change in oxidation state — that is the qualitative fingerprint that separates migratory insertion from oxidative addition/reductive elimination. The energetics are controlled almost entirely by how good an electrophile the inserting ligand's terminal atom is (carbonyl carbon vs. alkene carbon) and by the steric bulk that must be accommodated in the compact, four-centered transition state.

Example 1 (foundational): Electron counting through a full CO insertion cycle on Mn. Take $\text{CH}_3\text{Mn(CO)}_5$, an 18-electron, octahedral, $d^6$ Mn(I) complex (Mn(I) is $d^6$; count: Mn(I) contributes 6 d-electrons, five CO each donate 2e = 10e, methyl anion donates 2e, total $6+10+2=18$). Add one equivalent of PPh$_3$. Step 1: Alkyl migration onto a cis CO generates the five-coordinate intermediate $\text{CH}_3\text{C(O)Mn(CO)}_4$. Electron count of this intermediate: Mn(I) is still $d^6$ (oxidation state unchanged — the acyl carbon is still formally a two-electron $\sigma$-donor, exactly like the alkyl was), four remaining CO donate $4\times2=8e$, the acyl ligand donates 2e as an $\eta^1$-C-bound ligand,

This is a genuine 16-electron, coordinatively unsaturated intermediate — exactly the species required to be trapped. Step 2: PPh₃ binds at the vacated site (2e donor), giving

Oxidation state of Mn is Mn(I) throughout — never changes. This example demonstrates the core bookkeeping: migratory insertion is a $18\to16\to18$ electron-count round trip with the metal's oxidation state pinned at +1 the entire time, in contrast to oxidative addition (which would move Mn(I)→Mn(III) and 18→16→18 by a completely different route: adding two new ligands and two new bonds to the metal rather than merging two existing ligands).

Example 2 (applied): Regiochemistry and thermodynamics of propene insertion into Rh–H in a hydroformylation-relevant fragment. Consider the fragment $\text{HRh(CO)}_2(\text{PPh}_3)(\eta^2\text{-CH}_2\text{=CHCH}_3)$, a simplified Wilkinson/oxo-type intermediate. Propene can insert two regiochemical ways: Linear (1,2) insertion: H migrates to the terminal CH₂ carbon, giving the linear alkyl $\text{Rh–CH}_2\text{CH}_2\text{CH}_3$ (n-propyl). Branched (2,1) insertion: H migrates to the internal CH carbon, giving the secondary alkyl $\text{Rh–CH(CH}_3)_2$ (isopropyl). Sterically, the four-centered transition state places the bulkier alkyl substituent (CH₃) away from the crowded metal coordination sphere in the linear pathway, so linear insertion has the lower-energy TS whenever the metal center is sterically congested (bulky phosphines, as in Rh/PPh₃ oxo catalysts). Suppose the experimentally measured free-energy difference between the two competing transition states is

The kinetic product ratio follows from

This predicts a roughly 74:26 linear-to-branched selectivity from kinetics alone (before any subsequent steps of the catalytic cycle further amplify or dampen the ratio) — consistent with the qualitative fact that oxo-process catalysts engineered with bulky phosphines favor the linear aldehyde product downstream. The limiting case is instructive: if $\Delta\Delta G^\ddagger \to 0$ (no steric or electronic bias between the two insertion modes, as would be roughly true for a perfectly symmetric internal alkene like cis-2-butene where both carbons are equivalent), the ratio collapses to $1{:}1$, and no regioselectivity is possible — exactly what is observed for symmetric internal alkenes.

Example 3 (exam/competition level): Distinguishing insertion vs. external nucleophilic attack by a crossover/kinetic experiment, and computing $K_{eq}$ from a coupled equilibrium. A chemist proposes two possible mechanisms for the conversion

Mechanism A (migratory insertion/trapping, as derived above): rate is independent of $[^{13}\text{CO}]$ at saturating concentration, and the label from external $^{13}$CO ends up specifically at the site trans to the acyl group (the site vacated by the migrating methyl, now occupied by the trapping ligand), never in the acyl carbon itself. Mechanism B (bimolecular attack of external CO directly on the M–CH₃ bond, an $S_N2$-like process at the metal): rate would be first-order in $[^{13}\text{CO}]$, and the label could scramble into the acyl position. Given the experimentally measured rate law

and IR/NMR showing the $^{13}$CO label appears exclusively trans to the acyl carbonyl in the product (never inside the acyl group), Mechanism A is confirmed: the rate-determining step is the unimolecular alkyl migration, and the labeled CO enters only in a fast, subsequent trapping step at the vacated site. Now suppose the overall equilibrium constant for

is measured as $K_{eq} = 47$ for $L = \text{P(OPh)}_3$ at 313 K, while the intrinsic migration equilibrium constant for the unimolecular step alone (between the 18-electron alkyl and the 16-electron acyl intermediate, before trapping) is separately estimated as $K_1 = 3.1\times 10^{-4}$ (heavily disfavored, because the 16-electron acyl intermediate is high in energy). Because

where $K_2$ is the (very large) binding constant of L to the unsaturated 16-electron acyl intermediate, we can back out

This large $K_2$ demonstrates the essential thermodynamic trick of migratory insertion chemistry: the intrinsic migration step is often unfavorable (an 18-electron, coordinatively saturated alkyl is usually more stable than a 16-electron acyl), but the overall reaction is pulled strongly forward because the ligand trap $L$ removes the high-energy unsaturated intermediate from the equilibrium — Le Chatelier acting on a genuinely reversible elementary step. As a limiting-case check: if $L$ is a very poor donor (e.g., a weakly coordinating solvent) so that $K_2 \to 1$, then $K_{eq} \to K_1 \ll 1$ and essentially no acyl product accumulates — exactly the observed behavior when CO-insertion reactions are attempted in non-coordinating solvents without added trapping ligand, where the reaction reverts (decarbonylates) as fast as it forms.