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In organometallic chemistry, a migratory insertion is a type of reaction wherein two ligands on a metal complex combine. It is a subset of reactions that very closely resembles the insertion reactions, and both are differentiated by the mechanism that leads to the resulting stereochemistry of the products. However, often the two are used interchangeably because the mechanism is sometimes unknown. Therefore, migratory insertion reactions or insertion reactions, for short, are defined not by the mechanism but by the overall regiochemistry wherein one chemical entity interposes itself into an existing bond of typically a second chemical entity e.g.:^[1]

{\color {red}{\ce {A}}}+{\color {blue}{\ce {B-C}}}\longrightarrow {\color {blue}{\ce {B{-}}}}{\color {red}{\ce {A}}}{\color {blue}{\ce {-C}}}

Overview

In the migratory insertion, a ligand that is viewed as an anion (X) ligand in and a ligand that is viewed as neutral couple, generating a new anionic ligand. The anion and neutral ligands that react are adjacent. If the precursor complex is coordinatively saturated, migratory insertion often result in a coordinatively unsaturated product. A new (neutral) ligand can then react with the metal leading to a further insertion. The process can occur many times on a single metal, as in olefin polymerization.

The anionic ligand can be: H⁻ (hydride), R⁻ (alkyl), acyl, Ar⁻ (aryl), or OR⁻ (alkoxide). The ability of these groups to migrate is called their migratory aptitude. The neutral ligand can be CO, alkene, alkyne, or in some cases, even carbene.

Diverse reactions apply to the migratory insertion. One mechanism involves the attack of the anionic ligand on the electrophilic part of the neutral ligand (the anionic ligand migrates to the neutral ligand). The other mechanism involves the neutral ligand inserts itself between the metal and the anionic ligand.

CO insertion

The insertion of carbon monoxide into a metal-carbon bond to form an acyl group is the basis of carbonylation reactions, which provides many commercially useful products.

Mechanism

CO inserts into a metal-alkyl bond via migratory insertion. The key concept is that both the CO and the alkyl groups are ligands on the same metal. For example, the reaction of ¹³CO with Mn(CO)₅CH₃ exclusively form Mn(CO)₄(¹³CO)COCH₃. The alkyl group migrates intramolecularly to an adjacent CO ligand within the coordination sphere of the Mn(I) centre. Subsequent to the migration, the metal binds free CO (see figure below).^[2]^[3]

The equilibrium and the rates of the forward and reverse reactions in the system ${\ce {[MnCH3(CO)5] + CO <=> [Mn(CO)CH3(CO)5]}}$ were studied at atmospheric pressure of CO in various solvents and at several temperatures by Calderazzo and Cotton in 1961.^[4] The reaction proved to be first order in both MnCH₃(CO)₅ and CO and the effect of changing the solvent appeared to be mainly a function of the dielectric constant in stabilizing a somewhat polar transition state. The volume of CO absorbed or evolved at a given time was determined by gas-volumetric measurements. A scheme of the burette used for the experiments is shown in the figure 3. Once the volume of CO was measured, it was converted to moles of CO and the CO/Mn mole ratio was determined. By measuring the amount of CO absorbed as a function of time, it was possible to study the kinetics of the carbonylation reaction.

In 1967 Calderazzo and Noack re-examined the prototype system using ¹³CO, in gas phase 50% enriched.^[5] The reaction product was investigated by infrared spectroscopy, monitoring changes in terminal CO stretching bands. The compound MnL(CO)₅, where L can be a methyl or acetyl group, has three stretching active IR vibrations due to C_4v molecular symmetry.^[6] The key signals were the axial ¹²CO and ¹³CO stretching bands (trans to L), respectively at 1991 cm^-1 and 1949 cm^-1. Compared to ¹²CO, the 13-labelled molecule shows a C-O bond stretching absorption shifted to lower wavenumbers. First of all, it was observed that the incoming ¹³CO have not been inserted in between the metal carbon bond Mn-CH₃. They obtained a 100% cis isomer of [Mn(COCH₃)(¹³CO)(CO)₄] (scheme 1).

Two hypothetical mechanisms had been previously proposed: the CO or CH₃ intramolecular migration. Calderazzo and Noack designed an experiment based on the Principle of Microscopic Reversibility:^[7] in mechanistic terms, if a certain series of steps constitutes the mechanism of a reverse reaction, the mechanism of the forward reaction, under the same conditions, is given by the same steps traversed forwards. They examined the decarbonylation of acetyl complex, prepared either by about 50% enriched [MnCH₃(¹³CO)(CO)₄] or prepared from CH₃(¹³CO)CI and Na[Mn(CO)₅]. They found a mixture of cis and trans product [MnCH₃(CO)₄(¹³CO)], 50% and 25% respectively (in ratio of 2:1). They obtained also 25% of unlabelled CH₃Mn(CO)₅ (scheme 2). According to the Principle of Microscopic Reversibility, this result confirmed the intramolecular CH₃ migration hypothesis, because the carbonyl migration process should have shown only the cis product [MnCH₃(¹³CO)(CO)₄] with a probability of 75% and the free ¹³CO product with 25% .

Successive researches revealed also that cis-regiospecific alkyl or aryl migrations occur due to addition of neutral ligands (2 electron donor),^[8] and that a penta-coordinate intermediate, involved in the intramolecular mechanism, allows the retention of configuration (scheme 3). The kinetics are reminiscent of dissociative substitution except that the vacant site is formed at the metal during the migratory step, not by loss of a ligand. Using the steady-state method, the rate is given: ${\textstyle v=-{d[R] \over dt}=\left({\frac {k(1)k(2)[L][R]}{k(-1)+k(2)[L]}}\right)}$

It was reported that the rates of the carbonylation (and the decarbonylation) reaction are dependent on the alkyl group R. Plotting the rate constats versus the number of carbon atoms in the alkyl chain for the reaction of [Mn(R)(CO)₅] with PPh₃, it is confirmed an increase from methyl to n-propyl derivate, followed by a slowly decrease.^[9] Further experiments revealed that, not only Lewis acids are able to increase the reaction rate activating the carbonyl group, but also stabilizing the product and transition state. Other factors accelerating the reaction are polar, electron-withdrawing containing solvent, and bulky ligands L_n.^[10]^[11]

The involvement of migratory insertion in catalytic cycles of large industrial interest has contributed to maintain continuous interest in this mechanism.^[12] For instance, recently it has been highlighted the potential of alkyl Mn(I) carbonyl-based homogenous catalysts. New investigations have explored the catalyzed activation under milder conditions of nonpolar or moderately polar bond such as H−H, B-H, C-H and Si-H.^[13]

CO insertion does not always involve migration. Treatment of CpFe(L)(CO)CH₃ with ¹³CO yields a mix of both alkyl migration products and products formed by true insertion of bound carbonyls into the methyl group. Product distribution is influenced by the choice of solvent.^[14]

Alkyl derivatives of square planar complexes undergo CO insertions particularly readily. Insertion reactions on square planar complexes are of particular interest because of their industrial applications. Since square planar complexes are often coordinatively unsaturated, they are susceptible to formation of 5-coordinate adducts, which undergo migratory insertion readily.^[14] In most cases the in-plane migration pathway is preferred, but, unlike the nucleophilic pathway, it is inhibited by an excess of CO.^[15]

Effects on reaction rates

Steric effects strain – Increasing the steric strain of the chelate backbone in square planar complexes pushes the carbonyl and methyl groups closer together, increasing the reactivity of insertion reactions.^[15]
Oxidation state – Oxidation of the metal tends to increase insertion reaction rates. The main rate-limiting step in the mechanism is the migration of the methyl group onto a carbonyl ligand, oxidizing the metal by imparting a greater partial positive charge on the acetyl carbon, and thus increasing the rate of reaction.^[16]
Lewis acids – Lewis acids also increase the reaction rates, for reasons similar to metal oxidation increasing the positive charge on the carbon. Lewis acids bind to the CO oxygen and remove charge, increasing the electrophilicity of the carbon. This can increase the reaction rate by a factor of up to 10⁸, and the complex formed is stable enough that the reaction proceeds even without additional CO to bind to the metal.^[16]
Electronegativity of the leaving group - Increasing the electronegativity of the leaving alkyl group stabilizes the metal-carbon bond interaction and thus increases the activation energy required for migration, decreasing the reaction rate.^[17]
Trans-effect – Ligands in an octahedral or square planar complex are known to influence the reactivity of the group to which they are trans. This ligand influence is often referred to as the trans-influence, and it varies in intensity between ligands. A partial list of trans-influencing ligands is as follows, from highest trans-effect to lowest:^[14] aryl, alkyl > NR₃ > PR₃ > AsR₃ > CO > Cl. Ligands with a greater trans-influence impart greater electrophilicity to the active site. Increasing the electrophilicity of the CO group has been shown experimentally to greatly increase the reaction rate, while decreasing the electrophilicity of the methyl group slightly increases the reaction rate. This can be demonstrated by reacting a square planar [(PN)M(CO)(CH₃)] complex with CO, where PN is a bidentate phosphorus- or nitrogen-bound ligand. This reaction proceeds in much greater yield when the methyl group is trans-P and the CO trans-N, owing to the higher trans-influence of the more electronegative nitrogen.^[15]

Reverse reaction

Decarbonylation of aldehydes, the reverse of CO insertion, is a well-recognized reaction:

RCHO → RH + CO

The reaction is not widely practiced in part because the alkanes are less useful materials than are the aldehyde precursors. Furthermore, the reaction is not often conducted catalytically because the extruded CO can be slow to dissociate.^[18] Extrusion of CO from an organic aldehyde is most famously demonstrated using Wilkinson's catalyst:^[19]

RhCl(PPh₃)₃ + RCHO → RhCl(CO)(PPh₃)₂ + RH + PPh₃

Please see Tsuji-Wilkinson Decarbonylation Reaction for an example of this elementary organometallic step in synthesis

Insertion of other oxides

Many electrophilic oxides insert into metal carbon bonds; these include sulfur dioxide, carbon dioxide, and nitric oxide. These reactions have limited or no practical significance, but are of historic interest. With transition metal alkyls, these oxides behave as electrophiles and insert into the bond between metals and their relatively nucleophilic alkyl ligands. As discussed in the article on Metal sulfur dioxide complexes, the insertion of SO₂ has been examined in particular detail. SO₂ inserts to give both O-sulphinates and S-sulphinates, depending on the metal centre.^[20] With square planar alkyl complexes, a pre-equilibrium is assumed involving formation of an adduct.^[21]

Insertion of alkenes into metal-carbon bonds

The insertion of alkenes into both metal-carbon is important. The insertion of ethylene and propylene into titanium alkyls is the cornerstone of Ziegler–Natta catalysis, the main source of polyethylene and polypropylene. The majority of this technology involves heterogeneous catalysts, but it is widely assumed that the principles and observations on homogeneous systems are applicable to the solid-state versions. Related technologies include the Shell Higher Olefin Process which produces detergent precursors.

Steps in alkene polymerization. Step i involves binding of the monomer to the metal and step ii involves the migratory insertion step. These steps, which alternate from one side of the metal center to the other, are repeated many times for each polymer chain. The box represents a vacant (or extremely labile) coordination site.

Mechanism

Factors affecting the rate of olefin insertions include the formation of the cyclic, planar, four-center transition state involving incipient formation of a bond between the metal and an olefin carbon. From this transition state, it can be seen that a partial positive charge forms on the β-carbon with a partial negative charge formed on the carbon initially bonded to the metal. This polarization explains the subsequently observed formation of the bond between the negatively charged carbon/hydrogen and the positively charged β-carbon as well as the simultaneous formation of the metal-α-carbon bond. This transition state also highlights the two factors that most strongly contribute to the rate of olefin insertion reactions: (i) orbital overlap of the alkyl group initially attached to the metal and (ii) the strength of the metal-alkyl bond. With greater orbital overlap between the partially positive β-carbon and the partially negative hydrogen/alkyl group carbon, the formation of the new C-C bond is facilitated. With increasing strength of the metal-alkyl bond, the breaking of the bond between the metal and the hydrogen/alkyl carbon bond to form the two new bonds with the α-carbon and β-carbon (respectively) is slower, thus decreasing the rate of the insertion reaction.^[22]

Insertion of alkenes into M–H bonds

The insertion of alkenes into metal-hydrogen bonds is a key step in hydrogenation and hydroformylation reactions. The reaction involves the alkene and the hydride ligands combining within the coordination sphere of a catalyst. In hydrogenation, the resulting alkyl ligand combines with a second hydride to give the alkane. Analogous reactions apply to the hydrogenation of alkynes: an alkenyl ligand combines with a hydride to eliminate an alkene.

Migratory insertion of an alkene into a metal hydride. The middle image depicts the transition state. Notice that the less substituted carbon becomes bonded to the metal, at least usually. The reverse reaction is called beta-hydride elimination.

Mechanism

In terms of mechanism, the insertion of alkenes into M–H bond and into M–C bonds are described similarly. Both involve four-membered transition states that place the less substituted carbon on the metal.

The reverse of olefin insertion into a metal-hydrogen bond is β-hydride elimination. The Principle of Microscopic Reversibility requires that the mechanism of β-hydride elimination follow the same pathway as the insertion of alkenes into metal hydride bonds. The first requirement for β-hydride elimination is the presence of a hydrogen at a position that is β with respect to the metal. β-elimination requires a vacant coordination position on the metal that will accommodate the hydrogen that is abstracted.^[23]

Industrial applications

Carbonylation

Two widely employed applications of migratory insertion of carbonyl groups are hydroformylation and the production of acetic acid by carbonylation of methanol. The former converts alkenes, hydrogen, and carbon monoxide into aldehydes. The production of acetic acid by carbonylation proceeds via two similar industrial processes. More traditional is the Monsanto acetic acid process, which relies on a rhodium-iodine catalyst to transform methanol into acetic acid. This process has been superseded by the Cativa process which uses a related iridium catalyst, [Ir(CO)₂I₂]⁻ (1).^[24]^[25] By 2002, worldwide annual production of acetic acid stood at 6 million tons, of which approximately 60% is produced by the Cativa process.^[24]

The catalytic cycle of the Cativa process

The Cativa process catalytic cycle, shown above, includes both insertion and de-insertion steps. The oxidative addition reaction of methyl iodide with (1) involves the formal insertion of the iridium(I) centre into the carbon-iodine bond, whilst step (3) to (4) is an example of migratory insertion of carbon monoxide into the iridium-carbon bond. The active catalyst species is regenerated by the reductive elimination of acetyl iodide from (4), a de-insertion reaction.^[24]

Alkene polymerization

Industrial applications of alkene insertions include metal-catalyzed routes to polyethylene and polypropylene. Typically these conversions are catalyzed heterogeneously by titanium trichloride which are activated by aluminium alkyls. This technology is known as Ziegler–Natta catalysts.^[26] In these reactions, ethylene coordinates to titanium metal followed by its insertion. These steps can be repeated multiple times, potentially leading to high molecular weight polymers.

References

^ Elschenbroich, C. (2006). Organometallics. Weinheim: Wiley-VCH. ISBN 978-3-527-29390-2.
^ Hartwig, J. F. (2010). Organotransition Metal Chemistry, from Bonding to Catalysis. New York, NY: University Science Books. ISBN 978-1-891389-53-5.
^ Yadav, M. S. (2005). Quick Review in Inorganic Chemistry. Anmol Publications. p. 244. ISBN 978-81-261-1898-4.
^ F. Calderazzo, F. A. Cotton. “The Carbonylation of Methyl Manganese Pentacarbonyl and Decarbonylation of Acetyl Manganese Pentacarbonyl”. Inorg. Chem., 1962, 1, 30–36. https://doi.org/10.1021/ic50001a008.
^ See note 13.
^ L. E. Orgel. “The Infrared Spectra of Substituted Metal Carbonyls”. Inorg. Chem., 1962, 1, 25–29. https://doi.org/10.1021/ic50001a007.
^ R. L. Burwell Jr., R. G. Pearson. “The Principle of Microscopic Reversibility”. Phys. Chem., 1966, 70, 300–302. https://doi.org/10.1021/j100873a508.
^ K. Noack, M. Ruch, F. Calderazzo. “The Mechanism of Reaction of Methylmanganese Pentacarbonyl and Acetylmanganese Pentacarbonyl with Triphenylphosphine”. Inorg. Chem., 1968, 7, 345–349. https://doi.org/10.1021/ic50060a037.
^ M. Andersen, J. R. Moss. “Synthesis of an Extensive Series of Manganese Pentacarbonyl Alkyl and Acyl Compounds: Carbonylation and Decarbonylation Studies on [Mn(R)(CO)₅] and [Mn(COR)(CO)₅]”. Organometallics, 1994, 13, 5013–5020. https://doi.org/10.1021/om00024a051.
^ E. J. Kuhlmann, J. J. Alexander. “Carbon monoxide insertion into transition metal-carbon sigma-bonds”. Coord. Chem. Rev., 1980, 33, 195-225. https://doi.org/10.1016/S0010-8545(00)80454-3.
^ F. Calderazzo. “Synthetic and Mechanistic Aspects of Inorganic Insertion Reactions. Insertion of Carbon Monoxide”. Angew. Chem. 1977, 165, 299-311. https://doi.org/10.1002/anie.197702991.
^ M. T. Whited, G. E. Hofmeister. “Synthesis and Migratory-Insertion Reactivity of CpMo(CO)₃(CH₃): Small-Scale Organometallic Preparations Utilizing Modern Glovebox Techniques”. J. Chem. Educ. 2014, 91, 1050–1053. https://doi.org/10.1021/ed500114a.
^ S. Weber, K. Kirchner. “Manganese Alkyl Carbonyl Complexes: From Iconic Stoichiometric Textbook Reactions to Catalytic Applications”. Acc. Chem. Res. 2022, 55, 2740−2751. https://doi.org/10.1021/acs.accounts.2c00470.
^ ^a ^b ^c Anderson, G. K.; Cross, R. J. (1984). "Carbonyl-Insertion Reactions of Square Planar Complexes". Acc. Chem. Res. 17 (2): 67–74. doi:10.1021/ar00098a005.
^ ^a ^b ^c Cavell, K. J. (1996). "Recent Fundamental Studies on Migratory Insertion into Metal-Carbon Bonds". Coord. Chem. Rev. 155 (11): 209–243. doi:10.1016/S0010-8545(96)90182-4.
^ ^a ^b Alexander, J.J. (1985). "Insertions into transition metal-carbon bonds". In Hartley; Patai (eds.). The chemistry of the metal–carbon bond. Vol. 2. John Wiley & Sons. doi:10.1002/9780470771747.ch5.
^ Shusterman, A. J.; Tamir, I.; Pross, A. (1988). "The Mechanism of Organometallic Migration Reactions. A Configuration Mixing (CM)Approach". J. Organomet. Chem. 340 (2): 203–222. doi:10.1016/0022-328X(88)80076-7.
^ Fristrup, Peter; Kreis, Michael; Palmelund, Anders; Norrby, Per-Ola; Madsen, Robert (2008). "The Mechanism for the Rhodium-Catalyzed Decarbonylation of Aldehydes: A Combined Experimental and Theoretical Study". J. Am. Chem. Soc. 130 (15): 5206–5215. doi:10.1021/ja710270j. PMID 18303836. S2CID 207119793.
^ Ohno, K.; Tsuji, J. (1968). "Organic Synthesis by means of Noble Metal Compounds. XXXV. Novel Decarbonylation Reactions of Aldehydes and Acyl Halides using Rhodium Complexes". J. Am. Chem. Soc. 90 (1): 99–107. doi:10.1021/ja01003a018.
^ Douglas; McDaniel; Alexander (1994). Concepts and Models of Inorganic Chemistry (3rd ed.). John Wiley & Sons, Inc. ISBN 978-0-471-62978-8.
^ Puddephatt, R.A.; Stalteri, M.A. (1980). "Competition between Insertion of Sulfur Dioxide into the Methyl- or Phenyl- Transition Metal Bond". Journal of Organometallic Chemistry. 193: C27–C29. doi:10.1016/S0022-328X(00)86091-X.
^ Burger, B. J.; Thompson, M. E.; Cotter, W. D.; Bercaw, J. E. (1990). "Ethylene Insertion and β-hydrogen Elimination for Permethylscandocene Alkyl Complexes. A Study of the Chain Propagation and Termination Steps in Ziegler-Natta Polymerization of Ethylene". J. Am. Chem. Soc. 112 (4): 1566–1577. doi:10.1021/ja00160a041.
^ Crabtree, R. H. (2009). The Organometallic Chemistry of the Transition Metals. John Wiley and Sons. p. 192. ISBN 978-0-470-25762-3.
^ ^a ^b ^c Jones, Jane H. (2000). "The Cativa Process for the Manufacture of Acetic Acid". Platinum Metals Review. 44 (3): 94–105.
^ Sunley, G. J.; Watson, D. J. (2000). "High Productivity Methanol Carbonylation Catalysis using Iridium - The Cativa Process for the Manufacture of Acetic Acid". Catalysis Today. 58 (4): 293–307. doi:10.1016/S0920-5861(00)00263-7.
^ Kissin, Y. V. (2008). "Synthesis, Chemical Composition, and Structure of Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization". Alkene Polymerization Reactions with Transition Metal Catalysts. Amsterdam: Elsevier. pp. 207–290. ISBN 978-0-444-53215-2.

External links

[1] Elschenbroich, C. (2006). Organometallics. Weinheim: Wiley-VCH. ISBN 978-3-527-29390-2.

[Hartwig-2] Hartwig, J. F. (2010). Organotransition Metal Chemistry, from Bonding to Catalysis. New York, NY: University Science Books. ISBN 978-1-891389-53-5.

[3] Yadav, M. S. (2005). Quick Review in Inorganic Chemistry. Anmol Publications. p. 244. ISBN 978-81-261-1898-4.

[4] F. Calderazzo, F. A. Cotton. “The Carbonylation of Methyl Manganese Pentacarbonyl and Decarbonylation of Acetyl Manganese Pentacarbonyl”. Inorg. Chem., 1962, 1, 30–36. https://doi.org/10.1021/ic50001a008.

[5] See note 13.

[6] L. E. Orgel. “The Infrared Spectra of Substituted Metal Carbonyls”. Inorg. Chem., 1962, 1, 25–29. https://doi.org/10.1021/ic50001a007.

[7] R. L. Burwell Jr., R. G. Pearson. “The Principle of Microscopic Reversibility”. Phys. Chem., 1966, 70, 300–302. https://doi.org/10.1021/j100873a508.

[8] K. Noack, M. Ruch, F. Calderazzo. “The Mechanism of Reaction of Methylmanganese Pentacarbonyl and Acetylmanganese Pentacarbonyl with Triphenylphosphine”. Inorg. Chem., 1968, 7, 345–349. https://doi.org/10.1021/ic50060a037.

[9] M. Andersen, J. R. Moss. “Synthesis of an Extensive Series of Manganese Pentacarbonyl Alkyl and Acyl Compounds: Carbonylation and Decarbonylation Studies on [Mn(R)(CO)₅] and [Mn(COR)(CO)₅]”. Organometallics, 1994, 13, 5013–5020. https://doi.org/10.1021/om00024a051.

[10] E. J. Kuhlmann, J. J. Alexander. “Carbon monoxide insertion into transition metal-carbon sigma-bonds”. Coord. Chem. Rev., 1980, 33, 195-225. https://doi.org/10.1016/S0010-8545(00)80454-3.

[11] F. Calderazzo. “Synthetic and Mechanistic Aspects of Inorganic Insertion Reactions. Insertion of Carbon Monoxide”. Angew. Chem. 1977, 165, 299-311. https://doi.org/10.1002/anie.197702991.

[12] M. T. Whited, G. E. Hofmeister. “Synthesis and Migratory-Insertion Reactivity of CpMo(CO)₃(CH₃): Small-Scale Organometallic Preparations Utilizing Modern Glovebox Techniques”. J. Chem. Educ. 2014, 91, 1050–1053. https://doi.org/10.1021/ed500114a.

[13] S. Weber, K. Kirchner. “Manganese Alkyl Carbonyl Complexes: From Iconic Stoichiometric Textbook Reactions to Catalytic Applications”. Acc. Chem. Res. 2022, 55, 2740−2751. https://doi.org/10.1021/acs.accounts.2c00470.

[ACR-14] Anderson, G. K.; Cross, R. J. (1984). "Carbonyl-Insertion Reactions of Square Planar Complexes". Acc. Chem. Res. 17 (2): 67–74. doi:10.1021/ar00098a005.

[CCR-15] Cavell, K. J. (1996). "Recent Fundamental Studies on Migratory Insertion into Metal-Carbon Bonds". Coord. Chem. Rev. 155 (11): 209–243. doi:10.1016/S0010-8545(96)90182-4.

[Brown-16] Alexander, J.J. (1985). "Insertions into transition metal-carbon bonds". In Hartley; Patai (eds.). The chemistry of the metal–carbon bond. Vol. 2. John Wiley & Sons. doi:10.1002/9780470771747.ch5.

[Shusterman-17] Shusterman, A. J.; Tamir, I.; Pross, A. (1988). "The Mechanism of Organometallic Migration Reactions. A Configuration Mixing (CM)Approach". J. Organomet. Chem. 340 (2): 203–222. doi:10.1016/0022-328X(88)80076-7.

[18] Fristrup, Peter; Kreis, Michael; Palmelund, Anders; Norrby, Per-Ola; Madsen, Robert (2008). "The Mechanism for the Rhodium-Catalyzed Decarbonylation of Aldehydes: A Combined Experimental and Theoretical Study". J. Am. Chem. Soc. 130 (15): 5206–5215. doi:10.1021/ja710270j. PMID 18303836. S2CID 207119793.

[OHNO-19] Ohno, K.; Tsuji, J. (1968). "Organic Synthesis by means of Noble Metal Compounds. XXXV. Novel Decarbonylation Reactions of Aldehydes and Acyl Halides using Rhodium Complexes". J. Am. Chem. Soc. 90 (1): 99–107. doi:10.1021/ja01003a018.

[DMA-20] Douglas; McDaniel; Alexander (1994). Concepts and Models of Inorganic Chemistry (3rd ed.). John Wiley & Sons, Inc. ISBN 978-0-471-62978-8.

[Puddephatt-21] Puddephatt, R.A.; Stalteri, M.A. (1980). "Competition between Insertion of Sulfur Dioxide into the Methyl- or Phenyl- Transition Metal Bond". Journal of Organometallic Chemistry. 193: C27–C29. doi:10.1016/S0022-328X(00)86091-X.

[Burger_et_al.-22] Burger, B. J.; Thompson, M. E.; Cotter, W. D.; Bercaw, J. E. (1990). "Ethylene Insertion and β-hydrogen Elimination for Permethylscandocene Alkyl Complexes. A Study of the Chain Propagation and Termination Steps in Ziegler-Natta Polymerization of Ethylene". J. Am. Chem. Soc. 112 (4): 1566–1577. doi:10.1021/ja00160a041.

[Crabtree-23] Crabtree, R. H. (2009). The Organometallic Chemistry of the Transition Metals. John Wiley and Sons. p. 192. ISBN 978-0-470-25762-3.

[Cativa-24] Jones, Jane H. (2000). "The Cativa Process for the Manufacture of Acetic Acid". Platinum Metals Review. 44 (3): 94–105.

[25] Sunley, G. J.; Watson, D. J. (2000). "High Productivity Methanol Carbonylation Catalysis using Iridium - The Cativa Process for the Manufacture of Acetic Acid". Catalysis Today. 58 (4): 293–307. doi:10.1016/S0920-5861(00)00263-7.

[26] Kissin, Y. V. (2008). "Synthesis, Chemical Composition, and Structure of Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization". Alkene Polymerization Reactions with Transition Metal Catalysts. Amsterdam: Elsevier. pp. 207–290. ISBN 978-0-444-53215-2.

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@@ Line 18: / Line 18: @@
 CO inserts into a metal-[[alkyl]] bond via migratory insertion. The key concept is that both the CO and the alkyl groups are ligands on the same metal. For example, the reaction of <sup>13</sup>CO with Mn(CO)<sub>5</sub>CH<sub>3</sub> exclusively form Mn(CO)<sub>4</sub>(<sup>13</sup>CO)COCH<sub>3</sub>. The alkyl group migrates intramolecularly to an adjacent CO ligand within the [[coordination sphere]] of the [[Manganese|Mn]](I) centre. Subsequent to the migration, the metal binds free CO (see figure below).<ref name=Hartwig>{{cite book|last=Hartwig|first=J. F.|title=Organotransition Metal Chemistry, from Bonding to Catalysis|publisher=University Science Books|location=New York, NY|date=2010|isbn=978-1-891389-53-5}}</ref><ref>{{cite book|title = Quick Review in Inorganic Chemistry|last=Yadav|first= M. S.|url = https://books.google.com/books?id=qlwlWsaoLwcC&q=Mn%28CO%295CH3+insertion+reaction&pg=PA244|publisher = Anmol Publications|year = 2005|page = 244|isbn = 978-81-261-1898-4}}</ref>
 :[[File:Insertion reaction for an octahedral complex.png|thumb|center|606px|CO Insertion reaction pathway for an octahedral complex]]
+The [[Equilibrium constant|equilibrium]] and the rates of the forward and reverse reactions in the system <chem>[MnCH3(CO)5] + CO <=> [Mn(CO)CH3(CO)5] </chem> were studied at atmospheric pressure of CO in various solvents and at several temperatures by Calderazzo and Cotton in 1961.<ref>F. Calderazzo, F. A. Cotton. “The Carbonylation of Methyl Manganese Pentacarbonyl and Decarbonylation of Acetyl
+Manganese Pentacarbonyl”. '''''Inorg. Chem.''''', 1962, 1, 30–36. https://doi.org/10.1021/ic50001a008.</ref>                   The reaction proved to be first order in both MnCH<sub>3</sub>(CO)<sub>5</sub> and CO and the effect of changing the solvent appeared to be mainly a function of the [[dielectric constant]] in stabilizing a somewhat polar transition state. The volume of [[Carbon monoxide|CO]] absorbed or evolved at a given time was determined by gas-volumetric measurements. A scheme of the burette used for the experiments is shown in the figure 3. Once the volume of CO was measured, it was converted to [[Mole (unit)|moles]] of CO and the CO/Mn mole ratio was determined. By measuring the amount of CO absorbed as a function of time, it was possible to study the kinetics of the carbonylation reaction.
+[[File:Gas-volumetric burette.png|thumb|Figure 3. Gas-volumetric burette: '''A''' marks the 100 mL burette; '''B''' the Hg containing bulb; '''C''' the capillary tube connected to atmosphere; '''D''' the three-way stopcock (CO flux); '''E''' the 50 mL two-neck Erlenmayer flask (reaction system). Liquid Hg is colored Red and the Yellow meniscus is the same solvent contained in Erlenmayer flask. The apparatus is thermostatically controlled.|center|303x303px]]
+In 1967 Calderazzo and Noack re-examined the prototype system using <sup>13</sup>CO, in gas phase 50% enriched.<ref>See note 13.</ref> The reaction product was investigated by [[infrared spectroscopy]], monitoring changes in terminal CO stretching bands. The compound MnL(CO)<sub>5</sub>, where L can be a methyl or [[acetyl group]], has three stretching active IR vibrations due to C<sub>4v</sub> [[molecular symmetry]].<ref>L. E. Orgel. “The Infrared Spectra of Substituted Metal Carbonyls”. '''''Inorg. Chem.''''', 1962, 1, 25–29. https://doi.org/10.1021/ic50001a007.</ref> The key signals were the axial <sup>12</sup>CO and <sup>13</sup>CO stretching bands (trans to L), respectively at 1991 cm<sup>-1</sup> and 1949 cm<sup>-1</sup>. Compared to <sup>12</sup>CO, the 13-labelled molecule shows a C-O bond stretching absorption shifted to lower wavenumbers. First of all, it was observed that the incoming <sup>13</sup>CO have not been inserted in between the metal carbon bond Mn-CH<sub>3</sub>. They obtained a 100% cis isomer of [Mn(COCH<sub>3</sub>)(<sup>13</sup>CO)(CO)<sub>4</sub>] (scheme 1).
+[[File:CO insertion.png|thumb|Scheme 1. Note the solvent changing in the backward reaction: heptane is a high-boiling solvent, suitable for the temperature indicated.|center|364x364px]]
+Two hypothetical mechanisms had been previously proposed: the CO or CH<sub>3</sub> intramolecular migration. Calderazzo and Noack designed an experiment based on the Principle of [[Microscopic reversibility|Microscopic Reversibility]]:<ref>R. L. Burwell Jr., R. G. Pearson. “The Principle of Microscopic Reversibility”. '''''Phys. Chem.''''', 1966, 70, 300–302. https://doi.org/10.1021/j100873a508.</ref>  in mechanistic terms, if a certain series of steps constitutes the mechanism of a reverse reaction, the mechanism of the forward reaction, under the same conditions, is given by the same steps traversed forwards. They examined the decarbonylation of acetyl complex, prepared either by about 50% enriched [MnCH<sub>3</sub>(<sup>13</sup>CO)(CO)<sub>4</sub>] or prepared from CH<sub>3</sub>(<sup>13</sup>CO)CI and Na[Mn(CO)<sub>5</sub>]. They found a mixture of cis and trans product [MnCH<sub>3</sub>(CO)<sub>4</sub>(<sup>13</sup>CO)], 50% and 25% respectively (in ratio of 2:1). They obtained also 25% of unlabelled CH<sub>3</sub>Mn(CO)<sub>5</sub> (scheme 2). According to the Principle of Microscopic Reversibility, this result confirmed the intramolecular CH<sub>3</sub> migration hypothesis, because the carbonyl migration process should have shown only the cis product [MnCH<sub>3</sub>(<sup>13</sup>CO)(CO)<sub>4</sub>] with a probability of 75% and the free <sup>13</sup>CO product with 25% .
+[[File:Methyl migration.png|thumb|Scheme 2. Methyl migration.|center|507x507px]]
+Successive researches revealed also that cis-regiospecific alkyl or aryl migrations occur due to addition of neutral ligands (2 electron donor),<ref>K. Noack, M. Ruch, F. Calderazzo. “The Mechanism of Reaction of Methylmanganese Pentacarbonyl and Acetylmanganese Pentacarbonyl with Triphenylphosphine”. '''''Inorg. Chem.''''', 1968, 7, 345–349. https://doi.org/10.1021/ic50060a037.</ref> and that a penta-coordinate intermediate, involved in the intramolecular mechanism, allows the retention of configuration (scheme 3). The kinetics are reminiscent of [[dissociative substitution]] except that the vacant site is formed at the metal during the migratory step, not by loss of a [[ligand]]. Using the steady-state method, the rate is given: '''<math display="inline">v = -{d[R] \over dt}= \left( \frac{k(1)k(2)[L][R]}{k(-1)+k(2)[L]} \right)</math>'''
+It was reported that the rates of the carbonylation (and the decarbonylation) reaction are dependent on the alkyl group R. Plotting the rate constats versus the number of carbon atoms in the alkyl chain for the reaction of [Mn(R)(CO)<sub>5</sub>] with [[Triphenylphosphine|PPh<sub>3</sub>]], it is confirmed an increase from [[Methyl group|methyl]] to [[n-propyl]] derivate, followed by a slowly decrease.<ref>M. Andersen, J. R. Moss. “Synthesis of an Extensive Series of Manganese Pentacarbonyl Alkyl and Acyl Compounds: Carbonylation and Decarbonylation Studies on [Mn(R)(CO)<sub>5</sub>] and [Mn(COR)(CO)<sub>5</sub>]”. '''''Organometallics''''', 1994, 13, 5013–5020. https://doi.org/10.1021/om00024a051.</ref> Further experiments revealed that, not only Lewis acids are able to increase the reaction rate activating the carbonyl group, but also stabilizing the product and [[transition state]]. Other factors accelerating the reaction are polar, [[Electron-withdrawing group|electron-withdrawing]] containing solvent, and bulky ligands L<sub>n</sub>.<ref>E. J. Kuhlmann, J. J. Alexander. “Carbon monoxide insertion into transition metal-carbon sigma-bonds”. '''''Coord. Chem. Rev.''''', 1980, 33, 195-225. https://doi.org/10.1016/S0010-8545(00)80454-3.</ref><ref>F. Calderazzo. “Synthetic and Mechanistic Aspects of Inorganic Insertion Reactions. Insertion of Carbon Monoxide”. '''''Angew. Chem.''''' 1977, 165, 299-311. https://doi.org/10.1002/anie.197702991.</ref>
+[[File:Mechanism of migratory insertion.png|thumb|Scheme 3. Kinetic constants K(n) are colored orange.|center|455x455px]]
+The involvement of migratory insertion in catalytic cycles of large industrial interest has contributed to maintain continuous interest in this mechanism.<ref>M. T. Whited, G. E. Hofmeister. “Synthesis and Migratory-Insertion Reactivity of CpMo(CO)<sub>3</sub>(CH<sub>3</sub>): Small-Scale Organometallic Preparations Utilizing Modern Glovebox Techniques”. '''''J. Chem. Educ.''''' 2014, 91, 1050–1053. https://doi.org/10.1021/ed500114a.</ref> For instance, recently it has been highlighted the potential of alkyl [[Manganese|Mn]](I) carbonyl-based homogenous catalysts. New investigations have explored the catalyzed activation under milder conditions of nonpolar or moderately polar bond such as [[Hydrogen|H]]−H, [[Boron|B]]-H, [[Carbon|C]]-H and [[Silicon|Si]]-H.<ref>S. Weber, K. Kirchner. “Manganese Alkyl Carbonyl Complexes: From Iconic Stoichiometric Textbook Reactions to Catalytic Applications”. '''''Acc. Chem. Res''.''' 2022, 55, 2740−2751. https://doi.org/10.1021/acs.accounts.2c00470.</ref>
 CO insertion does not always involve migration. Treatment of CpFe(L)(CO)CH<sub>3</sub> with <sup>13</sup>CO yields a mix of both alkyl migration products and products formed by true insertion of bound [[carbonyls]] into the [[methyl]] group. Product distribution is influenced by the choice of [[solvent]].<ref name="ACR">{{cite journal|last1 = Anderson|first1 = G. K.|last2 = Cross|first2 = R. J.|title = Carbonyl-Insertion Reactions of Square Planar Complexes|journal = [[Acc. Chem. Res.]]|year = 1984|volume = 17|issue = 2|pages = 67–74|doi = 10.1021/ar00098a005}}</ref>