16.05.03 · inorgchem / organometallic

Homogeneous catalysis: Wilkinson's catalyst and Ziegler-Natta polymerization

stub3 tiersLean: nonepending prereqs

Anchor (Master): Hartwig — Organotransition Metal Chemistry (2010)

Intuition Beginner

A catalyst speeds up a chemical reaction without being consumed. The catalyst participates in the reaction but regenerates at the end of each cycle, so a tiny amount can convert enormous quantities of reactant. Organometallic catalysts use transition metals as the active centre, cycling through a repeating sequence of elementary steps.

Wilkinson's catalyst, RhCl(PPh $_{3}$ ) $_{3}$ , hydrogenates alkenes at room temperature and atmospheric pressure. The rhodium centre grabs H $_{2}$ , binds the alkene, inserts the alkene into a Rh-H bond, and then eliminates the alkane product. The catalyst returns to its original form, ready for the next molecule. This cycle repeats thousands of times per second.

Ziegler-Natta catalysts convert ethylene and propylene into polyethylene and polypropylene, the most-produced plastics on Earth. A titanium active site binds the alkene monomer, inserts it into a growing Ti-C polymer chain, and repeats. Each insertion adds one monomer unit. A single active site can chain together millions of monomers before the polymer is released.

Both catalysts work through cycles of the four elementary steps from the previous unit: oxidative addition, migratory insertion, reductive elimination, and ligand dissociation. The power of homogeneous catalysis lies in combining these simple steps into productive loops.

Visual Beginner

A two-panel diagram. Left panel: the Wilkinson hydrogenation cycle as a circular arrow diagram showing RhCl(PPh $_{3}$ ) $_{3}$ at the top, H $_{2}$ oxidative addition on the left, alkene coordination at the bottom, migratory insertion on the right, and reductive elimination returning to the top with alkane product. Right panel: the Ziegler-Natta Cossee-Arlman propagation cycle as a repeating linear sequence showing Ti-alkyl, ethylene coordination, migratory insertion to give Ti-alkyl extended by one CH $_{2}$ unit.

Two-panel catalytic cycle schematic: Wilkinson hydrogenation (left) and Cossee-Arlman Ziegler-Natta propagation (right).

Worked example Beginner

Wilkinson hydrogenation of cyclohexene.

Starting catalyst: RhCl(PPh $_{3}$ ) $_{3}$ , a square-planar Rh(I) complex with 16 valence electrons. The catalytic cycle has five stages:

Dissociation. One PPh $_{3}$ ligand falls off, leaving RhCl(PPh $_{3}$ ) $_{2}$ (14 electrons, open site).
Oxidative addition of H $_{2}$ . H $_{2}$ adds across the open site: RhCl(PPh $_{3}$ ) $_{2}$ + H $_{2}$ $\to$ RhCl(H) $_{2}$ (PPh $_{3}$ ) $_{2}$ . Rh goes from +1 to +3, electron count from 14 to 16.
Alkene coordination. Cyclohexene binds to the remaining open site, giving an 18-electron Rh(III) complex.
Migratory insertion. One hydride migrates to the coordinated alkene, forming a Rh-alkyl bond and opening a site. Electron count drops to 16.
Reductive elimination. The remaining hydride and the alkyl couple to form cyclohexane, which leaves. Rh drops from +3 back to +1, regenerating RhCl(PPh $_{3}$ ) $_{2}$ .

Net reaction: C $_{6}$ H $_{10}$ + H $_{2}$ $\to$ C $_{6}$ H $_{12}$ . The rhodium complex is unchanged after one full turn of the cycle.

Check your understanding Beginner

Exercise (medium, short-answer).

In the Wilkinson cycle, why must one PPh $_{3}$ ligand dissociate before H $_{2}$ can add? Use the 18-electron rule.

Hint

RhCl(PPh $_{3}$ ) $_{3}$ is a 16-electron complex. Oxidative addition adds 2 electrons and 2 ligands. What electron count would result if no dissociation occurred first?

Answer

RhCl(PPh $_{3}$ ) $_{3}$ already has 16 electrons. Without dissociation, oxidative addition of H $_{2}$ would produce a 20-electron complex, exceeding the 18-electron limit and creating a 7-coordinate intermediate that is geometrically and electronically disfavoured. Dissociation of one PPh $_{3}$ drops the count to 14, allowing OA to reach 16 electrons — still within the stable range. Alternatively, the dissociated complex directly coordinates H $_{2}$ as a sigma complex first (reaching 16), then undergoes OA to reach 18.

Formal definition Intermediate+

Catalytic turnover. A catalytic cycle is a closed sequence of elementary steps at a metal centre that converts substrates to products while regenerating the active catalyst. The turnover number (TON) is the number of substrate molecules converted per catalyst molecule before deactivation. The turnover frequency (TOF) is TON per unit time (s $^{- 1}$ ).

Wilkinson's catalyst cycle. RhCl(PPh $_{3}$ ) $_{3}$ catalyses the hydrogenation of alkenes at 25 °C and 1 atm H $_{2}$ . The accepted mechanism (Halpern, 1974) proceeds via the following steps:

Ligand dissociation. RhCl(PPh $_{3}$ ) $_{3}$ $⇌$ RhCl(PPh $_{3}$ ) $_{2}$ + PPh $_{3}$ . The 16-electron Rh(I) complex loses one phosphine to give a 14-electron species with a vacant coordination site. This equilibrium is rapid and the 14-electron species is the resting state.
Oxidative addition of H $_{2}$ . RhCl(PPh $_{3}$ ) $_{2}$ + H $_{2}$ $\to$ RhCl(H) $_{2}$ (PPh $_{3}$ ) $_{2}$ . Concerted three-centre addition. Rh(I) $\to$ Rh(III), electron count 14 $\to$ 16. The two hydrides are cis.
Alkene coordination. RhCl(H) $_{2}$ (PPh $_{3}$ ) $_{2}$ + alkene $\to$ RhCl(H) $_{2}$ (alkene)(PPh $_{3}$ ) $_{2}$ . The alkene occupies the remaining open site, giving an 18-electron Rh(III) dihydride alkene complex.
Migratory insertion (1,2-insertion). One hydride migrates to the coordinated alkene: RhCl(H)(alkyl)(PPh $_{3}$ ) $_{2}$ + open site. The electron count drops from 18 to 16 as the hydride and alkene merge into one alkyl ligand.
Reductive elimination. The remaining hydride and the alkyl couple: RhCl(H)(alkyl)(PPh $_{3}$ ) $_{2}$ $\to$ RhCl(PPh $_{3}$ ) $_{2}$ + alkane. Rh(III) $\to$ Rh(I), electron count 16 $\to$ 14. The catalyst is regenerated.

The net reaction is alkene + H $_{2}$ $\to$ alkane. The rate law under typical conditions is approximately rate = $k [Rh] [H_{2}] [alkene] [PPh_{3}]^{- 1}$ , confirming that dissociation of PPh $_{3}$ is a prerequisite.

Ziegler-Natta polymerisation: the Cossee-Arlman mechanism. Heterogeneous Ziegler-Natta catalysts (TiCl $_{3}$ or TiCl $_{4}$ /MgCl $_{2}$ activated by AlR $_{3}$ ) polymerise ethylene and propylene. The active site is a d $^{0}$ Ti(IV) centre with a vacant coordination site and a Ti-alkyl bond (formed by alkylation of Ti-Cl with AlR $_{3}$ ). The Cossee-Arlman mechanism:

Active site formation. TiCl $_{3}$ (surface) + AlR $_{3}$ $\to$ RTiCl $_{2}$ (surface) + AlR $_{2}$ Cl. A surface Ti-Cl bond is alkylated, creating the growing chain and leaving a chloride vacancy (the coordination site for monomer).
Monomer coordination. The alkene (ethylene or propylene) coordinates to the vacant site at Ti through its pi-electrons, forming a pi-complex.
Migratory insertion (1,2-insertion). The Ti-alkyl bond inserts into the coordinated alkene in a syn fashion. The alkyl migrates to the alkene carbon, extending the polymer chain by one monomer unit and regenerating the vacant site.

Ti-CH_{2} CH_{2} R + CH_{2} =CH_{2} ⟶ Ti-CH_{2} CH_{2} CH_{2} CH_{2} R

Steps 2 and 3 repeat, growing the polymer chain. Because Ti(IV) is d $^{0}$ , there is no oxidative addition or reductive elimination — the cycle consists solely of alternating coordination and migratory insertion. The oxidation state remains +4 throughout.

Chain termination. Beta-hydride elimination from the Ti-alkyl gives a Ti-H and releases the polymer with a terminal vinyl group. Alternatively, chain transfer to monomer or to AlR $_{3}$ terminates the chain.

Stereoselectivity: isotactic polypropylene. The key commercial achievement of Ziegler-Natta catalysis is isotactic polypropylene, where all methyl groups are on the same side of the polymer backbone. A chiral active site on the TiCl $_{3}$ surface (or on a chiral metallocene catalyst) controls the face of propylene coordination, ensuring that each monomer inserts with the same absolute configuration.

Key mechanism Intermediate+

The Wilkinson cycle alternates between Rh(I) and Rh(III) via oxidative addition and reductive elimination, with migratory insertion converting the coordinated alkene into a metal-alkyl intermediate. The Ziegler-Natta Cossee-Arlman cycle bypasses redox changes entirely — the d $^{0}$ Ti(IV) centre propagates through alternating alkene coordination and 1,2-migratory insertion with no change in oxidation state.

Catalytic cycle analysis Intermediate+

Proposition (steady-state rate of Wilkinson hydrogenation). Under conditions where substrate is in large excess and the catalyst resting state is RhCl(PPh $_{3}$ ) $_{2}$ , the rate of hydrogenation is first-order in catalyst, first-order in H $_{2}$ , first-order in alkene, and inverse-first-order in free PPh $_{3}$ concentration.

Derivation. The rate-determining step is the oxidative addition of H $_{2}$ to the 14-electron RhCl(PPh $_{3}$ ) $_{2}$ . The concentration of this species is governed by the pre-equilibrium:

K_{diss} = \frac{[ RhCl(PPh _{3} ) _{2} ] [ PPh _{3} ]}{[ RhCl(PPh _{3} ) _{3} ]}

The rate of OA is $r = k_{OA} [RhCl(PPh_{3})_{2}] [H_{2}]$ . Substituting the equilibrium expression:

r = \frac{k _{OA} K _{diss} [ Rh ] _{total} [ H _{2} ]}{[ PPh _{3} ] + K _{diss}}

When $[PPh_{3}] ≫ K_{diss}$ (excess phosphine), this simplifies to $r \approx (k_{OA} K_{diss} / [PPh_{3}]) [Rh]_{total} [H_{2}]$ , giving the observed inverse-first-order dependence on phosphine concentration. The alkene dependence enters at the coordination/insertion stage and is first-order under typical conditions. $■$

Exercises Intermediate+

Exercise 4 (hard, short-answer).

A metallocene catalyst Cp $_{2}$ ZrCl $_{2}$ /MAO polymerises propylene to give isotactic polypropylene when the catalyst has C $_{2}$ symmetry, but atactic polypropylene when the catalyst has no symmetry. Explain the stereochemical control.

Hint

The chiral environment of the metallocene determines which face of propylene coordinates. What happens if every monomer sees the same chiral environment vs. a racemic mixture?

Answer

A C $_{2}$ -symmetric metallocene (e.g., rac-ethylenebis(indenyl)ZrCl $_{2}$ ) has two enantiotopic coordination sites that are related by a C $_{2}$ axis but are locally homotopic. Propylene always coordinates with the same face orientation relative to the growing chain, producing isotactic insertion (all methyl groups on the same side). A meso or achiral metallocene has no such enantiocontrol: propylene inserts with random face selectivity, giving atactic polymer (random methyl orientation). The C $_{2}$ symmetry ensures that the two possible coordination sites present identical chiral environments, so there is no "mistake" site — every insertion has the same stereochemical outcome.

Exercise 5 (hard, short-answer).

Compare the Wilkinson hydrogenation cycle with the Wacker oxidation cycle (PdCl $_{2}$ /CuCl $_{2}$ /O $_{2}$ converting ethylene to acetaldehyde). Identify which elementary steps are shared and which differ.

Hint

Both cycles involve alkene coordination and migratory insertion. How is the Pd cycle re-oxidised? Does Wacker use H $_{2}$ ?

Answer

Shared steps: alkene coordination to the metal, and migratory insertion of the alkene into a M-X bond (M-H for Wilkinson, M-OH for Wacker).

Differences:

Wilkinson uses OA of H $_{2}$ and RE of alkane. Wacker uses no OA of H $_{2}$ — instead, water coordinates to Pd and the alkene inserts into the Pd-OH bond (migratory insertion into a Pd-O bond, forming a Pd-CH $_{2}$ CH $_{2}$ OH intermediate).
Wacker then undergoes beta-hydride elimination to release acetaldehyde and form Pd-H, which decomposes to Pd(0). The Pd(0) is re-oxidised to Pd(II) by CuCl $_{2}$ (a redox co-catalyst), which is in turn re-oxidised by O $_{2}$ .
Wilkinson is a closed cycle on Rh (Rh(I)/Rh(III) redox couple). Wacker requires an external oxidant (Cu(II)/O $_{2}$ ) to regenerate the Pd(II) catalyst — it is a net oxidation of ethylene, not a hydrogenation.

Homogeneous catalysis: advanced topics Master

Asymmetric hydrogenation

The Wilkinson hydrogenation of a prochiral alkene gives a racemic mixture. Introducing chiral ligands on the metal creates a chiral environment that differentiates the two enantiotopic faces of the alkene, enabling enantioselective hydrogenation. Knowles (2001 Nobel) demonstrated this with the L-DOPA synthesis using a chiral diphosphine Rh catalyst. Noyori (2001 Nobel) developed BINAP-Ru catalysts that achieve >99% ee for a wide range of substrates.

The enantioselectivity arises from a combination of steric and electronic effects in the transition state for migratory insertion. The chiral diphosphine creates a chiral pocket around the metal: one prochiral face of the alkene fits comfortably while the other suffers from steric repulsion with the ligand substituents. The energy difference between the two diastereomeric transition states (typically 2-4 kcal/mol) determines the enantiomeric excess.

Key mechanistic insight: for many Rh-diphosphine catalysts, the major product does not come from the major diastereomeric alkene-catalyst complex. The less stable diastereomeric complex is more reactive, so it reacts faster and delivers the major enantiomer. This "anti-Curtin-Hammett" behaviour was established by Halpern and Landis through kinetic studies of the Rh-DIPAMP system.

Metallocene catalysts for stereoregular polymerisation

Metallocene catalysts (Cp $_{2}$ MCl $_{2}$ activated by methylaluminoxane, MAO) offer single-site polymerisation with exquisite stereocontrol. Unlike conventional heterogeneous Ziegler-Natta catalysts (which have multiple active-site types and broad molecular-weight distributions), metallocenes provide a uniform active site, producing polymers with narrow dispersity.

The stereochemistry of polypropylene is controlled by the metallocene symmetry:

C $_{2}$ -symmetric metallocenes (e.g., rac-Et(Ind) $_{2}$ ZrCl $_{2}$ ) give isotactic polypropylene. The C $_{2}$ axis ensures that both coordination sites present the same chiral environment, so every insertion has the same sense of stereochemical induction.
C $_{2 v}$ -symmetric metallocenes (e.g., Cp $_{2}$ ZrCl $_{2}$ ) give atactic polypropylene. No chiral induction.
C $_{s}$ -symmetric metallocenes (e.g., Me $_{2}$ C(Cp)(Flu)ZrCl $_{2}$ ) give syndiotactic polypropylene. The mirror plane enforces alternating face selectivity.

The active species is a 14-electron [Cp $_{2}$ Zr-R] $^{+}$ cation. MAO serves as both alkylating agent (replacing Cl with Me) and abstractor (removing Me $^{-}$ to generate the cationic active site). The propagation cycle is identical to the Cossee-Arlman mechanism: alkene coordination followed by 1,2-migratory insertion, with no change in Zr(IV) oxidation state.

Wacker oxidation

The Wacker process converts ethylene to acetaldehyde using PdCl $_{2}$ , CuCl $_{2}$ , and O $_{2}$ in aqueous solution. The catalytic cycle involves:

Ethylene coordination to Pd(II).
Nucleophilic attack by water on the coordinated ethylene (anti addition), forming a Pd-OH-CH $_{2}$ CH $_{2}$ -OH intermediate (hydroxypalladation).
Beta-hydride elimination to give acetaldehyde and Pd-H.
Reductive elimination of HCl from Pd-H to give Pd(0).
Re-oxidation of Pd(0) to Pd(II) by CuCl $_{2}$ : Pd(0) + 2 CuCl $_{2}$ $\to$ PdCl $_{2}$ + 2 CuCl.
Re-oxidation of Cu(I) to Cu(II) by O $_{2}$ .

The key mechanistic feature is the anti-hydroxypalladation: water attacks the coordinated ethylene from the face opposite to Pd, generating a Pd-OH intermediate that undergoes beta-HE. The stereochemistry of this step was established by Smidt using cis- and trans-CHD=CHD as mechanistic probes.

Hydroformylation

Hydroformylation (oxo process) converts alkenes to aldehydes using CO and H $_{2}$ in the presence of a Co or Rh catalyst. It is the largest-volume industrial homogeneous catalytic process.

Cobalt cycle (Roelen, 1938): HCo(CO) $_{4}$ dissociates one CO to give the 16-electron HCo(CO) $_{3}$ . The alkene coordinates, undergoes migratory insertion to form a Co-alkyl, CO coordinates, CO inserts (1,1-insertion) to form a Co-acyl, H $_{2}$ undergoes oxidative addition to Co(I) giving Co(III) dihydride acyl, and reductive elimination releases the aldehyde. The linear-to-branched ratio is ~3:1.

Rhodium cycle (Wilkinson-type): HRh(CO)(PPh $_{3}$ ) $_{2}$ is the active species under modified conditions. The rhodium cycle is analogous but faster and operates at lower pressure (10-20 atm vs. 200-300 atm for Co). Modified rhodium catalysts with bulky phosphines give >95% linear aldehyde selectivity by steering the regiochemistry of alkene insertion toward the anti-Markovnikov product.

Catalytic C-H functionalisation

Direct activation and functionalisation of C-H bonds by organometallic catalysts bypasses the need for pre-functionalised substrates (halides, triflates). Key approaches:

Directed C-H activation. A coordinating directing group (DG) on the substrate chelates the metal, positioning it near a specific C-H bond. The metal undergoes concerted metalation-deprotonation (CMD) through a six-membered transition state, forming a metallacycle. Subsequent functionalisation (olefination, arylation, acetoxylation) delivers the product. Pd(II)/Pd(0) and Rh(III)/Rh(I) catalytic manifolds are most common.
Undirected C-H activation. Without a directing group, the metal must discriminate among many similar C-H bonds. Approaches include transient directing groups, electrochemical oxidation to generate highly reactive high-valent Pd(IV) or Pd(III) intermediates, and photoredox-assisted C-H activation using Ir or Ru photocatalysts.
C-H borylation. Ir catalysts with bidentate nitrogen ligands (e.g., [Ir(OMe)(COD)] $_{2}$ /dtbpy) catalyse the borylation of arene and alkane C-H bonds with B $_{2}$ pin $_{2}$ . The mechanism proceeds through oxidative addition of B-B to Ir(I), C-H oxidative addition to Ir(III), and reductive elimination of the C-B bond. The selectivity is governed by steric accessibility rather than C-H bond strength: the least sterically hindered C-H bond is borylated first.

Connections Master

Organometallic reaction types 16.05.02 pending. Every catalytic cycle in this unit is assembled from the four elementary steps (OA, RE, MI, beta-HE) plus ligand dissociation. Wilkinson hydrogenation uses OA + MI + RE. Ziegler-Natta uses MI only (with coordination). Wacker uses MI + beta-HE + external redox.
Electron counting and the 18-electron rule 16.05.01. The need for dissociation before OA in the Wilkinson cycle, the stability of intermediates at 16 and 18 electrons, and the d $^{0}$ restriction preventing OA in Ziegler-Natta all follow directly from electron-counting arguments.
Cross-coupling reactions 15.09.02 pending. Palladium-catalysed cross-coupling (Suzuki, Stille, Heck, Negishi) uses the same elementary steps as the catalytic cycles here: OA of R-X, transmetallation, and RE. The Heck reaction additionally uses migratory insertion and beta-hydride elimination, exactly analogous to the Wacker oxidation.
Coordination chemistry 16.04.01. The cis/trans geometry of intermediates, the trans-influence on reaction rates, and the steric control of enantioselectivity in asymmetric hydrogenation all build on the coordination geometry principles from the coordination chemistry chapter.
Bioinorganic chemistry 16.06.01. Hydrogenase enzymes catalyse H $_{2}$ oxidation using Fe-Fe or Ni-Fe active sites that undergo oxidative addition and reductive elimination of H $_{2}$ , mechanistically parallel to Wilkinson's catalyst. Cobalamin (vitamin B $_{1} 2$ ) uses Co-C bond homolysis and radical recombination analogous to RE and OA.

Historical notes Master

Wilkinson's catalyst. Geoffrey Wilkinson and J. F. Young reported RhCl(PPh $_{3}$ ) $_{3}$ in 1965 and demonstrated its catalytic hydrogenation of alkenes at ambient conditions. Wilkinson shared the 1973 Nobel Prize with E. O. W. Fischer for sandwich compounds and organometallic chemistry. The detailed mechanism was established by Halpern and co-workers (1974-1976), who showed that the dissociated 14-electron species RhCl(PPh $_{3}$ ) $_{2}$ is the active catalyst and that OA of H $_{2}$ is rate-determining. The anti-Curtin-Hammett behaviour in asymmetric hydrogenation was elucidated by Halpern and Landis in the early 1980s.

Ziegler-Natta polymerisation. Karl Ziegler discovered in 1953 that mixtures of TiCl $_{4}$ and AlEt $_{3}$ polymerise ethylene at mild conditions to give linear high-density polyethylene, replacing the high-pressure radical process. Giulio Natta extended this to propylene in 1954, producing isotactic polypropylene using TiCl $_{3}$ -based catalysts. Ziegler and Natta shared the 1963 Nobel Prize. The Cossee-Arlman mechanism was proposed by Cossee (1960) and Arlman (1964), establishing the active-site model. The development of MgCl $_{2}$ -supported high-activity catalysts in the 1970s (Montedison and Mitsui) transformed polypropylene production.

Metallocene catalysts. Kaminsky and Sinn discovered in 1980 that Cp $_{2}$ ZrCl $_{2}$ activated by methylaluminoxane (MAO) is an exceptionally active ethylene polymerisation catalyst. Brintzinger, Ewen, and Kaminsky then developed chiral ansa-metallocenes for stereoregular polypropylene (1984-1988). Ewen demonstrated the symmetry-selectivity relationships (C $_{2}$ for isotactic, C $_{s}$ for syndiotactic) in 1988.

Asymmetric hydrogenation. Knowles at Monsanto developed the first practical asymmetric hydrogenation for L-DOPA production (1970s), using a chiral diphosphine Rh catalyst. Noyori developed BINAP-Ru catalysts in the 1980s, achieving practical enantioselective hydrogenation of ketones and olefins. Knowles, Noyori, and Sharpless shared the 2001 Nobel Prize.

Wacker oxidation. Smidt and co-workers at Wacker Chemie developed the PdCl $_{2}$ /CuCl $_{2}$ process for acetaldehyde production from ethylene in the late 1950s. The anti-hydroxypalladation mechanism was established through stereochemical probes by Smidt and by Henry in the 1960s and 1970s.

Hydroformylation. Otto Roelen at Ruhrchemie discovered the cobalt-catalysed hydroformylation of alkenes (the "oxo process") in 1938. The rhodium-catalysed low-pressure process was developed by Wilkinson, Pruett, and co-workers at Union Carbide in the 1970s, using phosphine-modified Rh catalysts for improved linear selectivity.

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