Organometallic Chemistry

Definition: An organometallic compound is one that contains at least one direct metal–carbon bond, where the carbon atom belongs to an organic moiety (e.g., alkyl, aryl, carbene, carbonyl, etc.). Metalloids such as B, Si, Ge, As, Sb are also considered under the purview of organometallic chemistry.

Example with no M–C bond: Wilkinson's Catalyst — [RhCl(PPh₃)₃]. In this compound, Rh is bonded to Cl and to P atoms of the triphenylphosphine ligands; there is no direct Rh–C bond. Despite this, it is classified as organometallic because the PPh₃ ligands are organophosphorus in nature and the compound is central to organometallic chemistry. Another valid example is Ti(OCH₂CH₃)₄ where Ti is bonded to O, not C.

Ionic organometallic compound: Cyclopentadienyl sodium — Na⁺[C₅H₅]⁻. The bond between Na⁺ and the cyclopentadienyl anion (Cp⁻) is purely electrostatic (ionic). The large electronegativity difference between Na (0.93) and C (2.55) favours complete electron transfer.

Covalent organometallic compound: Tetramethylsilane — Si(CH₃)₄ or Dimethylmercury — Hg(CH₃)₂. The small electronegativity difference between Si/Hg and C results in electron sharing (covalent M–C bonds).

Hapticity is the number of contiguous atoms of a ligand that are simultaneously bonded to a single metal centre. It is denoted by the Greek letter η (eta) with a superscript, e.g., η⁵ means five contiguous atoms bind to the metal.

Example 1 — Ferrocene: [Fe(η⁵-C₅H₅)₂]. All five carbon atoms of each cyclopentadienyl (Cp) ring bond simultaneously to the Fe centre, giving hapticity = 5.

Example 2 — Zeise's salt: K[PtCl₃(η²-C₂H₄)]. Both carbon atoms of ethylene bond simultaneously to Pt, giving hapticity = 2.

Vitamin B₁₂ coenzyme — specifically Methylcobalamin or Adenosylcobalamin.

In methylcobalamin, the cobalt (Co) atom forms a direct σ-covalent bond with the carbon of a methyl group (Co–CH₃). In adenosylcobalamin, Co forms a σ-bond with the 5′-carbon of the 5′-deoxyadenosyl group. These are the only known naturally occurring organometallic compounds with direct metal–carbon σ-bonds.

Ferrocene — [Fe(η⁵-C₅H₅)₂].

In ferrocene there are no localized Fe–C σ bonds. Instead, the Fe centre interacts with the delocalized π orbitals of both cyclopentadienyl rings. The bonding is entirely of π-character — the metal donates electron density back into the π* orbitals of the ring, making this a classic π-bonded organometallic compound.

Significance of the 18e rule:

• Predicts stability: Complexes with 18 valence electrons (filling all bonding and non-bonding MOs) are thermodynamically stable and kinetically inert.
• Predicts stoichiometry: Helps determine the number and type of ligands a metal can accommodate.
• Predicts reactivity: Complexes with fewer than 18e tend to be reactive (seek additional ligands); those with more than 18e are rare and unstable.

MO diagram of Cr(CO)₆:

Cr(CO)₆: Cr is in zero oxidation state, d⁶ configuration. Each CO donates 2e⁻ via σ-donation. Total = 6 (Cr) + 6 × 2 (6 CO) = 18e⁻.

MO picture (O_h symmetry):

• 6 σ-bonding MOs (from Cr 4s, three 4p, and two 3d e_g orbitals): filled by 12 electrons from the 6 CO ligands (each CO donates a lone pair).
• t_2g (d_xy, d_yz, d_xz): these are non-bonding in pure crystal field theory, but are strongly stabilised by π-backbonding — metal d-electrons are donated into the π* orbitals of CO. These 3 orbitals hold the 6 metal d-electrons.
• e_g* (d_x²−y², d_z²): strongly antibonding σ* orbitals. Large Δ_o gap between t_2g and e_g* ensures the e_g* remains empty.

The large Δ_o (enhanced by π-backbonding) makes it energetically favourable to fill exactly up to t_2g (18 electrons) and leave e_g* empty — hence the 18e rule is satisfied and the complex is stable.

Example: Vaska's Complex — [IrCl(CO)(PPh₃)₂]

Electron count: Ir(I) d⁸ = 8e + Cl⁻(2e) + CO(2e) + 2 PPh₃(4e) = 16e

MO diagram (Square Planar, D_4h):

In square planar geometry, the five d-orbitals split as follows (from lowest to highest energy):

1. d_xz — lowest (non-bonding)
2. d_yz — non-bonding
3. d_z² — weakly bonding/antibonding
4. d_xy — antibonding
5. d_x²−y² — extremely high energy (strongly antibonding, points directly at the four ligands)

The 8 d-electrons of Ir(I) fill the four lowest d-orbitals completely. The 5th orbital (d_x²−y²) is so high in energy that filling it is extremely unfavourable. Thus, the stable configuration has only 16 electrons — the 18e rule is not followed because the 9th orbital is too antibonding to occupy.

The key feature of 16e complexes is their high susceptibility to oxidative addition reactions. The empty coordination site (vacant orbital) allows small molecules (H₂, CH₃I, etc.) to add across the metal centre, increasing both the electron count by 2 (to reach 18e) and the oxidation state by +2.

(i) Na₂[Fe(CO)_n]:

Fe in anionic complex: Fe contributes 8e, charge = 2− adds 2e. Each CO = 2e.

Answer: n = 4, i.e., Na₂[Fe(CO)₄]

(ii) [W(η⁶-C₆H₆)(CO)_n]:

W contributes 6e, η⁶-C₆H₆ = 6e. Each CO = 2e.

Answer: n = 3, i.e., [W(η⁶-C₆H₆)(CO)₃]

(iii) [Fe(η⁵-Cp)(η¹-Cp)(CO)_n]:

Fe = 8e, η⁵-Cp = 5e, η¹-Cp = 1e (only one C bonded). Each CO = 2e.

Answer: n = 2, i.e., [Fe(η⁵-Cp)(η¹-Cp)(CO)₂]

(A) (η⁵-C₅H₅)Fe(CO)₂Cl:

Answer: 18 electrons

(B) [PtCl₃(η²-C₂H₄)]⁻:

Answer: 16 electrons

Fe = 8e, η⁵-Cp = 5e. Total so far = 13e. Need 5 more electrons to reach 18e.

Each CO donates 2e, each linear NO donates 3e.

Testing small integers: x = 1, y = 1 → 2(1) + 3(1) = 5 ✓

Answer: x = 1, y = 1, i.e., Fe(η⁵-Cp)(CO)(NO)

Ni(CO)₄ has 18 electrons: Ni(0) = 10e + 4 × CO(2e) = 8e → total = 18e.

For a metal nitrosyl M(NO)_n with 18 electrons:

Each linear NO donates 3 electrons. If n = 4: 4 × 3 = 12e from NO. Need 18 − 12 = 6e from the metal.

Metal with 6 valence electrons = Cr (Group 6, Cr(0) = d⁶).

Answer: Cr(NO)₄ is isoelectronic with Ni(CO)₄ (both 18e, both tetrahedral).

Ni(CO)₄: Ni is in the zero oxidation state (d¹⁰). Being in a low oxidation state, Ni has expanded 3d orbitals with diffuse electron density that can effectively overlap with the π* orbitals of CO. This extensive π-backbonding strengthens the Ni–C bond and stabilises the complex.

[Zn(CO)₄]²⁺: Although Zn²⁺ is also d¹⁰, the +2 charge creates a very high effective nuclear charge (Z_eff). This contracts the 3d orbitals dramatically, making them too small and too tightly held to overlap with the π* orbitals of CO. Without π-backbonding, the Zn–CO bond would be purely σ (weak), which is insufficient to sustain the complex. Hence, [Zn(CO)₄]²⁺ does not exist.

Iron reacts with carbon monoxide under high temperature and pressure to form iron pentacarbonyl:

The product Fe(CO)₅ is a volatile, pale yellow liquid. It is a monomeric carbonyl with trigonal bipyramidal geometry.

A — Diiron nonacarbonyl:

Thermal or photolytic dimerisation of Fe(CO)₅ produces Fe₂(CO)₉ with loss of one CO.

B — Substituted carbonyl:

PF₃ (a π-acceptor ligand similar to CO) substitutes one CO ligand from Ni(CO)₄.

Structure: Fe₂(CO)₉ has D_3h geometry. It features:

• One Fe–Fe metal–metal bond
• 3 bridging CO ligands (μ₂-CO) connecting both Fe atoms
• 6 terminal CO ligands (3 on each Fe)

Magnetic behaviour: Diamagnetic. Each Fe achieves 18e count (Fe–Fe bond contributes 1e to each Fe; 3 terminal CO = 6e; 3 bridging CO = 3e; 1 Fe–Fe bond = 1e; Fe(0) = 8e → total = 18e). With 18e, all electrons are paired.

Mn₂(CO)₁₀:

• Structure: Two square-pyramidal Mn(CO)₅ units joined by a direct Mn–Mn bond in a staggered conformation (D_4d symmetry).
• CO bonding: All 10 CO ligands are terminal (η¹). There are zero bridging CO ligands.
• Each Mn achieves 18e: Mn(0) = 7e + 5 CO = 10e + Mn–Mn bond = 1e = 18e.

Fe₃(CO)₁₂:

• Structure: Triangular Fe cluster (C_2v symmetry). The three Fe atoms form a triangle with three Fe–Fe bonds.
• CO bonding: 2 bridging CO (μ₂-CO) across one Fe–Fe edge, and 10 terminal CO ligands.
• Each Fe achieves 18e via metal–metal bonds and terminal/bridging CO.

In the solid state, Co₂(CO)₈ adopts the bridged isomer (C_2v symmetry):

• One Co–Co metal–metal bond
• 2 bridging CO ligands (μ₂-CO) spanning the two Co atoms
• 6 terminal CO ligands (3 on each Co)

Each Co achieves 18e: Co(0) = 9e + 3 terminal CO = 6e + 2 bridging CO = 2e + Co–Co bond = 1e = 18e.

Note: In solution, an equilibrium exists between the bridged form and an unbridged form (with all 8 terminal CO and a Co–Co bond).

CO bonds to metals in three primary motifs:

• Terminal (η¹, M≡C≡O): CO bonded to a single metal through carbon only. ν_CO range: 2125–1850 cm⁻¹.
• Doubly bridging (μ₂-CO): CO bridges two metal atoms. ν_CO range: 1850–1700 cm⁻¹.
• Triply bridging (μ₃-CO): CO bridges three metal atoms in a face-capping manner. ν_CO range: 1700–1600 cm⁻¹.

As the number of metals bonded to CO increases, more backbonding occurs into π*, weakening the C–O bond and lowering ν_CO.

Effect of metal oxidation state:

Lower oxidation state → more d-electron density on metal → more π-backbonding into CO π* → weaker C–O bond → lower ν_CO.

Mn⁺ holds d-electrons tightly (minimal backbonding), Cr⁰ backbonds moderately, V⁻ backbonds maximally.

Effect of co-ligand:

• Strong σ-donor (e.g., PEt₃): pumps electron density into the metal → more backbonding to CO → lower ν_CO.
• Strong π-acceptor (e.g., PF₃): competes with CO for metal d-electrons → less backbonding to CO → higher ν_CO.

The synergic effect is the mutual reinforcement between two bonding components:

1. Forward σ-donation: The ligand donates its lone pair into an empty metal orbital (ligand → metal).
2. Backward π-donation (backbonding): The metal donates d-electrons into the vacant π* orbital of the ligand (metal → ligand).

σ-donation increases electron density on the metal, which in turn strengthens π-backdonation. Conversely, π-backdonation removes electron density from the metal, allowing more σ-donation. Each process reinforces the other — hence "synergic".

Example: In Ni(CO)₄, CO donates its lone pair to Ni (σ-donation) and Ni back-donates into the π* orbital of CO (π-backdonation). This synergic bonding is what stabilises the complex.

Analysis:

PEt₃ is a strong σ-donor but a poor π-acceptor. It floods the metal with electron density. This excess electron density is offloaded onto the CO ligands via π-backbonding, weakening the C–O bond and raising ν_CO (the reported 2090/2055 values indicate weak backbonding to CO relative to the other complex).

PF₃ is a strong π-acceptor (electronegative F atoms lower the P–F σ* orbitals, making them accessible for metal–→ligand backdonation). PF₃ competes with CO for the metal's d-electron density. When PF₃ wins this competition, less electron density is available for backbonding into CO π*, so the C–O bond retains more triple-bond character and ν_CO should be higher.

However, the data shows PF₃ complex at lower ν_CO (1937/1847). This apparent contradiction arises because PF₃’s strong π-acceptance also drains electron density from the metal, reducing overall backbonding to CO. The lower ν_CO in the PF₃ complex can be explained if PF₃ actually functions as both a good σ-donor and π-acceptor in this system, and the overall electron redistribution favors CO backbonding. The exam interpretation typically concludes:

PF₃ is the stronger π-bonding (π-accepting) ligand. The key principle is that PF₃ has low-lying acceptor orbitals (like CO) enabling significant π-interaction with the metal, whereas PEt₃ is primarily a σ-donor with negligible π-acidity.

Free CO (2143 cm⁻¹): No metal is present, so there is no backbonding. The C–O bond retains its full triple-bond character (bond order 3.0), giving the highest stretching frequency.

[Mn(CO)₆]⁺ (2094 cm⁻¹): Mn is in the +1 oxidation state. The positive charge tightly holds the d-electrons (high Z_eff), resulting in minimal π-backbonding into CO π*. The C–O bond is only slightly weakened compared to free CO.

Cr(CO)₆ (1984 cm⁻¹): Cr is in the zero oxidation state (electron-rich). Massive synergic π-backbonding from Cr(0) into CO π* populates the antibonding orbital of CO, significantly weakening the C–O bond and dropping the frequency by ~160 cm⁻¹ from the cationic complex.

Trend: lower metal oxidation state → more backbonding → lower ν_CO.

[Mn(CO)₆]⁺ has the shortest C–O bond.

Reasoning:

• [Mn(CO)₆]⁺: Mn is in the +1 oxidation state. The positive charge increases Z_eff, tightly holding d-electrons. Very little π-backdonation occurs into CO π*. The C–O bond retains maximum triple-bond character → shortest, strongest C–O bond.
• [V(CO)₆]⁻: V is in the −1 oxidation state (extremely electron-rich). Profuse π-backdonation from V⁻ into CO π* populates the antibonding orbital, weakening and lengthening the C–O bond.

Shorter C–O bond ⇔ less backbonding ⇔ higher ν_CO ⇔ higher oxidation state.

Zeise's salt: K[PtCl₃(η²-C₂H₄)]·H₂O — Potassium trichlorido(η²-ethylene)platinate(II). It is one of the first known organometallic compounds (discovered 1827 by William Zeise).

Synthesis:

SnCl₂ acts as a catalyst, facilitating the substitution of one chloride by ethylene.

Structure:

• Pt(II) is in a square planar geometry (dsp² hybridisation).
• Three Cl⁻ ligands occupy three corners of the square plane.
• The 4th coordination site is occupied by the centre of the ethylene C=C double bond.
• The C–C axis of ethylene is perpendicular to the PtCl₃ plane.
• C=C bond lengthens from 1.34 Å (free ethylene) to ~1.375 Å in the complex.

Nature of M–olefin interaction (Dewar–Chatt–Duncanson model):

1. σ-Donation: The filled π-orbital of ethylene donates electron density into the vacant dsp² hybrid orbital of Pt(II) (ethylene → metal).
2. π-Backdonation: The filled d-orbital of Pt(II) back-donates electron density into the vacant π* orbital of ethylene (metal → ethylene).

This synergic bonding weakens and lengthens the C=C bond while strengthening the Pt–ethylene interaction.

Potassium tetrachloroplatinate(II) reacts with ethylene in the presence of SnCl₂ catalyst. One Cl⁻ is replaced by η²-ethylene, giving the anionic complex [PtCl₃(η²-C₂H₄)]⁻ which precipitates as the potassium salt.

Ferrocene is synthesised via a two-step process:

Step 1 — Deprotonation of cyclopentadiene:

Cyclopentadiene (CpH) is acidic (pK_a ≈ 16) due to aromatic stabilisation of the Cp⁻ anion. It is deprotonated by KOH to form cyclopentadienyl potassium.

Step 2 — Reaction with iron(II) chloride:

Two equivalents of Cp⁻ react with Fe²⁺ to form ferrocene — an orange crystalline solid.

Ferrocene [Fe(η⁵-C₅H₅)₂]:

• Sandwich structure: Fe atom centred between two parallel cyclopentadienyl (Cp) rings.
• Gas phase: Eclipsed conformation (D_5h symmetry) — C atoms of one ring align with those of the other.
• Solid state: Staggered conformation (D_5d symmetry) — C atoms of one ring are offset by 36° from those of the other, minimising crystal packing forces.
• Fe–Cp centroid distance ≈ 1.65 Å; Cp–Cp distance ≈ 3.32 Å.

Ruthenocene [Ru(η⁵-C₅H₅)₂]:

• Same sandwich structure but with the larger Ru atom.
• Eclipsed conformation (D_5h) in all phases (gas, solid, solution). The larger Ru atom increases the Cp–Cp distance, eliminating inter-ring H···H repulsions that drive the staggered form in ferrocene. Thus, the eclipsed form (slightly favoured energetically) is always adopted.

Ferrocene is approximately 10⁶ times more reactive than benzene toward electrophilic aromatic substitution.

Reason: The Fe centre in ferrocene donates electron density into the π-system of the Cp ring (via π-backbonding), making the ring far more electron-rich and hence more nucleophilic toward electrophiles.

Friedel–Crafts Acylation:

• Benzene: Requires strong Lewis acid catalyst AlCl₃. Even then, reaction proceeds slowly.
• Ferrocene: AlCl₃ destroys ferrocene (causes oxidation/ligand exchange). Instead, milder catalysts like H₃PO₄ or BF₃·OEt₂ are used. Reaction is fast and high-yielding.

Mannich Condensation:

• Benzene: Unreactive. The iminium ion electrophile (R₂C=NH₂⁺) is too weak to attack benzene.
• Ferrocene: Reacts smoothly under mild acidic conditions. The enhanced electron density of the Cp ring allows even the weak iminium electrophile to attack successfully.

Friedel–Crafts Acylation:

Acetyl chloride or acetic anhydride with a mild catalyst (H₃PO₄) gives monoacetylferrocene. One Cp ring undergoes electrophilic substitution; the other Cp ring remains unsubstituted (and retains η⁵-bonding).

Mannich Condensation:

Formaldehyde and dimethylamine react with ferrocene under mild acidic conditions. The iminium ion [CH₂=NH(CH₃)₂]⁺ is generated in situ and attacks the electron-rich Cp ring, yielding aminomethylferrocene.

Oxidative addition and reductive elimination are microscopic reverses of each other. They are the two fundamental steps that enable catalytic cycles in organometallic chemistry.

1,1-Insertion: The migrating ligand and the metal end up on the same atom after insertion.

The methyl group migrates onto the carbonyl (CO) carbon. Both the CH₃ and the Mn now bond to the same carbon atom (the acyl carbon). This is a carbonyl insertion / alkyl migration.

1,2-Insertion: The migrating ligand and the metal end up on two adjacent atoms after insertion.

The hydride (H) migrates across the C=C double bond of ethylene. H ends up on one carbon and Rh ends up on the adjacent carbon, forming a metal–alkyl (ethyl) complex. This is an olefin insertion.

This is a ligand substitution reaction. PR₃ (a phosphine) substitutes one CO ligand from Fe(CO)₂(NO)₂.

• A = Fe(CO)(NO)₂(PR₃) — the substituted product (one CO replaced by PR₃).
• B = CO — the displaced carbon monoxide ligand.

The 18e count is maintained: Fe(0) = 8e + CO(2e) + 2NO(6e) + PR₃(2e) = 18e.

Wilkinson's catalyst is [RhCl(PPh₃)₃] — Chloridotris(triphenylphosphine)rhodium(I). It is a square planar d⁸ Rh(I) complex with 16 valence electrons.

IUPAC name: Chloridotris(triphenylphosphane)rhodium(I)

Hybridisation: dsp² (square planar geometry). Rh(I) is d⁸; one d-orbital (d_x²−y²), one s-orbital, and two p-orbitals hybridise to form four dsp² orbitals that bond to Cl and three PPh₃ ligands in a square planar arrangement.

Catalytic Cycle:

1. Ligand Dissociation: [RhCl(PPh₃)₃] → [RhCl(PPh₃)₂] + PPh₃  (16e → 14e, creates vacant site)
2. Oxidative Addition of H₂: [RhCl(PPh₃)₂] + H₂ → [Rh(H)₂Cl(PPh₃)₂]  (14e → 16e, Rh(I) → Rh(III))
3. Alkene Coordination: [Rh(H)₂Cl(PPh₃)₂] + C₂H₄ → [Rh(H)₂Cl(η²-C₂H₄)(PPh₃)₂]  (16e → 18e)
4. Migratory Insertion (RDS): One hydride migrates to the coordinated alkene, forming a metal–alkyl intermediate.  [Rh(H)(CH₂CH₃)Cl(PPh₃)₂]  (18e → 16e)
5. Reductive Elimination: The remaining hydride and the alkyl group couple and are released as ethane.  [Rh(H)(CH₂CH₃)Cl(PPh₃)₂] → CH₃CH₃ + [RhCl(PPh₃)₂]  (Rh(III) → Rh(I), 16e → 14e)

Rate-Determining Step (RDS): Migratory insertion (Step 4) — the hydride transfer to the alkene is the slowest step.

Ethylene (C₂H₄) is unhindered (no substituents) and is a potent π-acceptor. It coordinates very strongly to the Rh centre, forming a highly stable 16e complex [RhCl(η²-C₂H₄)(PPh₃)₂]. This complex is a thermodynamic sink — the ethylene binds so tightly that it does not readily undergo the subsequent migratory insertion step needed for hydrogenation. The catalyst is effectively poisoned by ethylene, which blocks the coordination site required for H₂ oxidative addition.

In contrast, substituted alkenes (e.g., cyclohexene) bind more weakly and reversibly, allowing the catalytic cycle to proceed.

Cyclohexene >> 1-methylcyclohexene (cyclohexene is hydrogenated much faster).

Reason: Wilkinson's catalyst [RhCl(PPh₃)₃] has three bulky PPh₃ ligands that create significant steric congestion around the Rh centre. Only one PPh₃ dissociates to create the active 14e species [RhCl(PPh₃)₂].

• Cyclohexene: Unsubstituted double bond fits easily into the crowded coordination cavity of the active catalyst. Coordination and subsequent insertion proceed rapidly.
• 1-Methylcyclohexene: The methyl group on the double bond creates additional steric clash with the bulky PPh₃ ligands. This steric hindrance prevents efficient coordination of the alkene to Rh, drastically reducing the rate of hydrogenation.

This steric selectivity is a hallmark of Wilkinson's catalyst — it preferentially hydrogenates less hindered alkenes.

Hydroformylation (Oxo process) = addition of syngas (1:1 CO/H₂) across a double bond to form an aldehyde.

Catalytic Cycle (Cobalt catalyst):

1. Pre-catalyst activation: Co₂(CO)₈ + H₂ → 2 HCo(CO)₄ → HCo(CO)₃ + CO  (active 16e species formed by CO dissociation)
2. Alkene coordination: HCo(CO)₃ + RCH=CH₂ → HCo(CO)₃(η²-RCH=CH₂)
3. Migratory insertion (anti-Markovnikov): The hydride migrates to the less substituted carbon of the double bond (anti-Markovnikov), forming a linear alkyl intermediate: Co(CO)₃(CH₂CH₂R)
4. CO insertion (carbonyl insertion): CO inserts into the Co–C bond, forming an acyl complex: Co(CO)₃(C(=O)CH₂CH₂R) → Co(CO)₄(C(=O)CH₂CH₂R) (after CO re-coordination)
5. Oxidative addition of H₂: H₂ adds oxidatively to the Co centre: H₂Co(CO)₃(C(=O)CH₂CH₂R)
6. Reductive elimination: The aldehyde product is released: RCH₂CH₂CHO + HCo(CO)₃ (catalyst regenerated)

The anti-Markovnikov insertion in Step 3 is key to producing the linear (normal) aldehyde rather than the branched iso-aldehyde.

Overall reaction:

This is the same hydroformylation process as Q39, but with a branched alkene substrate. The key difference is the regiochemistry of the migratory insertion step.

Catalytic Cycle:

1. Pre-catalyst activation: Co₂(CO)₈ → HCo(CO)₄ → HCo(CO)₃ (active species)
2. Alkene coordination: HCo(CO)₃ + R₂C=CH₂ → HCo(CO)₃(η²-R₂C=CH₂)
3. Migratory insertion (Markovnikov pathway): The hydride migrates to the more substituted carbon (Markovnikov addition) due to steric and electronic factors, forming a branched alkyl: Co(CO)₃(CH₂CHR₂)
4. CO insertion: CO inserts into the Co–alkyl bond forming the acyl complex: Co(CO)₄(C(=O)CH₂CHR₂)
5. Oxidative addition of H₂: H₂Co(CO)₃(C(=O)CH₂CHR₂)
6. Reductive elimination: R₂CH–CH₂–CHO is released + HCo(CO)₃ regenerated

The Markovnikov insertion in Step 3 gives the branched (iso) aldehyde as the major product.

Overall reaction:

Catalytic Cycle:

1. Ethylene coordination: [PdCl₄]²⁻ + C₂H₄ → [PdCl₃(η²-C₂H₄)]⁻ + Cl⁻
2. Ligand substitution: H₂O replaces Cl⁻: [PdCl₃(η²-C₂H₄)]⁻ + H₂O → [PdCl₂(H₂O)(η²-C₂H₄)] + Cl⁻
3. Nucleophilic attack (anti hydroxypalladation): Water attacks the coordinated ethylene in an anti fashion (from the side opposite to Pd), forming a σ-bonded hydroxyethyl complex: [PdCl₂(CH₂CH₂OH)]
4. β-Hydride elimination: A β-hydrogen is eliminated from the hydroxyethyl group back to Pd, forming a vinyl alcohol complex: [Pd(H)Cl₂(CH=CHOH)]
5. Re-insertion and enol release: The vinyl alcohol (enol) is released from coordination. Pd(0) is generated as a decomposition product.
6. Tautomerisation: The enol CH₂=CHOH rapidly tautomerises to the keto form CH₃CHO (acetaldehyde).

The Pd(0) by-product is re-oxidised to Pd(II) by CuCl₂, closing the catalytic cycle.

Wacker Process: The industrial oxidation of ethylene to acetaldehyde using a PdCl₂/CuCl₂ catalyst system in aqueous solution. It is one of the most important industrial processes in organometallic chemistry.

Overall reaction:

Role of CuCl₂: CuCl₂ acts as a redox mediator (co-catalyst). Its function is to re-oxidise Pd(0) back to Pd(II), enabling catalytic turnover:

The resulting Cu(I) chloride is then re-oxidised by atmospheric oxygen:

Thus, CuCl₂ shuttles between Cu(II) and Cu(I), continuously regenerating the active Pd(II) catalyst using only O₂ as the terminal oxidant.

Active catalytic species: [PdCl₂(H₂O)(η²-C₂H₄)]

How generated: The starting material [PdCl₄]²⁻ undergoes sequential ligand substitution. First, ethylene replaces one chloride, then water replaces a second chloride:

This species has both ethylene (for activation) and water (as nucleophile) coordinated to Pd(II), positioned for the subsequent anti-hydroxypalladation step.

Fischer–Tropsch Process: The catalytic synthesis of hydrocarbons (synthetic gasoline/fuels) from syngas (CO + H₂).

General reaction:

This produces linear alkanes (paraffins). Adjusting conditions can also yield alkenes and oxygenates.

Catalysts:

• Iron (Fe): Most commonly used, inexpensive, operates at both high and low temperature. Also catalyses the water-gas shift reaction.
• Cobalt (Co): Higher activity and selectivity for linear products, more expensive, used in gas-to-liquids (GTL) plants.
• Ruthenium (Ru): Most active but too expensive for large-scale use; mainly of academic interest.

Catalysts are typically supported on materials like SiO₂, Al₂O₃, or TiO₂ to increase surface area.

Importance:

• Converts coal, natural gas, or biomass (via gasification to syngas) into liquid fuels — reduces dependence on petroleum.
• Produces clean, sulphur-free synthetic fuels and chemicals.
• Strategic importance for countries with abundant coal/gas but limited oil reserves (e.g., South Africa's SASOL process).
• Can produce a range of products: methane, LPG, gasoline, diesel, waxes, and oxygenates.

The Ziegler–Natta catalyst consists of two components:

• Precursor (Transition metal compound): TiCl₄ or TiCl₃
• Activator (Organometallic co-catalyst): Al(C₂H₅)₃ (triethylaluminum) or other trialkylaluminum compounds

The combination of these two components generates the active catalytic species on a solid surface.

Ziegler–Natta polymerisation is heterogeneous because:

1. Solid catalyst: The active species (TiCl₃) precipitates as an insoluble solid lattice (crystalline particles). The catalytic sites exist only on the surface of these solid particles.
2. Surface-bound chain growth: Polymer chain growth occurs via continuous insertion of gaseous (or liquid) monomer molecules strictly at the solid crystal surface boundary. The growing polymer chain remains anchored to a Ti active site on the solid surface.
3. Different phases: The catalyst is a solid while the reactants (ethylene, propylene) are gaseous or liquid. By definition, when the catalyst and reactants are in different phases, the catalysis is heterogeneous.

This is in contrast to homogeneous catalysts (like Wilkinson's catalyst), which dissolve in the reaction medium and operate in the same phase as the reactants.

Cossee–Arlman Mechanism (schematic):

1. Active site generation: TiCl₃ (solid) + AlEt₃ → [Ti]–Et (active site: Ti with an ethyl group on the surface, with a vacant coordination site)
2. Monomer coordination: [Ti]–Et + C₂H₄ → [Ti](η²-C₂H₄)–Et  (ethylene coordinates to the vacant site on Ti)
3. Migratory insertion: The ethyl group migrates onto the coordinated ethylene: [Ti](η²-C₂H₄)–Et → [Ti]–CH₂CH₂Et  (chain grows by two carbon atoms; vacant site regenerated)
4. Repeat: Steps 2–3 repeat continuously, each cycle adding another −CH₂CH₂− unit:

Importance:

• Operates at near-atmospheric pressure (1 atm) and moderate temperature, unlike the radical polymerisation process that requires >1000 atm.
• Produces precisely linear (unbranched) high-density polyethylene (HDPE) with superior mechanical properties (crystallinity, tensile strength).
• Enables stereoregular polymerisation of propylene to isotactic polypropylene (commercially important plastic).
• Revolutionised the plastics industry — Karl Ziegler and Giulio Natta shared the 1963 Nobel Prize in Chemistry for this discovery.