Arrange the C–O stretching frequencies of the isoelectronic carbonyls in increasing order.

[V(CO)₆]⁻ < Cr(CO)₆ < [Mn(CO)₆]⁺

[Mn(CO)₆]⁺ < Cr(CO)₆ < [V(CO)₆]⁻

Cr(CO)₆ < [V(CO)₆]⁻ < [Mn(CO)₆]⁺

[V(CO)₆]⁻ < [Mn(CO)₆]⁺ < Cr(CO)₆

Which description fits a Fischer carbene complex such as (CO)₅Cr=C(OMe)Ph?

Low-valent metal, π-acceptor co-ligands, electrophilic carbene carbon

High-valent early metal, nucleophilic carbene carbon

The carbene carbon carries only H and alkyl groups

It reacts with ketones like a Wittig reagent

Why is (Me₃SiCH₂)₄Ti far more thermally stable than (CH₃CH₂)₄Ti?

The CH₂SiMe₃ group has no β-hydrogen, so β-hydride elimination is impossible

The silyl group is a π-acceptor

In the Monsanto acetic acid process catalysed by cis-[Rh(CO)₂I₂]⁻, which step is rate-determining?

Oxidative addition of CH₃I to Rh(I)

Migratory insertion of methyl to CO

Reductive elimination of CH₃COI

In the Wacker process, ethylene is oxidised to

acetaldehyde, with the oxygen coming from water

ethylene oxide, with the oxygen coming from O₂

acetic acid, with the oxygen coming from O₂

Which statements about hydroformylation are correct?

It converts an alkene, CO and H₂ into an aldehyde with one more carbon

The acyl group forms by migratory insertion of an alkyl onto CO

HCo(CO)₄ is the active species in cobalt-catalysed hydroformylation

It converts an alkene, CO and H₂ into an aldehyde with one more carbon

The acyl group forms by migratory insertion of an alkyl onto CO

HCo(CO)₄ is the active species in cobalt-catalysed hydroformylation

Only the branched aldehyde is formed

In the Cossee–Arlman mechanism of Ziegler–Natta polymerisation, chain growth occurs by

coordination of the alkene at a vacant Ti site followed by migratory insertion into the Ti–C bond

free-radical addition initiated by AlEt₃

cationic polymerisation at aluminium

Why does the Fischer–Tropsch reaction give a broad range of hydrocarbons rather than a single product?

Chain growth on the surface is a repeated probabilistic addition of C₁ units, giving an Anderson–Schulz–Flory distribution

The catalyst decomposes during the reaction

CO does not adsorb on iron or cobalt

The reaction is reversible at every step

Organometallic Chemistry and Catalysis

The Organometallics sub-heading of Section 2 of the GATE Chemistry (CY) paper, in the syllabus’s own order: the 18-electron rule; metal–alkyl, metal–carbonyl, metal–olefin and metal–carbene complexes and metallocenes; fluxionality in organometallic complexes; the types of organometallic reactions; homogeneous catalysis — hydrogenation, hydroformylation, the methanol-to-acetic-acid process, olefin metathesis and the Wacker oxidation; and heterogeneous catalysis — the Fischer–Tropsch reaction and Ziegler–Natta polymerisation. Electron counts use the neutral-ligand (covalent) convention throughout, with the ionic convention noted where it differs; both give the same total.

1. The 18-electron rule and electron counting

Stable low-valent organometallic complexes of the d-block usually have 18 valence electrons around the metal, filling its nine valence orbitals (five d, one s, three p). In the neutral (covalent) method the metal contributes its group number, and each ligand the electrons it brings as a neutral species; the overall charge is then subtracted (or added for an anion). In the ionic method the ligands are counted as closed-shell ions (H⁻, CH₃⁻, Cl⁻, Cp⁻ as 2, 2, 2 and 6) and the metal as its dⁿ ion. The two methods always agree on the total.

Ligand electron counts (neutral method)
Ligand	Electrons	Ligand	Electrons
H, CH₃, Cl, aryl (X-type)	1	η³-allyl	3
CO, PR₃, NH₃, H₂O (L-type)	2	η⁴-butadiene	4
η²-alkene, η²-alkyne	2	η⁵-cyclopentadienyl (Cp)	5
carbene =CR₂ (Fischer)	2	η⁶-benzene	6
NO, linear	3	M–M bond	1 to each metal

Worked counts: Cr(CO)₆ = 6 + 12 = 18; Fe(CO)₅ = 8 + 10 = 18; Ni(CO)₄ = 10 + 8 = 18; ferrocene Fe(η⁵-C₅H₅)₂ = 8 + 5 + 5 = 18; Cr(η⁶-C₆H₆)₂ = 6 + 12 = 18; CpMn(CO)₃ = 7 + 5 + 6 = 18. The 17-electron Mn(CO)₅ dimerises through a Mn–Mn bond to Mn₂(CO)₁₀ (18 each), while V(CO)₆ (17) is a stable exception because six CO ligands crowd out a dimer. Square planar d⁸ complexes are stable at 16 electrons — Wilkinson’s RhCl(PPh₃)₃, Vaska’s trans-IrCl(CO)(PPh₃)₂, [PtCl₄]²⁻ — and much catalysis runs through 16- and 18-electron intermediates in alternation. Metallocenes beyond iron exceed 18: cobaltocene (19) is readily oxidised to the 18-electron cobaltocenium ion, and nickelocene (20) has two unpaired electrons. For clusters, the M–M bond count follows from the deficit: M_x needs 18x electrons, and each M–M bond supplies two to the count.

2. Metal alkyls, carbonyls, olefins, carbenes, metallocenes and fluxionality

Metal alkyls are kinetically unstable mainly because of β-hydride elimination, which needs a hydrogen on the β-carbon and a vacant cis site: M–CH₂CH₃ → M(H)(CH₂=CH₂). Alkyls without β-hydrogens — methyl, benzyl, neopentyl CH₂CMe₃, CH₂SiMe₃ — or with coordinatively saturated metals are far more robust. Metal carbonyls bind by σ-donation from the carbon lone pair into an empty metal orbital and π-back-donation from filled metal d orbitals into CO π*. Back-donation weakens the C–O bond, so the IR stretching frequency falls below free CO (2143 cm⁻¹) as the metal becomes more electron-rich: [Mn(CO)₆]⁺ about 2090, Cr(CO)₆ about 2000, [V(CO)₆]⁻ about 1860 cm⁻¹. Terminal CO absorbs near 1850–2120, doubly bridging μ₂-CO near 1700–1860 cm⁻¹.

Metal–olefin bonding follows the Dewar–Chatt–Duncanson model, the same σ-donation (from the C=C π orbital) plus π-back-donation (into C=C π*): in Zeise’s salt K[PtCl₃(C₂H₄)] the C=C lengthens slightly and the hydrogens bend back from the metal; strong back-donation turns the alkene into a metallacyclopropane. Carbene complexes M=CR₂ come in two classes. Fischer carbenes — low-valent, late metals with π-acceptor co-ligands and a heteroatom (OR, NR₂) on the carbene carbon, as in (CO)₅Cr=C(OMe)Ph — have an electrophilic carbene carbon. Schrock carbenes (alkylidenes) — high-valent, early metals with alkyl or H substituents, as in Ta(=CHCMe₃)(CH₂CMe₃)₃ — have a nucleophilic carbon, and act as Wittig-like reagents towards carbonyls.

Metallocenes M(η⁵-C₅H₅)₂: ferrocene is an air-stable orange 18-electron sandwich whose rings undergo electrophilic substitution (Friedel–Crafts acylation) far faster than benzene and which is reversibly oxidised to ferrocenium, a standard for non-aqueous electrochemistry. Fluxionality is rapid, reversible rearrangement that exchanges chemically distinct positions faster than the NMR timescale. Fe(CO)₅ has two axial and three equatorial carbonyls but shows one ¹³C signal at room temperature, because Berry pseudorotation interchanges them; in (η¹-C₅H₅)(η⁵-C₅H₅)Fe(CO)₂ the σ-bonded ring "ring-whizzes" by 1,2-shifts, so its five protons give one ¹H signal at room temperature and split into separate signals on cooling. Variable-temperature NMR is the tool: coalescence gives the exchange rate.

3. Types of organometallic reactions

The elementary steps of organometallic chemistry
Step	Change in oxidation state	Change in electron count	Example
Ligand substitution	0	0 (D for 18 e, A for 16 e)	Ni(CO)₄ + PR₃ → Ni(CO)₃(PR₃) + CO
Oxidative addition	+2	+2	Vaska’s complex + H₂ → Ir(III) dihydride
Reductive elimination	−2	−2	cis-M(H)(R) → M + R–H
Migratory insertion (CO)	0	−2	CH₃Mn(CO)₅ → CH₃C(O)Mn(CO)₄
1,2-Insertion of an alkene	0	−2	M–H + C₂H₄ → M–CH₂CH₃
β-Hydride elimination	0	+2	reverse of 1,2-insertion

Oxidative addition breaks an X–Y bond across the metal, raising oxidation state and coordination number by two; it needs a coordinatively unsaturated, electron-rich metal (16-electron d⁸ Rh(I), Ir(I), Pd(0) after ligand loss). H₂ adds concertedly and cis; MeI adds by an S_N2-type attack of the metal on carbon, often trans. Reductive elimination is the reverse and forms the C–C or C–H bond in most catalytic cycles; the groups must be cis. Migratory insertion of CO moves the alkyl group onto an adjacent cis CO: labelling shows that in CH₃Mn(CO)₅ the methyl migrates rather than CO inserting, and no labelled CO enters the acyl. Ligands on the metal can also be attacked directly by nucleophiles (the Wacker step) or electrophiles, and transmetallation swaps an organic group from a main-group metal onto the transition metal (the key step of cross-coupling).

4. Homogeneous catalysis

Hydrogenation with Wilkinson’s catalyst RhCl(PPh₃)₃: loss of PPh₃ gives a 14-electron RhCl(PPh₃)₂, oxidative addition of H₂ gives a Rh(III) dihydride, the alkene coordinates and inserts into Rh–H to form an alkyl, and reductive elimination of the alkane regenerates the catalyst. It reduces unhindered, terminal and cis alkenes selectively and leaves C=O, NO₂ and esters alone; with chiral bisphosphines (the rhodium–DIPAMP and ruthenium–BINAP systems) the same chemistry gives asymmetric hydrogenation. Hydroformylation (the oxo process) adds H and CHO across an alkene, RCH=CH₂ + CO + H₂ → RCH₂CH₂CHO (linear) plus branched isomer. With HCo(CO)₄ (from Co₂(CO)₈ and H₂) the cycle is CO loss, alkene coordination, 1,2-insertion, CO coordination, migratory insertion to the acyl, oxidative addition of H₂ and reductive elimination of the aldehyde; rhodium with excess PPh₃ works under much milder conditions and gives a higher linear:branched ratio.

Methanol to acetic acid (the Monsanto process): CH₃OH + CO → CH₃COOH with cis-[Rh(CO)₂I₂]⁻ and HI. HI converts methanol to CH₃I; oxidative addition of CH₃I to the Rh(I) anion is rate-determining; methyl migration to CO gives an acyl, CO adds, reductive elimination releases CH₃COI, and hydrolysis gives the acid and regenerates HI. The iridium-based Cativa process works the same way. Olefin metathesis exchanges alkylidene fragments between alkenes, R¹CH=CHR¹ + R²CH=CHR² ⇌ 2R¹CH=CHR², by the Chauvin mechanism: a metal carbene and an alkene undergo [2 + 2] cycloaddition to a metallacyclobutane, which cleaves the other way. Grubbs ruthenium and Schrock molybdenum carbenes make ring-closing, ring-opening and cross metathesis routine. The Wacker oxidation converts ethylene to acetaldehyde with PdCl₂, CuCl₂ and O₂: water attacks the Pd-bound alkene (anti-nucleophilic attack), β-hydride elimination and tautomerisation release CH₃CHO and Pd(0), Cu(II) re-oxidises Pd(0), and O₂ re-oxidises Cu(I). The oxygen in the product comes from water, not from O₂.

5. Heterogeneous catalysis: Fischer–Tropsch and Ziegler–Natta

The Fischer–Tropsch reaction turns synthesis gas into hydrocarbons over iron or cobalt catalysts, nCO + (2n + 1)H₂ → CₙH₂ₙ₊₂ + nH₂O, at about 200–350 °C. CO dissociates on the metal surface to carbide and oxide; the carbide is hydrogenated to surface CHₓ species, which couple and grow chains by successive insertion; chains leave by β-hydride elimination (alkenes) or hydrogenation (alkanes). Chain growth is a repeated probability of adding one more carbon, so the products follow the Anderson–Schulz–Flory distribution, which is why Fischer–Tropsch gives a broad mixture rather than one product.

Ziegler–Natta polymerisation makes polyethylene and stereoregular polypropylene at low pressure with TiCl₄ or TiCl₃ activated by an aluminium alkyl such as AlEt₃ or Et₂AlCl. In the Cossee–Arlman mechanism the aluminium alkyl alkylates a titanium on the crystal surface, leaving a vacant coordination site; the alkene coordinates there and undergoes migratory insertion into the Ti–C bond, which regenerates a vacant site on the other side, and the cycle repeats. The chiral environment of the surface site orients each incoming propylene the same way, giving isotactic polypropylene. Homogeneous metallocene catalysts (Cp₂ZrCl₂ activated by methylaluminoxane) work by the same insertion and allow the tacticity to be designed through the ligand symmetry.

Key takeaways

Count electrons: metal group number + ligand contributions − charge; 18 for stable carbonyls and metallocenes, 16 for square planar d⁸; V(CO)₆ (17), cobaltocene (19) and nickelocene (20) are the classic exceptions.
σ-donation plus π-back-donation binds CO and alkenes; more back-donation means lower ν(CO); Fischer carbenes are electrophilic, Schrock carbenes nucleophilic; alkyls without β-H resist β-hydride elimination.
Oxidative addition raises oxidation state and electron count by 2, reductive elimination lowers them; migratory insertion moves the alkyl onto a cis CO; fluxional Fe(CO)₅ shows one ¹³C signal.
Wilkinson hydrogenation: OA of H₂, insertion, RE; hydroformylation via acyl migratory insertion; Monsanto: OA of CH₃I is rate-determining; metathesis via a metallacyclobutane; Wacker: the O comes from water.
Fischer–Tropsch gives an Anderson–Schulz–Flory product spread; Ziegler–Natta inserts alkenes into Ti–C bonds at a vacant site (Cossee–Arlman) and gives isotactic polypropylene.

Practice questions (18)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

What is the valence electron count of the iron atom in ferrocene, Fe(η⁵-C₅H₅)₂?
Numerical answer — type the value.
Show answer
Answer: 18
Neutral method: Fe (group 8) gives 8 and each η⁵-C₅H₅ gives 5, total 18. Ionic method: Fe²⁺ is d⁶ and each Cp⁻ gives 6, again 18. Counting Cp as 6 in the neutral method double-counts and gives 20.
What is the valence electron count of vanadium in V(CO)₆?
Numerical answer — type the value.
Show answer
Answer: 17
V (group 5) gives 5 and six CO give 12, total 17. It does not dimerise like Mn(CO)₅ because six carbonyls leave no room for a V–V bond; it is a paramagnetic radical that is easily reduced to the 18-electron [V(CO)₆]⁻.
For the neutral binary carbonyl Ni(CO)ₓ to obey the 18-electron rule, what must x be?
Numerical answer — type the value.
Show answer
Answer: 4
Ni (group 10) gives 10, so it needs 8 more electrons from 2-electron CO ligands: x = 4, the tetrahedral Ni(CO)₄. By the same arithmetic Fe needs 5 and Cr needs 6.
For the anion [Co(CO)ₓ]⁻ to obey the 18-electron rule, what must x be?
Numerical answer — type the value.
Show answer
Answer: 4
Co (group 9) gives 9 and the negative charge adds 1, so 10 + 2x = 18 and x = 4: the tetrahedral [Co(CO)₄]⁻, isoelectronic with Ni(CO)₄. Forgetting the charge gives x = 4.5, a sign that something was missed.
What is the valence electron count of rhodium in Wilkinson’s catalyst, RhCl(PPh₃)₃?
Numerical answer — type the value.
Show answer
Answer: 16
Rh (group 9) gives 9, Cl 1 and three PPh₃ 6: 16. It is a square planar d⁸ Rh(I) complex, stable at 16 electrons, and that vacancy is what lets it add H₂. Assuming every complex must be 18 is the trap.
Using the 18-electron rule, how many Co–Co bonds are present in the tetrahedral cluster Co₄(CO)₁₂?
Numerical answer — type the value.
Show answer
Answer: 6
Total electrons = 4 × 9 + 12 × 2 = 60. Four metals obeying the rule need 4 × 18 = 72, and each M–M bond supplies 2 electrons to the count, so (72 − 60)/2 = 6 bonds — the six edges of the Co₄ tetrahedron. Fe₃(CO)₁₂ by the same method has 3.
Which of the following complexes obey the 18-electron rule?
1. Fe(CO)₅
2. Cr(η⁶-C₆H₆)₂
3. Co(η⁵-C₅H₅)₂
4. Ni(η⁵-C₅H₅)₂
Show answer
Answer: A — Fe(CO)₅; B — Cr(η⁶-C₆H₆)₂
Fe(CO)₅: 8 + 10 = 18; Cr(η⁶-C₆H₆)₂: 6 + 12 = 18. Cobaltocene is 9 + 10 = 19 and is easily oxidised to cobaltocenium; nickelocene is 10 + 10 = 20 with two unpaired electrons in antibonding e₁g* orbitals.
Arrange the C–O stretching frequencies of the isoelectronic carbonyls in increasing order.
1. [V(CO)₆]⁻ < Cr(CO)₆ < [Mn(CO)₆]⁺
2. [Mn(CO)₆]⁺ < Cr(CO)₆ < [V(CO)₆]⁻
3. Cr(CO)₆ < [V(CO)₆]⁻ < [Mn(CO)₆]⁺
4. [V(CO)₆]⁻ < [Mn(CO)₆]⁺ < Cr(CO)₆
Show answer
Answer: A — [V(CO)₆]⁻ < Cr(CO)₆ < [Mn(CO)₆]⁺
The anionic vanadium centre is the most electron-rich and back-donates most into CO π*, weakening C–O (about 1860 cm⁻¹); neutral Cr about 2000; the cationic Mn back-donates least (about 2090), close to free CO at 2143 cm⁻¹.
Which description fits a Fischer carbene complex such as (CO)₅Cr=C(OMe)Ph?
1. Low-valent metal, π-acceptor co-ligands, electrophilic carbene carbon
2. High-valent early metal, nucleophilic carbene carbon
3. The carbene carbon carries only H and alkyl groups
4. It reacts with ketones like a Wittig reagent
Show answer
Answer: A — Low-valent metal, π-acceptor co-ligands, electrophilic carbene carbon
A Fischer carbene has a heteroatom on the carbene carbon and a low-valent metal with CO ligands, so the carbon is electron-poor and is attacked by nucleophiles (amines replace OMe). The high-valent, nucleophilic, Wittig-like alkylidene is the Schrock type.
Why is (Me₃SiCH₂)₄Ti far more thermally stable than (CH₃CH₂)₄Ti?
1. The CH₂SiMe₃ group has no β-hydrogen, so β-hydride elimination is impossible
2. Si makes the Ti–C bond ionic
3. It obeys the 18-electron rule
4. The silyl group is a π-acceptor
Show answer
Answer: A — The CH₂SiMe₃ group has no β-hydrogen, so β-hydride elimination is impossible
The β-atom of CH₂SiMe₃ is silicon, which carries methyls, not hydrogens, so the low-energy β-hydride decomposition route of the ethyl compound is closed. Ti(IV) alkyls are 8-electron species, far from 18, so the rule is not the reason.
Fe(CO)₅ is trigonal bipyramidal with two axial and three equatorial CO groups. How many ¹³C NMR signals does it show for the carbonyls in solution at room temperature?
Numerical answer — type the value.
Show answer
Answer: 1
Berry pseudorotation exchanges axial and equatorial carbonyls faster than the NMR timescale, so all five appear equivalent and give one averaged signal. The static structure would give two signals in a 2:3 ratio, which is the tempting answer.
Vaska’s complex, trans-IrCl(CO)(PPh₃)₂, adds H₂. What is the oxidation state of iridium in the product?
Numerical answer — type the value.
Show answer
Answer: 3
Ir starts as Ir(I), d⁸, 16 electrons. Oxidative addition gives the cis dihydride IrH₂Cl(CO)(PPh₃)₂, with two hydrides counted as H⁻: Ir(III), oxidation state +3, d⁶, 18 electrons. The oxidation state rises by two, not one.
In the Monsanto acetic acid process catalysed by cis-[Rh(CO)₂I₂]⁻, which step is rate-determining?
1. Oxidative addition of CH₃I to Rh(I)
2. Migratory insertion of methyl to CO
3. Reductive elimination of CH₃COI
4. Hydrolysis of CH₃COI
Show answer
Answer: A — Oxidative addition of CH₃I to Rh(I)
The rate is first order in [Rh] and [CH₃I] and zero order in CO and methanol, consistent with oxidative addition of CH₃I to the anion as the slow step; migration, CO uptake and reductive elimination are fast. Hydrolysis happens off the metal.
In the Wacker process, ethylene is oxidised to
1. acetaldehyde, with the oxygen coming from water
2. ethylene oxide, with the oxygen coming from O₂
3. acetic acid, with the oxygen coming from O₂
4. ethanol, by hydration
Show answer
Answer: A — acetaldehyde, with the oxygen coming from water
Water attacks Pd(II)-bound ethylene, and β-hydride elimination and tautomerisation give CH₃CHO; labelling with H₂¹⁸O puts ¹⁸O in the aldehyde. O₂ only re-oxidises Cu(I) to Cu(II), which re-oxidises Pd(0). Ethylene oxide is made on silver, a different process.
The key intermediate in the Chauvin mechanism of olefin metathesis is
1. a metallacyclobutane
2. a metal hydride
3. a free carbocation
4. a π-allyl complex
Show answer
Answer: A — a metallacyclobutane
A metal carbene and an alkene undergo [2 + 2] cycloaddition to a four-membered metallacyclobutane, which then cleaves along the other pair of bonds to give a new carbene and a new alkene. Hydride and π-allyl species belong to isomerisation, not metathesis.
Which statements about hydroformylation are correct?
1. It converts an alkene, CO and H₂ into an aldehyde with one more carbon
2. The acyl group forms by migratory insertion of an alkyl onto CO
3. HCo(CO)₄ is the active species in cobalt-catalysed hydroformylation
4. Only the branched aldehyde is formed
Show answer
Answer: A — It converts an alkene, CO and H₂ into an aldehyde with one more carbon; B — The acyl group forms by migratory insertion of an alkyl onto CO; C — HCo(CO)₄ is the active species in cobalt-catalysed hydroformylation
RCH=CH₂ + CO + H₂ gives RCH₂CH₂CHO and RCH(CH₃)CHO; after 1,2-insertion of the alkene into Co–H, the alkyl migrates to a cis CO to give the acyl, and H₂ then cleaves it. Both isomers form, and phosphine-modified rhodium favours the linear one.
In the Cossee–Arlman mechanism of Ziegler–Natta polymerisation, chain growth occurs by
1. coordination of the alkene at a vacant Ti site followed by migratory insertion into the Ti–C bond
2. free-radical addition initiated by AlEt₃
3. cationic polymerisation at aluminium
4. metathesis of the alkene
Show answer
Answer: A — coordination of the alkene at a vacant Ti site followed by migratory insertion into the Ti–C bond
The aluminium alkyl alkylates surface titanium, leaving a vacant site; propylene binds there and inserts into Ti–R, and the vacancy moves to the other side, ready for the next monomer. The fixed geometry of the site repeats the same face selection, giving isotactic chains. No radicals or carbocations are involved.
Why does the Fischer–Tropsch reaction give a broad range of hydrocarbons rather than a single product?
1. Chain growth on the surface is a repeated probabilistic addition of C₁ units, giving an Anderson–Schulz–Flory distribution
2. The catalyst decomposes during the reaction
3. CO does not adsorb on iron or cobalt
4. The reaction is reversible at every step
Show answer
Answer: A — Chain growth on the surface is a repeated probabilistic addition of C₁ units, giving an Anderson–Schulz–Flory distribution
At each growth step a surface chain either adds another CHₓ unit or desorbs, with roughly constant probability, so chain lengths follow a geometric (Anderson–Schulz–Flory) distribution. CO adsorbs and dissociates readily on Fe and Co, which is why they are the catalysts.