VKC Notes · Organometallics · Civil Services P-I · IFoS P-I · Geo-Scientist P-I

13.4 Organometallics: 18-Electron Rule, Oxidative Addition, Wilkinson's Catalyst

★★★ 2021–2025: 19 parts, 180 marks (Civil Services 6 parts / 70 m · IFoS 7 / 55 m · Geo-Scientist 6 / 55 m). Wilkinson's catalyst is a Geo-Scientist favourite; fluxional molecules and oxidative addition come in Civil Services and IFoS.

Syllabus lines. Civil Services and IFoS: “Complexes with aromatic systems, synthesis, structure and bonding in metal olefin complexes, alkyne complexes and cyclopentadienyl complexes; coordinative unsaturation, oxidative addition reactions, insertion reactions, fluxional molecules and their characterization; compounds with metal-metal bonds and metal atom clusters”. Geo-Scientist: “18-electron rule and its applications to carbonyls and nature of bonding involved therein. Simple examples of metal-metal bonded compounds and metal clusters. Wilkinson's catalyst”.

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CSE 2024 Q7(a) = asked there; tap to open the solved answerSN = line kept in the short notes

13.4.1 The 18-electron rule and electron counting ★★★

Stable d-block organometallics usually have 18 valence electrons (filled s, p and d shells, like a noble gas). Square-planar d⁸ complexes of Rh(I), Ir(I), Pd(II), Pt(II) are stable at 16.SN
Ionic count: d electrons of the metal in its oxidation state + 2 per neutral ligand (CO, PR₃, alkene, alkyne) + 2 per X⁻ (Cl⁻, H⁻, CH₃⁻) + 6 per η⁵-C₅H₅⁻ + 6 per η⁶-arene + 4 per η³-allyl⁻ + 1 per M–M bond.SN

Solved PYQ CSE 2022 Q6(c) 10m

Which d⁶ centre among Co(0), Mn(I), Fe(II), Fe(III) forms a neutral mixed sandwich with the ring pairs C₅H₅⁻/C₆H₆ or C₅H₅⁻/C₄H₄?

d⁶ centres: Mn(I) and Fe(II) (Co(0) is d⁹, Fe(III) d⁵).
C₅H₅⁻ + C₆H₆ give 6 + 6 = 12 e, charge −1; C₅H₅⁻ + C₄H₄ give 10 e.
Mn(I): 6 + 12 = 18 and charge +1 − 1 = 0: (η⁵-C₅H₅)Mn(η⁶-C₆H₆). Fe(II) gives the 18-e cation [CpFe(C₆H₆)]⁺; with C₄H₄ both reach only 16.SN

Compound	Count	Total
Ni(CO)₄ · Fe(CO)₅ · Cr(CO)₆		18
Mn₂(CO)₁₀ (each Mn)		18
Ferrocene, Fe(η⁵-C₅H₅)₂		18
RhCl(PPh₃)₃, IrCl(CO)(PPh₃)₂		16
[Co(CN)₅]³⁻		17 (radical-like)

13.4.2 Alkene and alkyne complexes ★★★

Fig. 13.4.1 Dewar–Chatt–Duncanson bonding.

Draw it in 60 s: two panels: C=C with π lobes pointing at M (σ donation arrow); four-lobed metal d orbital and the π* lobes of C=C (back-donation arrow).

Zeise's salt, K[PtCl₃(η²-C₂H₄)]·H₂O, the first organometallic (1827): ethene into K₂[PtCl₄] in dilute HCl (SnCl₂ catalyst). Pt(II) square planar (3 Cl + midpoint of C=C); C=C perpendicular to the PtCl₃ plane; C=C 137.5 pm (free 133.7), ν(C=C) 1623 → ≈ 1520 cm⁻¹; Pt–Cl trans to ethene is longer (trans influence); H atoms bend away from Pt CSE 2024 Q5(d) 10m.SN
DCD model: σ donation weakens the C=C a little; π back-donation into π* weakens it more. Strong back-donation (electron-rich M, electron-poor alkene) gives the metallacyclopropane limit: C sp³-like, substituents bent far back.SN
[Pt(η²-PhC≡CPh)(PPh₃)₂]: Pt(0), d¹⁰, 16 e, trigonal planar. C–C 1.32 Å (free alkyne 1.20 Å, close to a C=C) and C–C–Ph 140° (180° when free): heavy back-donation, so best drawn as a Pt(II) metallacyclopropene IFoS 2023 Q7(c) 10m.SN
[Co₂(CO)₆(PhC≡CPh)] (from Co₂(CO)₈ + alkyne, − 2CO): the alkyne lies across the Co–Co bond, perpendicular to it; its two π bonds each donate to one Co, giving a tetrahedral C₂Co₂ core (a dimetallatetrahedrane). Each Co: 9 + 6 (3 CO) + 1 (Co–Co) + 2 (alkyne) = 18. C–C lengthens to ≈ 1.36 Å, C–C–Ph bends to ≈ 140° IFoS 2025 Q8(a)(i) 10m.SN

13.4.3 Cyclopentadienyl compounds ★★

Ferrocene, Fe(η⁵-C₅H₅)₂: 18 e, orange, sandwich with parallel rings (eclipsed in the gas phase), stable to 400 °C; made from NaCp + FeCl₂. The rings are aromatic and electron-rich: Friedel–Crafts acylation runs about 10⁶ times faster than for benzene.SN
Acetylferrocene: CH₃COCl/AlCl₃ (or Ac₂O + H₃PO₄); the acetyl group deactivates its own ring, so mono-substitution is easy. Ferrocenecarboxylic acid: lithiate with n-BuLi or t-BuLi → FcLi; + CO₂; H₃O⁺. Do not use HNO₃ or other oxidants: they give the blue ferrocenium ion, [Fe(C₅H₅)₂]⁺ CSE 2023 Q8(a) 10m.SN

Fig. 13.4.2 Three structures UPSC asks you to draw.

Draw it in 60 s: two parallel pentagons with Fe between; a square plane (parallelogram) with Pt, three Cl and a vertical C=C at the fourth corner; Pt with two PPh₃ and a three-membered ring to C=C, Ph bent back at 140°.

13.4.4 Oxidative addition and reductive elimination ★★★

(13.4.1)

Needs coordinative unsaturation: a 16-e (or 14-e) metal with a vacant site and an accessible higher oxidation state (Rh(I), Ir(I), Pd(0), Pt(0)). Reductive elimination is the exact reverse: two cis groups couple, oxidation state and CN fall by 2 Geo 2024 Q7(a) 10m.SN
Vaska's complex trans-[IrCl(CO)(PPh₃)₂] (Ir(I), d⁸, 16 e) + H₂ → cis-dihydride [IrH₂Cl(CO)(PPh₃)₂], Ir(III), d⁶, octahedral, 18 e IFoS 2024 Q7(b)(i) 5m.SN

Fig. 13.4.3 Oxidative addition of H₂ to Vaska's complex.

Draw it in 60 s: square-planar Ir with P top and bottom, Cl and CO sideways; arrow + H₂; octahedron with the two P axial and the two H on a wedge and a hash, cis to each other.

Mechanism	Substrates	Signs
Concerted (three-centre TS)	H₂, Si–H, C–H, non-polar	two new groups cis; retention at carbon; little solvent effect
S_N2 (metal as nucleophile)	CH₃I, RX, polar	rate ∝ [M][RX]; faster in polar solvents; inversion at C; cationic intermediate; groups may end trans
Radical (one electron per metal)	RX with 17-e metals	2[Co(CN)₅]³⁻ + CH₃I → [Co(CN)₅CH₃]³⁻ + [Co(CN)₅I]³⁻

[Co(CN)₅]³⁻ (Co(II), 17 e) can gain only one electron, so two Co share the addition: each goes +2 → +3, 5 → 6-coordinate, 18 e. [Ru(CO)₃(PPh₃)₂] (Ru(0), 18 e) attacks CH₃I by S_N2 → [Ru(CH₃)(CO)₃(PPh₃)₂]⁺; I⁻ then replaces one CO: [Ru(CH₃)I(CO)₂(PPh₃)₂], Ru(II), 18 e CSE 2025 Q8(e) 10m IFoS 2023 Q8(a) 10m.SN

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