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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper I covers inorganic chemistry.

UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper I

Q9(b). Reaction of [Mn(CO)₅]⁻ with allyl chloride

  • 10 marks
  • Topic: Organometallic chemistry
  • Answered
Question

Find the products (K to N) in the following reaction and show the metal to ligand bond connectivities, only for organometallic species: [Mn(CO)₅]⁻ + C₃H₅Cl → K + L, then Δ or hν: K→ M+N.

Answer in brief

K=(η¹-C₃H₅)Mn(CO)₅ (Mn–CH₂ σ bond), L=Cl⁻ (as NaCl or KCl); on heating or photolysis M=(η³-C₃H₅)Mn(CO)₄ (Mn bonded to all three allyl carbons) and N=CO.

Key points

  • nucleophilic substitution. [Mn(CO)₅]⁻ (18e, Mn(-I), a strong nucleophile) attacks the CH₂ carbon of allyl chloride and displaces Cl⁻: [Mn(CO)₅]⁻ + CH₂=CH-CH₂Cl → [(η¹-CH₂CH=CH₂)Mn(CO)₅] + Cl⁻
  • η¹ (monohapto) allyl, 1-electron donor: 7+1+5(2)=18 e. Bond connectivity: Mn–C1 σ bond; the CH=CH₂ part is free (uncoordinated).
  • CO loss. On heating or UV light one CO is expelled; the vacant site is filled by the pendant C=C: the allyl changes from η¹ to η³ (3-electron donor, π-allyl): (η¹-C₃H₅)Mn(CO)₅ →(η³-C₃H₅)Mn(CO)₄ + CO
  • 7+3+4(2)=18 e. Bond connectivity: Mn bonded to C1, C2 and C3 (all three carbons equivalent, delocalised allyl, C–C ≈1.40 Å).

The full model answer

Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.

  1. Step 1: nucleophilic substitution. [Mn(CO)₅]⁻ (18e, Mn(-I), a strong nucleophile) attacks the CH₂ carbon of allyl chloride and displaces Cl⁻: [Mn(CO)₅]⁻ + CH₂=CH-CH₂Cl → [(η¹-CH₂CH=CH₂)Mn(CO)₅] + Cl⁻
  2. K: η¹ (monohapto) allyl, 1-electron donor: 7+1+5(2)=18 e. Bond connectivity: Mn–C1 σ bond; the CH=CH₂ part is free (uncoordinated).
  3. Step 2: CO loss. On heating or UV light one CO is expelled; the vacant site is filled by the pendant C=C: the allyl changes from η¹ to η³ (3-electron donor, π-allyl): (η¹-C₃H₅)Mn(CO)₅ →(η³-C₃H₅)Mn(CO)₄ + CO
  4. M: 7+3+4(2)=18 e. Bond connectivity: Mn bonded to C1, C2 and C3 (all three carbons equivalent, delocalised allyl, C–C ≈1.40 Å).
  5. Reverse: (η³-C₃H₅)Mn(CO)₄ + CO → (η¹-C₃H₅)Mn(CO)₅ under CO pressure: a classic η¹⇌η³ interconversion (a test of the 18-electron rule).

the η³-allyl–metal unit is the core of Pd-catalysed allylic substitution (Tsuji–Trost); the η¹/η³ switch lets the metal stay at 18 or 16 electrons.

Model answer to Q9(b), 2026 Mains Paper I: solution page 20
Solution page 20 of 26
Read the whole 2026 solution2026 question paper

Coversη¹ and η³ allyl complexesReaction of [Mn(CO)₅]⁻ with allyl chloride

All parts of Question 9