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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2025 · Paper I

Q6(a). Rigid or fluxional: an allyl palladium complex

Question

The following molecule shows the rigid or fluxional behaviour at higher temperature or in the presence of a base. Justify the answer with the help of ¹H NMR spectrum. [structure of an η³-allyl palladium complex with Cl and PPh₃ ligands, allyl protons labelled Hx, HA, HA′, HB, HB′, given in the paper]

Answer in brief

Fluxional: the η³-allyl group is rigid only at low temperature; on warming, or with a base, it changes η³ ⇌ η¹ ⇌ η³ fast enough to average the four terminal protons.

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Topic notes: organometallics: 18-electron rule, oxidative addition, fluxional molecules and Wilkinson's catalyst, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversfluxional moleculesη3-allyl palladiumsyn-anti exchangevariable temperature NMRhapticity changeAX4 pattern

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