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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 · 2021 · Paper I

Q7(b). Bonding in two fluxional molecules from variable-temperature ¹H NMR

Question

How would you account for bonding in the following fluxional molecules based on ¹H NMR spectral studies at variable temperatures? (i) (C₅H₅)₄Ti (ii) C₃(CH₃)₄Fe(CO)₄

Answer in brief

(i) (η⁵-C₅H₅)₂(η¹-C₅H₅)₂Ti has two π-bonded and two σ-bonded rings, giving two lines of equal area at about −30 °C; (ii) in (η²-Me₂C=C=CMe₂)Fe(CO)₄ iron sits on one C=C bond, giving three methyl lines 1 : 1 : 2 at about −60 °C.

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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 moleculesvariable-temperature NMRring whizzinghapticitytetramethylallene iron tetracarbonylcoalescence

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