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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

Q7(e). Substitution in [PtCl₃(C₂H₄)]⁻ and the trans effect

Question

Complete the following reaction and explain the mechanism with the help of π-bonding theory: [PtCl₃(C₂H₄)]⁻ + NH₃ → [structure of the square-planar complex given in the paper]

Answer in brief

[PtCl₃(C₂H₄)]⁻ + NH₃ → trans-[PtCl₂(C₂H₄)(NH₃)] + Cl⁻; the chloride trans to ethylene is replaced because ethylene is a strong π acceptor and so a strong trans director.

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Coverstrans effectZeise's saltsquare planar substitutionπ back-bondingassociative mechanismtrans-directing series

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