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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 · 2026 · Paper I

Q5(f). Chelate effect and chelate ring size

  • 5 marks
  • Answered
Question

What is chelate effect? How is it affected by the ring size?

Answer in brief

A chelate complex is more stable than one with the same number of similar monodentate donors. Five-membered rings give the largest effect; six-membered suit conjugated ligands; other sizes are weaker.

Key points

  • Both Ni(II) complexes have six amine N donors, yet β for the en complex is about 10^(9.7) times larger. For [Ni(NH₃)₆]²+ + 3en → [Ni(en)₃]²+ + 6NH₃, K=β₃(en)/β₆(NH₃), so log K=18.3-8.6=9.7 and Δ G^°=-2.303RTlog K=-55 kJ mol⁻¹.
  • [Ni(H₂O)₆]²+ + 3en gives 7 particles from 4, while 6NH₃ gives 7 from 7. The increase in the number of particles makes Δ S^° positive: for the exchange above Δ H^° is only about -17 kJ mol⁻¹, so most of Δ G^° comes from TΔ…
  • Five-membered rings (en, oxalate, glycinate) are the most stable: the bite of the ligand and the angles in the ring fit an octahedral metal with no ring strain.
  • Six-membered rings are reasonably stable and preferred for conjugated ligands such as acac⁻ (sp² carbons, delocalised ring).

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

  1. Both Ni(II) complexes have six amine N donors, yet β for the en complex is about 10^(9.7) times larger. For [Ni(NH₃)₆]²+ + 3en → [Ni(en)₃]²+ + 6NH₃, K=β₃(en)/β₆(NH₃), so log K=18.3-8.6=9.7 and Δ G^°=-2.303RTlog K=-55 kJ mol⁻¹.
  2. Entropy: [Ni(H₂O)₆]²+ + 3en gives 7 particles from 4, while 6NH₃ gives 7 from 7. The increase in the number of particles makes Δ S^° positive: for the exchange above Δ H^° is only about -17 kJ mol⁻¹, so most of Δ G^° comes from TΔ S^°≈+38 kJ mol⁻¹. Also, once one end is bound, the other is held close, so ring closure is likely.
  3. Five-membered rings (en, oxalate, glycinate) are the most stable: the bite of the ligand and the angles in the ring fit an octahedral metal with no ring strain.
  4. Six-membered rings are reasonably stable and preferred for conjugated ligands such as acac⁻ (sp² carbons, delocalised ring).
  5. Three-, four-membered rings (bidentate carbonate) are strained; seven-membered and larger rings put the second donor far away, so the effect fades.

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Model answer to Q5(f), 2026 Paper I: solution page 31
Solution page 31 of 55
Read the whole 2026 solution2026 question paper

All parts of Question 5