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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

Q6(a). Silver cyanide complex and its crystal field splitting

  • 10 marks
  • Topic: Coordination compounds
  • Answered
Question

Consider the following equilibrium: KCN (excess) + AgNO₃ ⇌ I + J. Find the compounds I and J, and draw the crystal field splitting diagram (with labels and filled electrons of orbitals) of silver complex (I).

Answer in brief

I=K[Ag(CN)2] (potassium dicyanoargentate(I)); J=KNO3. In the linear [Ag(CN)₂]⁻ (Ag⁺, d¹⁰) all five d orbitals are filled: (d_(xy),d_(x²-y²))⁴(d_(xz),d_(yz))⁴(d_(z²))².

Key points

  • with a little cyanide a white precipitate forms, Ag⁺ + CN⁻ → AgCN ↓; with excess it dissolves: AgCN + CN⁻ → [Ag(CN)₂]⁻: AgNO₃ + 2KCN ⇌ K[Ag(CN)₂] + KNO₃
  • β₂≈10²¹, so the reaction is practically complete to the right; the equilibrium sign is notional.
  • Splitting in a linear field (ligands on the z axis): the repulsion order is d_(z²) (the lobes point at the ligands) > d_(xz),d_(yz) > d_(xy),d_(x²-y²) (lobes perpendicular to the axis, lowest).
  • Ag⁺ is 4d¹⁰: (d_(xy),d_(x²-y²))⁴ (d_(xz),d_(yz))⁴ (d_(z²))². There is no CFSE (full shell); diamagnetic and colourless: no d–d transition is possible.

The full model answer

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

  1. Reaction: with a little cyanide a white precipitate forms, Ag⁺ + CN⁻ → AgCN ↓; with excess it dissolves: AgCN + CN⁻ → [Ag(CN)₂]⁻: AgNO₃ + 2KCN ⇌ K[Ag(CN)₂] + KNO₃
  2. Position of equilibrium: β₂≈10²¹, so the reaction is practically complete to the right; the equilibrium sign is notional.
  3. Splitting in a linear field (ligands on the z axis): the repulsion order is d_(z²) (the lobes point at the ligands) > d_(xz),d_(yz) > d_(xy),d_(x²-y²) (lobes perpendicular to the axis, lowest).
  4. Electrons: Ag⁺ is 4d¹⁰: (d_(xy),d_(x²-y²))⁴ (d_(xz),d_(yz))⁴ (d_(z²))². There is no CFSE (full shell); diamagnetic and colourless: no d–d transition is possible.
  5. Why linear, not tetrahedral: Ag⁺ has a 4d¹⁰5s⁰ shell with a low-lying 5s–4d_(z²) mixing which keeps the two ligands trans (sp hybridisation).

the argentocyanide process: gold and silver ores are leached with NaCN + O₂, giving [Ag(CN)₂]⁻ and [Au(CN)₂]⁻; Zn dust then displaces the metal.

Model answer to Q6(a), 2026 Mains Paper I: solution page 16
Solution page 16 of 26
Read the whole 2026 solution2026 question paper

Coverslinear complexesSilver cyanide complex and its crystal field splitting

All parts of Question 6