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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 · 2016 · Paper I

Q6(c). IUPAC names of K₃[Fe(CN)₅NO] and [Co(NO₂)₃(NH₃)₃]

  • 10 marks
  • Answered
Question

Write IUPAC nomenclature of the following:
(i) K₃[Fe(CN)₅NO]
(ii) [CO(NO₂)₃(NH₃)₃]

Answer in brief

(i) potassium pentacyanidonitrosylferrate(II) [or pentacyanidonitrosylferrate(3-)] (ii) triamminetrinitrito-κ N-cobalt(III)

Key points

  • (i) Cation named first (potassium, no prefix needed). Ligands in alphabetical order: cyanido (c) before nitrosyl (n); the multiplying prefix penta- is ignored in the order. The complex is an anion, so iron becomes ferrate.
  • potassium pentacyanonitrosylferrate(II). Here NO is counted as a neutral ligand (nitrosyl), so iron is +2; counted as NO⁺, the same ion would be written as Fe(I). Because NO can be counted as NO⁺, NO or NO⁻, IUPAC also allows…
  • (ii) Neutral complex, written as one word. Ammine (a) precedes nitrito (n). NO₂⁻ bound through N is nitrito-κ N (older: nitro); bound through O it would be nitrito-κ O (nitrito), a linkage isomer.
  • [Co(NO₂)₃(NH₃)₃] is a non-electrolyte (no ions outside the sphere) and exists as fac and mer geometrical isomers (panel).

The full model answer

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

Reading: “CO” in (ii) is a misprint for Co (cobalt); [Co(NO₂)₃(NH₃)₃] is meant.

CompoundIUPAC nameOxidation state
(i)K₃[Fe(CN)₅NO]pentacyanidonitrosylferrate(II)NO counted neutral: x+5(-1)+0=-3, Fe(II); as NO⁺ it would be Fe(I)
(ii)[Co(NO₂)₃(NH₃)₃]-κ N-cobalt(III)x+3(-1)+3(0)=0, Co(III)
  1. (i) Cation named first (potassium, no prefix needed). Ligands in alphabetical order: cyanido (c) before nitrosyl (n); the multiplying prefix penta- is ignored in the order. The complex is an anion, so iron becomes ferrate.
  2. In older usage: potassium pentacyanonitrosylferrate(II). Here NO is counted as a neutral ligand (nitrosyl), so iron is +2; counted as NO⁺, the same ion would be written as Fe(I). Because NO can be counted as NO⁺, NO or NO⁻, IUPAC also allows the charge number in place of the oxidation number: potassium pentacyanidonitrosylferrate(3-), which is unambiguous. (The familiar nitroprusside is the 2- ion, Na₂[Fe(CN)₅NO].)
  3. (ii) Neutral complex, written as one word. Ammine (a) precedes nitrito (n). NO₂⁻ bound through N is nitrito-κ N (older: nitro); bound through O it would be nitrito-κ O (nitrito), a linkage isomer.
  4. [Co(NO₂)₃(NH₃)₃] is a non-electrolyte (no ions outside the sphere) and exists as fac and mer geometrical isomers (panel).

Application. nitroprusside gives the purple colour with sulfide ions (test for S²-) and is used in medicine as a source of NO (a vasodilator); the nitro/nitrito pair of [Co(NH₃)₅(NO₂)]²+ was the first example of linkage isomerism.

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Model answer to Q6(c), 2016 Paper I: solution page 34
Solution page 34 of 46
Read the whole 2016 solution2016 question paper

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