Question 192 marksVery Short AnswerSection B

(a) Write IUPAC names of the following coordination compounds: (2 × 1)

(i) [Ag(NH₃)₂] [Ag(CN)₂]

(ii) K₃[Fe(C₂O₄)₃]

OR

(b) (i) Give a chemical test to show that [Co(NH₃)₅SO₄]Cl and [Co(NH₃)₅Cl]SO₄ are ionisation isomers. (2 × 1)

(ii) What is meant by the 'Chelate effect'? Give an example.

Coordination Compounds
IUPAC nomenclature and isomerism of coordination compounds

Official answer

Part (a) — IUPAC names


  1. [Ag(NH₃)₂][Ag(CN)₂] — both cation and anion are complexes of Ag in +1 state:
  • Cation [Ag(NH₃)₂]⁺ = diamminesilver(I)
  • Anion [Ag(CN)₂]⁻ = dicyanidoargentate(I)
  • Name: Diamminesilver(I) dicyanidoargentate(I)

  1. K₃[Fe(C₂O₄)₃] — oxalate (C₂O₄²⁻ = oxalato) is bidentate; Fe is +3:
  • Name: Potassium trioxalatoferrate(III) (also accepted: potassium tris(oxalato)ferrate(III))

OR — Part (b)


  1. Test for ionisation isomers:
  • [Co(NH₃)₅SO₄]Cl releases free Cl⁻ → gives a white precipitate of AgCl with AgNO₃ (no ppt with BaCl₂).
  • [Co(NH₃)₅Cl]SO₄ releases free SO₄²⁻ → gives a white precipitate of BaSO₄ with BaCl₂ (no ppt with AgNO₃).

  1. Chelate effect: the extra stability a complex gains when a bidentate/polydentate ligand forms a ring (chelate) with the metal, compared with similar monodentate ligands. Example: [Ni(en)₃]²⁺ is far more stable than [Ni(NH₃)₆]²⁺.
Answer keywords:diamminesilver(I) dicyanidoargentate(I)potassium trioxalatoferrate(III)ionisation isomersAgNO₃ test for Cl⁻BaCl₂ test for SO₄²⁻chelate effectbidentate ligandethylenediamine (en)

Marking scheme

  1. 1.1 mark: correct IUPAC name of [Ag(NH₃)₂][Ag(CN)₂] as diamminesilver(I) dicyanidoargentate(I).
  2. 2.1 mark: correct IUPAC name of K₃[Fe(C₂O₄)₃] as potassium trioxalatoferrate(III) (accept tris(oxalato)).
  3. 3.OR (b)(i) 1 mark: AgNO₃ gives AgCl ppt with the -Cl salt and BaCl₂ gives BaSO₄ ppt with the -SO₄ salt.
  4. 4.OR (b)(ii) 1 mark: definition of chelate effect + a valid example such as [Ni(en)₃]²⁺ vs [Ni(NH₃)₆]²⁺.

Hint

Name ligands alphabetically, add -ate to the metal in the anion, and show oxidation state; for isomers, AgNO₃ detects free Cl⁻ and BaCl₂ detects free SO₄²⁻.

Quick oral answer

In [Ag(NH₃)₂][Ag(CN)₂] the cyanido complex is the anion so it is named diamminesilver(I) dicyanidoargentate(I); K₃[Fe(C₂O₄)₃] is potassium trioxalatoferrate(III) because three oxalates make iron +3.

Common mistakes

  1. 1.Naming the anionic complex without the -ate suffix (writing 'silver' instead of 'argentate').
  2. 2.Assigning the wrong oxidation state to Fe — forgetting each oxalate is 2−, giving Fe(III) not Fe(II).
  3. 3.Swapping the isomer tests — using BaCl₂ for the chloride salt or AgNO₃ for the sulphate salt.

Analysis and explanation

Concept — naming rules


  • Ligands are named alphabetically before the metal; anionic ligands take the metal suffix -ate.
  • Oxidation state is shown in Roman numerals in brackets.
  • In a salt of two complex ions, the cation is named first, then the anion (which ends in -ate).

Working out the two names


  • In [Ag(NH₃)₂][Ag(CN)₂], overall charge is zero and both silvers are +1; the cyanido complex, being the anion, becomes 'argentate'.
  • In K₃[Fe(C₂O₄)₃], the three K⁺ balance a 3− complex; with three oxalate (each 2−) the metal must be Fe(III).

Exam trap


  • The current NCERT convention is 'cyanido' and 'oxalato' (not the older 'cyano'/'oxalato' spellings) — use cyanido to be safe.
  • For ionisation isomers, students confuse which reagent tests which ion: AgNO₃ detects free Cl⁻, BaCl₂ detects free SO₄²⁻.

Real-world link


The chelate effect underlies why EDTA (a hexadentate chelating agent) is used in water softening, in titrations for hardness, and even as an antidote in heavy-metal poisoning.

Interesting facts

The Latin stem for silver, 'argentum', survives only in the anionic name 'argentate' — the symbol Ag comes from the same root.

The word 'chelate' comes from the Greek 'chele' meaning a crab's claw, describing how a bidentate ligand grips the metal from two sides.

Ionisation isomers such as these cobalt salts give literally different-coloured solutions of the freed anions, which is how Werner distinguished them in the 1890s.

Question details

Q19 of 37 questions

Board
CBSE
Class
Class 12
Subject
Chemistry
Year
2026
Section
Section B
Marks
2
Type
Very Short Answer
Chapter
Coordination Compounds
Topic
IUPAC nomenclature and isomerism of coordination compounds
Bloom's level
understand
Syllabus topic
Unit: Coordination Compounds — IUPAC nomenclature, ionisation isomerism and chelate effect
Author
CBSE
Reviewed by
Padhantu Academic Team

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Frequently asked questions

Why is the cyanido complex named 'argentate' and not 'silver'?

When a complex ion carries a negative charge, the central metal takes the Latinised name with an -ate suffix. For silver the Latin root is argentum, so the anion [Ag(CN)₂]⁻ becomes dicyanidoargentate(I), while the cationic [Ag(NH₃)₂]⁺ keeps the ordinary name silver(I).

How does a single AgNO₃/BaCl₂ test prove ionisation isomerism?

Ionisation isomers differ in which ion is inside the coordination sphere and which is free in solution. Only the free (counter) ion reacts. AgNO₃ precipitates only free Cl⁻ (as AgCl) and BaCl₂ precipitates only free SO₄²⁻ (as BaSO₄), so the two salts give opposite results, proving they release different ions.