Q19
2 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⁺.
diamminesilver(I) dicyanidoargentate(I)potassium trioxalatoferrate(III)ionisation isomersAgNO₃ test for Cl⁻BaCl₂ test for SO₄²⁻chelate effectbidentate ligandethylenediamine (en)

Marking Scheme

  • 11 mark: correct IUPAC name of [Ag(NH₃)₂][Ag(CN)₂] as diamminesilver(I) dicyanidoargentate(I).
  • 21 mark: correct IUPAC name of K₃[Fe(C₂O₄)₃] as potassium trioxalatoferrate(III) (accept tris(oxalato)).
  • 3OR (b)(i) 1 mark: AgNO₃ gives AgCl ppt with the -Cl salt and BaCl₂ gives BaSO₄ ppt with the -SO₄ salt.
  • 4OR (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.

Analysis & 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.

Common Mistakes

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

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.

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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.