9 min readConcept sheet1-mark MCQ and assertion-reason, and 2-mark 'name the type of isomerism' or 'how will you distinguish' questions

Linkage, coordination, ionisation and solvate isomerism

Structural isomers differ in which atoms are bonded: through a different donor atom, ligands swapped between metals, or ions and solvent swapped across the bracket.

Linkage isomerismAn ambidentate ligand bonded through different donor atoms, e.g. -NO₂ vs -ONO

Coordination isomerismLigands exchanged between a complex cation and a complex anion

Ionisation isomerismAn ion inside the sphere swaps with a counter ion outside; the isomers give different ions in water

Solvate (hydrate) isomerismWater is inside the sphere in one isomer and outside it, as water of crystallisation, in another

The four types at a glance

Type	Example pair	What changes
Linkage	[Co(NH₃)₅(NO₂)]Cl₂ (yellow) and [Co(NH₃)₅(ONO)]Cl₂ (red)	NO₂⁻ bonded through N or through O
Coordination	[Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆]	NH₃ and CN⁻ change partners between Co³⁺ and Cr³⁺
Ionisation	[Co(NH₃)₅(SO₄)]Br and [Co(NH₃)₅Br]SO₄	SO₄²⁻ inside and Br⁻ outside, or the other way round
Solvate (hydrate)	[Cr(H₂O)₆]Cl₃ (violet) and [Cr(H₂O)₅Cl]Cl₂·H₂O (grey-green)	One water inside the sphere is replaced by a Cl⁻

Linkage isomerism

Linkage isomerism happens in complexes with an ambidentate ligand. In [Co(NH₃)₅(NO₂)]Cl₂ the nitrite ion can bind through nitrogen (nitrito-N, the yellow form) or through oxygen (nitrito-O, written ONO, the red form). Thiocyanate behaves in the same way: it can bind as M-SCN (thiocyanato-S) or as M-NCS (thiocyanato-N).

Coordination and solvate isomerism

Coordination isomerism needs both a complex cation and a complex anion. The ligands are shared out differently between the two metals: [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆]. Solvate isomerism is called hydrate isomerism when the solvent is water. CrCl₃·6H₂O exists as violet [Cr(H₂O)₆]Cl₃ and grey-green [Cr(H₂O)₅Cl]Cl₂·H₂O. The violet form gives 3 mol of AgCl per mol; the grey-green form gives only 2.

How to tell ionisation isomers apart in the lab

Make solutions of [Co(NH₃)₅(SO₄)]Br and [Co(NH₃)₅Br]SO₄ and test each with two reagents.

1
[Co(NH₃)₅(SO₄)]Br + AgNO₃ → [Co(NH₃)₅(SO₄)]NO₃ + AgBr
Pale yellow precipitate of AgBr: Br⁻ is outside the bracket.
2
[Co(NH₃)₅Br]SO₄ + BaCl₂ → [Co(NH₃)₅Br]Cl₂ + BaSO₄
White precipitate of BaSO₄: SO₄²⁻ is outside the bracket.
3★
The first gives no precipitate with BaCl₂; the second gives no precipitate with AgNO₃.
Ions inside the coordination sphere do not react.

★ key step

Using AgCl data to identify a hydrate isomer

Problem

0.010 mol of a chromium(III) complex of formula CrCl₃·6H₂O is treated with excess AgNO₃ solution. It gives 1.435 g of AgCl (molar mass 143.5 g mol⁻¹). Write the formula of this isomer.

Working

Moles of AgCl = 1.435 g ÷ 143.5 g mol⁻¹ = 0.010 mol.
Ionisable Cl⁻ per formula unit = 0.010 mol ÷ 0.010 mol = 1.
So two Cl⁻ are inside the sphere. Cr³⁺ has coordination number 6, so four H₂O are also inside.
The remaining two H₂O are water of crystallisation, outside the sphere.

Answer

[Cr(H₂O)₄Cl₂]Cl·2H₂O, a hydrate isomer of [Cr(H₂O)₆]Cl₃.

In real life

Water-testing labs use the same AgNO₃ and BaCl₂ tests to check the chloride and sulphate in drinking water from wells and borewells.

Exam angle

2 marks: “Name the type of isomerism shown by [Co(NH₃)₅(NO₂)]Cl₂ and [Co(NH₃)₅(ONO)]Cl₂. Why does it arise?” Linkage isomerism. NO₂⁻ is ambidentate and can bind through N (yellow form) or through O (red form).

2 marks: “How would you distinguish [Co(NH₃)₅(SO₄)]Br from [Co(NH₃)₅Br]SO₄?” With AgNO₃, only the first gives a pale yellow precipitate (AgBr). With BaCl₂, only the second gives a white precipitate (BaSO₄).

1 marks: “Write a coordination isomer of [Cu(NH₃)₄][PtCl₄].” [Pt(NH₃)₄][CuCl₄].

Remember it

Link through another atom; Coordinate by swapping metals; Ionise by swapping across the bracket; Solvate by moving water.