Lesson 6 of 13 · 5 min
Structural isomerism
NCERT §5.4.3–5.4.6
Farhan's teacher shows him a red and a yellow sample, both [Co(NH₃)₅(NO₂)]Cl₂. This time the bonds themselves differ.
The lesson in notes
In short
Linkage isomerism needs an ambidentate ligand. Thiocyanate may bind as M–NCS or M–SCN.
Jørgensen found [Co(NH₃)₅(NO₂)]Cl₂ in two forms: red, with nitrite bound through oxygen (–ONO), and yellow, with nitrite bound through nitrogen (–NO₂).
Coordination isomerism arises when ligands swap between the cationic and anionic entities of two different metals: [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆].
Ionisation isomerism arises when a counter ion that could itself be a ligand trades places with a ligand: [Co(NH₃)₅(SO₄)]Br and [Co(NH₃)₅Br]SO₄.
Ionisation isomers give different ions in water, so they answer different tests: [Co(NH₃)₅Br]SO₄ gives white BaSO₄ with Ba²⁺, while [Co(NH₃)₅(SO₄)]Br gives AgBr with Ag⁺.
Solvate isomerism (hydrate isomerism when the solvent is water) depends on whether solvent is bonded to the metal or just sits in the crystal: [Cr(H₂O)₆]Cl₃ is violet, while [Cr(H₂O)₅Cl]Cl₂·H₂O is grey-green.