Which of the following isomerism is not exhibited by complex [Co(NH3)4(NO2)2]Cl?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Hydrate
Coordination compounds show two main kinds of isomerism:
Structural isomerism: the isomers differ in which atoms are bonded to the metal. It includes linkage (an ambidentate ligand bonds through different atoms), ionisation (a ligand and a counter ion exchange places), coordination (ligands exchange between a complex cation and a complex anion) and solvate or hydrate isomerism (water moves between the coordination sphere and the crystal lattice).
Stereoisomerism: the same bonds, but a different arrangement in space. It includes geometrical (cis/trans, fac/mer) and optical isomerism.
Option (1) Hydrate — Not exhibited by this complex. \( \ce{[Co(NH3)4(NO2)2]Cl} \) contains no water molecule, either as a ligand or as water of crystallisation, so hydrate (solvate) isomerism is not possible.
Option (2) Linkage — Exhibited by this complex. \( \ce{NO2^{-}} \) is an ambidentate ligand. It can bond through nitrogen (nitro, –\( \ce{NO2} \)) or through oxygen (nitrito, –ONO), so linkage isomers exist.
Option (3) Ionisation — Exhibited by this complex. The counter ion \( \ce{Cl^{-}} \) can exchange places with an \( \ce{NO2^{-}} \) ligand, giving the ionisation isomer \( \ce{[Co(NH3)4(NO2)Cl]NO2} \).
Option (4) Geometrical — Exhibited by this complex. The complex ion is octahedral of the type [MA4B2], so the two \( \ce{NO2^{-}} \) ligands can be cis (adjacent) or trans (opposite).
NCERT (2026-27 edition):
Linkage Isomerism – “Linkage isomerism arises in a coordination compound containing ambidentate ligand. A simple example is provided by complexes containing the thiocyanate ligand, NCS–, which may bind through the nitrogen to give M–NCS or through sulphur to give M–SCN. Jørgensen discovered such behaviour in the complex [Co(NH3)5(NO2)]Cl2, which is obtained as the red form, in which the nitrite ligand is bound through oxygen (–ONO), and as the yellow form, in which the nitrite ligand is bound through nitrogen (–NO2).”
Solvate Isomerism
