[Co(NH3)5(NO2)]Cl2 exhibits....
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Linkage isomerism
Isomerism in coordination compounds is of two main kinds:
Stereoisomerism: same bonds, different arrangement in space — geometrical (cis/trans, fac/mer) and optical (non-superimposable mirror images).
Structural isomerism: different bonds — linkage (an ambidentate ligand binds through a different atom), coordination (ligands exchanged between a complex cation and a complex anion), ionisation (a ligand exchanged with a counter ion) and solvate/hydrate (water inside or outside the coordination sphere).
In \( \ce{[Co(NH3)5(NO2)]Cl2} \), cobalt is +3 (x + 5(0) + (−1) = +2, so x = +3). The nitrite ion is an ambidentate ligand: it can bind through nitrogen (nitro, –\( \ce{NO2} \)) or through oxygen (nitrito, –ONO).
So the compound shows linkage isomerism.
Why the other options are incorrect
Option (1) Optical isomerism — Incorrect. An octahedral MA5B complex has planes of symmetry, so it is superimposable on its mirror image.
Option (2) Coordination isomerism — Incorrect. It needs both the cation and the anion to be complex ions; here the anion is simple \( \ce{Cl^{-}} \).
Option (4) Hydrate isomerism — Incorrect. It needs water molecules that can move into or out of the coordination sphere; this compound has no water.
NCERT (2026-27 edition):
Linkage Isomerism
