Which kind of isomerism exists between [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6]?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Coordination isomerism
Coordination isomerism arises when the ligands are interchanged between the cationic and anionic entities of different metal ions in a complex salt. In \( \ce{[Co(NH3)6][Cr(CN)6]} \) the \( \ce{NH3} \) ligands are bound to \( \ce{Co^{3+}} \) and the \( \ce{CN^{-}} \) ligands to \( \ce{Cr^{3+}} \); in its isomer \( \ce{[Cr(NH3)6][Co(CN)6]} \) they have changed places.
Why the other options are incorrect
Option (1) Solvate isomerism — Incorrect. The isomers differ in whether a solvent (water) molecule is bonded to the metal or present only in the lattice, e.g. \( \ce{[Cr(H2O)6]Cl3} \) and \( \ce{[Cr(H2O)5Cl]Cl2} \).\( \ce{H2O} \).
Option (2) Linkage isomerism — Incorrect. An ambidentate ligand binds through different atoms, e.g. –\( \ce{NO2} \) and –ONO.
Option (4) Ionisation isomerism — Incorrect. A counter ion and a ligand exchange places, e.g. \( \ce{[Co(NH3)5(SO4)]Br} \) and \( \ce{[Co(NH3)5Br]SO4} \).
NCERT (2026-27 edition):
Coordination Isomerism
