In the coordination compound \([Cr(NH_3)_6][Co(CN)_6]\), what type of isomerism arises?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Coordination Isomerism
\( \ce{[Cr(NH3)6][Co(CN)6]} \) contains both a complex cation and a complex anion. The ligands can be interchanged between the two metal ions: the isomer \( \ce{[Co(NH3)6][Cr(CN)6]} \) has the \( \ce{NH3} \) ligands on cobalt and the \( \ce{CN^{-}} \) ligands on chromium. Isomerism arising from such an interchange of ligands between the cationic and anionic entities is coordination isomerism.
Why the other options are incorrect
Option (1) Ionisation Isomerism — Incorrect. Needs a counter ion that can itself act as a ligand and exchange with a ligand, as in \( \ce{[Co(NH3)5SO4]Br} \) and \( \ce{[Co(NH3)5Br]SO4} \); here both ions are complex entities.
Option (3) Linkage Isomerism — Incorrect. Needs an ambidentate ligand binding through different atoms (e.g. \( \ce{NO2^{-}} \) / \( \ce{ONO^{-}} \)); \( \ce{NH3} \) and the cyanide ligands here bind the same way in both isomers.
Option (4) Solvate Isomerism — Incorrect. Needs solvent (water) molecules that are either ligands or lattice molecules; no water is present.
NCERT (2026-27 edition):
Coordination Isomerism – “This type of isomerism arises from the interchange of ligands between cationic and anionic entities of different metal ions present in a complex. An example is provided by [Co(NH3)6][Cr(CN)6], in which the NH3 ligands are bound to Co3+ and the CN– ligands to Cr3+. In its coordination isomer [Cr(NH3)6][Co(CN)6], the NH3 ligands are bound to Cr3+ and the CN– ligands to Co3+.”