Which kind of isomerism is shown by following compounds?
\(M-SCN, M-NCS\)
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Linkage isomerism
Linkage isomerism arises in a coordination compound that contains an ambidentate ligand, a ligand with two different donor atoms that can bind through either one. The thiocyanate ion is such a ligand: it can bind through sulphur (M–SCN, thiocyanato) or through nitrogen (M–NCS, isothiocyanato). M–SCN and M–NCS have the same formula but a different donor atom attached to the metal, so they are linkage isomers. Another example is M–NO2 (nitro) and M–ONO (nitrito).
Why the other options are incorrect
Option (1) Ionisation isomerism — Incorrect. The isomers exchange an ion between the coordination sphere and the outside, e.g. \( \ce{[Co(NH3)5SO4]Br} \) and \( \ce{[Co(NH3)5Br]SO4} \); here the same ligand is only bound through a different atom.
Option (3) Co-ordination isomerism — Incorrect. Ligands are interchanged between a complex cation and a complex anion, e.g. \( \ce{[Co(NH3)6][Cr(CN)6]} \) and \( \ce{[Cr(NH3)6][Co(CN)6]} \).
Option (4) Solvate isomerism — Incorrect. Water (solvent) is a ligand in one isomer and lattice water in the other, e.g. \( \ce{[Cr(H2O)6]Cl3} \) and \( \ce{[Cr(H2O)5Cl]Cl2} \)·\( \ce{H2O} \) (hydrate isomerism).
NCERT (2026-27 edition):
Linkage Isomerism
