Isomerism shown by the complex pair \( [Cr(H_2O)_6]Cl_3 \) and \( [Cr(H_2O)_5Cl]Cl_2.H_2O \) is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Hydrate isomerism
The two complexes have the same formula, \( \ce{CrCl3.6H2O} \), but water is placed differently:
\( \ce{[Cr(H2O)6]Cl3} \) (violet): six water molecules are bonded to \( \ce{Cr^3+} \), and all three \( \ce{Cl-} \) ions are free; it gives 3 mol of AgCl with excess \( \ce{AgNO3} \).
\( \ce{[Cr(H2O)5Cl]Cl2.H2O} \) (grey-green): one \( \ce{Cl-} \) has entered the coordination sphere in place of a water molecule, which is now water of crystallisation; only two \( \ce{Cl-} \) ions are free, giving 2 mol of AgCl.
Water inside the coordination sphere in one isomer and outside it in the other is hydrate isomerism.
Why the other options are incorrect
Option (2) Coordination isomerism — Incorrect. It needs both a complex cation and a complex anion, which exchange ligands, as in \( \ce{[Co(NH3)6][Cr(CN)6]} \) and \( \ce{[Cr(NH3)6][Co(CN)6]} \).
Option (3) Linkage isomerism — Incorrect. It needs an ambidentate ligand bonded through different atoms, as in \( \ce{[Co(NH3)5(NO2)]Cl2} \) (through N) and \( \ce{[Co(NH3)5(ONO)]Cl2} \) (through O).
Option (4) Geometrical isomerism — Incorrect. It is a difference in the spatial arrangement of the same ligands (cis and trans), as in \( \ce{[Pt(NH3)2Cl2]} \); here the ligands themselves differ.
NCERT (2026-27 edition):
Solvate Isomerism
