Statement I: Coordination isomerism occurs when both the cation and t…

The correct answer is Option (2) → Both Statement I and statement II are true but statement II is not the correct explanation for statement I

Statement I: Coordination isomerism occurs when both the cation and the anion are complex ions. — True. Coordination isomers differ in how the ligands are shared between the cationic and anionic complex entities. In \( \ce{[Co(NH3)6][Cr(CN)6]} \) the \( \ce{NH3} \) ligands are on Co and the \( \ce{CN^-} \) ligands on Cr; in its isomer \( \ce{[Cr(NH3)6][Co(CN)6]} \) they are interchanged. This is possible only when both ions are complexes.

Statement II: Complexes with coordination number 6 are octahedral and involve either sp³d² or d²sp³ hybridisation. — True. By valence bond theory, six hybrid orbitals point to the corners of an octahedron. They come from outer 4d orbitals (sp³d², outer orbital complex) or from inner 3d orbitals (d²sp³, inner orbital complex).

Why Statement II does not explain Statement I?

• Statement I is about a kind of structural isomerism: whether ligands can be exchanged between two complex ions of a salt. Statement II is about the geometry and hybridisation of one complex entity.

• Octahedral geometry does not create coordination isomerism: \( \ce{[Co(NH3)6]Cl3} \) is octahedral but has only one complex ion, so it has no coordination isomer. The condition is the presence of a complex cation and a complex anion, which hybridisation does not explain.

NCERT (2026-27 edition):
5.4.4 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⁺.”

5.5.1 Valence Bond Theory