Isomerism shown by the complex pair $[Co(NH_3)_5(NO_3)]SO_4$ and $[Co(NH_3)_5(SO_4)]NO_3$ is

Ionization isomerism

The correct answer is Option (2) → Ionization isomerism

The complexes are:

• [Co(NH₃)₅(NO₃)]SO₄
• [Co(NH₃)₅(SO₄)]NO₃

These differ by the ion outside the coordination sphere:

• In the first complex → NO₃⁻ is inside the coordination sphere, SO₄²⁻ is counter-ion.
• In the second complex → SO₄²⁻ is inside the coordination sphere, NO₃⁻ is counter-ion.

This is a classic example of ionization isomerism, where the ions inside and outside the coordination sphere swap places.

1. Hydrate isomerism

This involves interchange between water molecules inside and outside the coordination sphere (e.g., coordinated $H_2O$ vs water of crystallization). Since no water molecules are involved here, this type does not apply.

3. Coordination isomerism

This occurs in salts containing both cationic and anionic complex ions where ligands exchange between two metal centers. Here, only one complex ion is present in each compound, so this does not apply.

4. Linkage isomerism

This arises when an ambidentate ligand binds through different donor atoms (e.g., $NO_2^-$ as nitro or nitrito). In this case, the ligands $NO_3^-$ and $SO_4^{2-}$ are simply swapping positions, not changing donor atoms.