Question

The type of isomerism exhibited by the complexes [Co(NH$_3$)$_5$(SO$_4$)]Br and [Co(NH$_3$)$_5$Br]SO$_4$ is

• A. Ionisation isomerism
• B. Linkage isomerism
• C. Coordinate isomerism
• D. Geometrical isomerism
• E. Optical isomerism

Hint:

Chemistry Tip: If inside-sphere ligand and outside counter ion swap places, think Ionisation Isomerism immediately.

Answer 1

The Correct Option is A

Concept:
Coordination compounds can show different types of isomerism when ligands or ions exchange positions. Ionisation isomerism occurs when:

• One ion inside coordination sphere exchanges place with counter ion outside sphere.
• Both compounds give different ions in solution.

Step 1: Write both complexes clearly.
First complex: $$[\text{Co(NH}_3)_5(\text{SO}_4)]Br$$ Here:

• SO$_4^{2-}$ is inside coordination sphere.
• Br$^-$ is outside as counter ion.

Second complex: $$[\text{Co(NH}_3)_5Br]\text{SO}_4$$ Here:

• Br$^-$ is inside coordination sphere.
• SO$_4^{2-}$ is outside as counter ion.

Step 2: Observe the interchange.
The ligand inside and ion outside have exchanged positions: $$\text{SO}_4^{2-} \leftrightarrow Br^-$$ This is the defining feature of ionisation isomerism.
Step 3: Why other options are wrong?}}

• Linkage isomerism: same ligand binds through different donor atoms.
• Coordinate isomerism: ligand exchange between cationic and anionic complexes.
• Geometrical isomerism: cis-trans / fac-mer arrangement.
• Optical isomerism: non-superimposable mirror images.

None of these apply here.
Step 4: Final answer.
Therefore the complexes exhibit: $$\boxed{\text{Ionisation isomerism}}$$ Hence correct option is (A). :contentReference[oaicite:0]{index=0}