The complex [Cr(NH₃)₅Br]Cl and [Cr(NH₃)₅Cl]Br can show which of the following?

Ionisation isomerism

The correct answer is option 2. Ionisation isomerism.

Ionisation isomerism occurs in coordination compounds when two isomers yield different ions in solution. This type of isomerism arises when the counter ion (the ion outside the coordination sphere) exchanges places with a ligand within the coordination sphere.

For the given complexes \([Cr(NH_3)_5Br]Cl\) and \([Cr(NH_3)_5Cl]Br\):

\([Cr(NH_3)_5Br]Cl\): In this complex, the \(Cr^{3+}\) ion is surrounded by five \(NH_3\) molecules and one \(Br^-\) ion, with \(Cl^-\) as the counter ion. In aqueous solution, it dissociates into \([Cr(NH_3)_5Br]^{2+}\) and \(Cl^-\) ions.

\([Cr(NH_3)_5Cl]Br\): In this complex, the \(Cr^{3+}\) ion is surrounded by five \(NH_3\) molecules and one \(Cl^-\) ion, with \(Br^-\) as the counter ion. In aqueous solution, it dissociates into \([Cr(NH_3)_5Cl]^{2+}\) and \(Br^-\) ions.

When these complexes dissolve in water, they produce different ions:

\([Cr(NH_3)_5Br]Cl \longrightarrow [Cr(NH_3)_5Br]^{2+} + Cl^-\)

\([Cr(NH_3)_5Cl]Br \longrightarrow [Cr(NH_3)_5Cl]^{2+} + Br^-\)

This difference in the ions produced in solution is the hallmark of ionisation isomerism. Both complexes have the same overall composition but differ in the identity of the ion that is outside the coordination sphere.