Two or more compounds having the same molecular formula but different arrangement of atoms are called isomers and the phenomenon is called isomerism.

Because of different arrangement of atoms, isomers differ in one or more physical or chemical properties. Isomers can be broadly classified into two major categories:

(A) Structural isomers (B) Stereoisomers.

These are further sub-divided as shown below:

\([Co(NH_3)_5Br]SO_4\) and \([Co(NH_3)_5SO_4]Br\)are the examples of

Ionization isomerism

The correct answer is (3) ionization isomerism.

The two complexes \([Co(NH_3)_5Br]SO_4\) and \([Co(NH_3)_5SO_4]Br\) have the same chemical formula, but they have different counter ions. The first complex has a bromide ion as the counter ion, while the second complex has a sulfate ion as the counter ion. This difference in counter ions gives rise to ionization isomerism.

Linkage isomerism occurs when a complex compound has the same ligands but different donor atoms. For example, the complex \([Co(NH_3)_5Cl]SO_4\) exhibits linkage isomerism because the chloride ligand can donate either its chlorine atom or its nitrogen atom to the metal ion.

Coordination isomerism occurs when a complex compound has different metal ions in the coordination sphere. For example, the complex \([Co(NH_3)_4Cl_2]NO_2\) exhibits coordination isomerism because it can have either Co²⁺ or Co³⁺ as the metal ion.

Ligand isomerism occurs when a complex compound has different ligands in the coordination sphere. For example, the complex \([Co(NH_3)_4Cl_2]NO_2\) exhibits ligand isomerism because it can have either Cl^– or NO₂^– as one of the ligands.