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:

The two compounds pentammine sulphate cobalt (III) chloride and pentammine sulphate cobalt (III) bromide represent

No isomerism

The correct answer is (4) no isomerism.

The two compounds pentammine sulphate cobalt (III) chloride and pentammine sulphate cobalt (III) bromide have the same chemical formula, \([Co(NH_3)_5SO_4]X\), where X is Cl or Br. The only difference between the two compounds is the identity of the counter ion. This does not give rise to any type of 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.

Ionization isomerism occurs when a complex compound has different counter ions. For example, the complex \([Co(NH_3)_5Cl]NO_3\) exhibits ionization isomerism because it can have either Cl^– or NO₃^– as the counter 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.