73. Among the following pairs of co-ordination compounds, the pair(s) which represent(s) a case of “ionization isomerism” is (are):

(A) [Pt(en)₂Cl₂]Br₂ and [Pt(en)₂Br₂]Cl₂

(B) [Cr(NH₃)₄ClBr]NO₂ and [Cr(NH₃)₄ClNO₂]Br

(C)[Co(NH₃)₆][Cr(CN)₆] and[Cr(NH₃)₆][Co(CN)₆]

(D)[Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅ SO₄]Br

Ionization Isomerism in Coordination Compounds

In this question, four different pairs of coordination compounds are given. Some represent ionization isomerism, while others belong to different classes of isomerism. The objective is to identify the pair or pairs that exhibit ionization isomerism.

What is Ionization Isomerism?

Ionization isomerism occurs when an anion present inside the coordination sphere exchanges its position with an anion present outside the coordination sphere. Although the molecular formula remains exactly the same, the ions produced when the compounds dissolve in water become different.

As a result, ionization isomers exhibit different chemical reactions toward reagents because they release different ions into solution.

For example, one isomer may liberate chloride ions, while the other liberates nitrate or bromide ions.

This type of isomerism is unique because it directly affects the ions detected during qualitative analysis.

Detailed Analysis of Every Option

Option (A): [Pt(en)₂Cl₂]Br₂ and [Pt(en)₂Br₂]Cl₂

In the first complex, chloride ions are coordinated to platinum, while bromide ions remain outside the coordination sphere as counter ions.

In the second complex, bromide ions are coordinated, whereas chloride ions become the counter ions.

Thus, chloride and bromide exchange their positions between the coordination sphere and the ionization sphere.

This is the exact definition of ionization isomerism.

Therefore, Option (A) is correct.

Option (B): [Cr(NH₃)₄ClBr]NO₂ and [Cr(NH₃)₄ClNO₂]Br

In the first compound, bromide is coordinated while nitrite remains outside the coordination sphere.

In the second compound, nitrite enters the coordination sphere and bromide moves outside.

Again, an anion inside the coordination sphere exchanges its position with the counter ion outside the coordination sphere.

This is another classic example of ionization isomerism.

Therefore, Option (B) is also correct.

Option (C): [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆]

This pair does not involve exchange between coordinated and ionizable anions.

Instead, the entire complex cation and complex anion exchange their metal centers. In one compound, cobalt forms the cationic complex and chromium forms the anionic complex, whereas in the other compound the roles are reversed.

This phenomenon is known as coordination isomerism, not ionization isomerism.

Therefore, Option (C) is incorrect.

Option (D): [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br

In the first complex, bromide is coordinated while sulfate exists as the counter ion.

In the second complex, sulfate becomes coordinated and bromide becomes the ionizable counter ion.

This exchange occurs between a coordinated ligand and the counter ion.

Hence, this pair also exhibits ionization isomerism.

Therefore, Option (D) is correct.

Correct Answer

The compounds showing ionization isomerism are:

(A), (B) and (D)

Why Option (C) is Different?

Students often confuse coordination isomerism with ionization isomerism because both involve two coordination complexes. However, the distinguishing feature is very simple.

In ionization isomerism, only a ligand inside the coordination sphere exchanges with a counter ion outside the sphere.

In coordination isomerism, the ligands remain coordinated, but the metal ions exchange between the complex cation and the complex anion.

Therefore, Option (C) belongs to an entirely different category of structural isomerism.

Difference Between Ionization Isomerism and Coordination Isomerism

Ionization isomerism changes the ions produced in aqueous solution because different counter ions are released. Consequently, the compounds give different results in qualitative analysis and precipitation reactions.

Coordination isomerism, on the other hand, involves redistribution of ligands between two complex ions. The counter ions do not simply exchange positions with coordinated ligands.

Recognizing this distinction allows students to identify the correct type of isomerism within seconds.

Final Answer

Correct Options: (A), (B) and (D)