Q.7 Among the following pairs of co-ordination compounds, the pair(s) which represent(s) a case of
“ionization isomerism” is (are):
(A) [Pt(en) 2 Cl 2 ]Br 2 and [Pt(en)2 Br 2 ]Cl 2
(B) [Cr(NH3 ) 4 ClBr]NO2 and [Cr(NH3 ) 4 ClNO2 ]Br
(C) [Co(NH 3 ) 6 ][Cr(CN)6 ] and [Cr(NH3 ) 6 ][Co(CN) 6 ]
(D) [Co(NH 3 ) 5 Br]SO4 and [Co(NH3 ) 5 SO4 ]Br

Ionization isomerism occurs in coordination compounds when ligands and counter ions exchange positions between the coordination sphere and the outer sphere, yielding different ions in solution. This multiple-choice question tests identification of such pairs among given options.​

Ionization Isomerism Defined

Ionization isomers have the same molecular formula but differ in ions produced upon dissociation. A ligand inside the coordination sphere swaps with a counter ion outside. Classic test: one gives white AgBr precipitate with AgNO₃, the other AgCl.​

Option Analysis

Option A: [Pt(en)₂Cl₂]Br₂ and [Pt(en)₂Br₂]Cl₂
Cl⁻ and Br⁻ ions exchange positions. First dissociates as [Pt(en)₂Cl₂]²⁺ + 2Br⁻; second as [Pt(en)₂Br₂]²⁺ + 2Cl⁻. This shows ionization isomerism.​

Option B: [Cr(NH₃)₄ClBr]NO₂ and [Cr(NH₃)₄ClNO₂]Br
NO₂⁻ exchanges with Br⁻. First yields NO₂⁻ ions; second yields Br⁻ ions in solution. This represents ionization isomerism.​

Option C: [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆]
Ligands NH₃ and CN⁻ swap between two metal centers (Co³⁺ and Cr³⁺). Both are neutral overall with no counter ion exchange. This is coordination isomerism.​

Option D: [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br
Br⁻ and SO₄²⁻ exchange. First gives SO₄²⁻ ions (no Ag⁺ precipitate); second gives Br⁻ ions (white AgBr precipitate). Clear ionization isomerism.​

Correct Answer

Options A, B, and D show ionization isomerism.​

Introduction to Ionization Isomerism Coordination Compounds

Ionization isomerism coordination compounds arise when counter ions outside the coordination sphere exchange with similar ligands inside, producing different ions in aqueous solution. Common in exams like JEE NEET CSIR NET, these isomers test understanding of sphere distinction. Key phrase “ionization isomerism coordination compounds” highlights pairs like [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br.​

Characteristics and Identification

These isomers share molecular formula but yield distinct ions: one with X⁻ outside gives AgX precipitate, counterpart does not. Applies to charged complexes where anion acts as ligand. No change in geometry; purely structural.​

• Dissociation test differentiates: e.g., BaCl₂ confirms SO₄²⁻.​

• Seen in octahedral [MA₅B]X and square planar [M(AA)₂B₂]X₂ types.​

Solved Examples from Question

Answer: A, B, D.​

Exam Tips for CSIR NET JEE NEET

Practice ion tests: AgNO₃ for halides, BaCl₂ for sulfate. Differentiate from coordination isomerism (both spheres complexed). Revise 10+ examples for multi-correct questions.​​