AIIMS 2005 Chemistry Isomerism MCQ Question

To determine which pairs of complexes exhibit optical isomerism, we first need to understand what optical isomerism is in the context of coordination compounds. Optical isomers, or enantiomers, are non-superimposable mirror images of each other, typically arising in chiral molecules. For a coordination complex to show optical isomerism, it must lack an internal plane of symmetry and possess chirality.

Analyzing Each Option

Option A: (a) cis-[Cr(C₂O₄)₂Cl₂]³⁻, cis-[Co(NH₃)₄Cl₂]

1. cis-[Cr(C₂O₄)₂Cl₂]³⁻: This complex has a square planar geometry. Due to the presence of two bidentate oxalate ligands ($C_2O_4^{2-}$) which occupy two positions, the remaining two positions are occupied by chloride ions. However, this complex possesses a plane of symmetry and does not exhibit optical isomerism.

2. cis-[Co(NH₃)₄Cl₂]: This complex has an octahedral geometry. The cis arrangement of the ligands does not create chirality, as it can be superimposed on its mirror image. Therefore, it also does not show optical isomerism.

Conclusion: Neither complex exhibits optical isomerism.

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Option B: (b) [Co(en)₂Cl₂]⁺, cis-[Co(en)₂Cl₂]Cl

1. [Co(en)₂Cl₂]⁺: This complex contains two bidentate ethylenediamine ($en$) ligands and two chloride ions in an octahedral configuration. The arrangement of the $en$ ligands in a cis position creates a chiral environment, resulting in two non-superimposable mirror images (optical isomers).

2. cis-[Co(en)₂Cl₂]Cl: This complex has a similar structure as the first one but includes an additional chloride ion. The presence of the two bidentate ethylenediamine ligands still allows for chirality in the octahedral arrangement, leading to optical isomerism as well.

Conclusion: Both complexes in this option exhibit optical isomerism.

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Option C: (c) [PtCl(dien)]Cl, [NiCl₂Br₂]²⁻

1. [PtCl(dien)]Cl: This complex has a square planar geometry with a bidentate diethylenetriamine ($dien$) ligand. It can potentially exhibit optical isomerism. However, since it typically has a non-chiral arrangement due to the square planar nature, it does not show optical isomerism.

2. [NiCl₂Br₂]²⁻: This complex has a tetrahedral or square planar geometry but possesses a plane of symmetry, thus it does not exhibit optical isomerism.

Conclusion: Neither complex exhibits optical isomerism.

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Option D: (d) [Co(NO₂)₃(NH₃)₃], cis-[Pt(en)₂Cl₂]

1. [Co(NO₂)₃(NH₃)₃]: This complex is octahedral, and while it can have several arrangements depending on the ligands, the presence of three identical ligands ($NO_2$) and three identical $NH_3$ ligands means that it is symmetric and does not have optical isomers.

2. cis-[Pt(en)₂Cl₂]: This complex does exhibit optical isomerism due to the two bidentate ethylenediamine ligands being arranged in a way that creates chirality.

Conclusion: Only one complex in this pair shows optical isomerism.

Summary

• Correct Answer: B because both complexes in this pair exhibit optical isomerism due to their chiral arrangements.
• Incorrect Answers: Options A, C, and D do not have both complexes exhibiting optical isomerism due to either symmetry or lack of chirality.

This analysis is crucial for understanding the concept of isomerism in coordination compounds, as it combines geometric arrangements, ligand types, and the implications of chirality.