Problem 173 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following has an optical isomer? [2010] (a) \(\left[\mathrm{Zn}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2}\right]^{2+}\) (b) \(\left[\mathrm{Co}(\mathrm{en})_{3}\right]^{3+}\) (c) \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}(\mathrm{en})\right]^{3+}\) (d) \(\left[\mathrm{Zn}(\mathrm{en})_{2}\right]^{2+}\) (en = ethylenediamine)

Step-by-Step Solution

Answer

Complex (b) \( \left[\mathrm{Co}(\mathrm{en})_{3}\right]^{3+} \) has optical isomers.

Step 1: Understanding Optical Isomerism

Optical isomerism occurs when a compound has non-superimposable mirror images, typically because of chiral centers, which aren't present in symmetric complexes. The presence of such chiral centers leads to enantiomers.

Step 2: Analyzing the Structure of Complex (a)

The complex \( \left[\mathrm{Zn}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2}\right]^{2+} \) consists of a central Zn ion with two ammonia and one ethylenediamine ligands. This set-up does not produce chiral centers or lack symmetry to result in optical isomerism.

Step 3: Analyzing the Structure of Complex (b)

The complex \( \left[\mathrm{Co}(\mathrm{en})_{3}\right]^{3+} \) has a central Co ion surrounded octahedrally by three ethylenediamine ligands. Ethylenediamine being a bidentate ligand can create a chiral configuration, thus allowing for optical isomers.

Step 4: Analyzing the Structure of Complex (c)

The compound \( \left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}(\mathrm{en})\right]^{3+} \) contains one bidentate ligand and four water molecules, which doesn't create an environment prone to optical isomers due to its symmetry.

Step 5: Analyzing the Structure of Complex (d)

The complex \( \left[\mathrm{Zn}(\mathrm{en})_{2}\right]^{2+} \) features two bidentate ethylenediamine ligands around a Zn ion, however, symmetry is present, resulting in no optical isomerism evolution.

Step 6: Conclusion: Identifying the optical isomer

Based on the examination of each option, Complex (b) \( \left[\mathrm{Co}(\mathrm{en})_{3}\right]^{3+} \) demonstrates the possibility of optical isomerism due to the configuration of three bidentate ligands around the Co ion, allowing for chirality.

Key Concepts

Chiral Centers

Chiral centers play a crucial role in optical isomerism. A chiral center is a specific type of atom within a molecule that is bonded to four different atoms or groups. This unique bonding pattern means that the molecule cannot be superimposed on its mirror image, much like our left and right hands. This leads to enantiomers, two mirror image forms of the same molecule.

In coordination chemistry, the presence of chiral centers often arises from specific arrangements of ligands around the central atom. Not every molecule with a chiral center will show optical isomerism unless the overall arrangement leads to non-superimposable mirror images. For example, complex (b) \( \left[ \mathrm{Co}(\mathrm{en})_{3}\right]^{3+} \) creates chiral centers due to its configuration, making optical isomerism possible, unlike complex (a) or (d), where symmetry negates this possibility.

Coordination Complexes

Coordination complexes are fascinating structures where central metal atoms bind with surrounding atoms or groups of atoms, called ligands, to form a stable entity. These complexes can take various shapes, influencing the properties of the complex significantly.

The geometry and nature of these ligands play a vital role in determining whether a complex can exhibit optical isomerism. In our example, complex \( \left[ \mathrm{Co}(\mathrm{en})_{3}\right]^{3+} \) takes an octahedral shape with three bidentate ligands. This arrangement lacks symmetry, allowing it to have non-superimposable mirror images and, therefore, enabling optical isomerism. In contrast, complexes like \( \left[\mathrm{Co}(\mathrm{H}_{2}\mathrm{O})_{4}(\mathrm{en})\right]^{3+} \) maintain a symmetrical arrangement that undermines optical isomerism due to their rigid structure.

Bidentate Ligands

Bidentate ligands are particularly interesting as they have two atom groups capable of forming bonds with a central metal ion. This characteristic allows them to form more stable rings with the metal ion, influencing the overall configuration of the coordination complex.

Ethylene diamine (en) is a classic example of a bidentate ligand. When en wraps around a metal center like cobalt in \( \left[ \mathrm{Co}(\mathrm{en})_{3}\right]^{3+} \), it not only secures the metal but also creates a geometric configuration susceptible to forming chiral centers. This specific alignment results in a complex capable of exhibiting optical isomerism. Such is not possible with simpler, monodentate ligands that do not wrap around the metal, like ammonia in \( \left[ \mathrm{Zn}(\mathrm{en})(\mathrm{NH}_3)_2 \right]^{2+} \). Here, the bidentate nature provides the potential for three-dimensional complexity within the structure.