Problem 171 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following has an optical isomer? [2009] (a) \(\left[\mathrm{CO}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2}\right]^{2+}\) (b) \(\left[\mathrm{CO}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}(\mathrm{en})\right]^{3+}\) (c) \(\left[\mathrm{CO}(\mathrm{en})_{2}\left(\mathrm{NH}_{3}\right)_{2}\right]^{3+}\) (d) \(\left[\mathrm{CO}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}\right]^{+}\)

Step-by-Step Solution

Answer

Option (c) \\([\mathrm{CO}(\mathrm{en})_{2}(\mathrm{NH}_{3})_{2}]^{3+}\\) has an optical isomer.

Step 1: Identify the Components

Examine the given complexes to identify their components: ethylenediamine (en), water (H₂O), ammonia (NH₃), and chloride (Cl⁻). Pay attention to the bidentate ligand ethylenediamine (en), as it contributes to geometric arrangements that can lead to optical isomerism.

Step 2: Evaluate Geometric Arrangement

Optical isomerism occurs when a complex has a non-superimposable mirror image. Focus on complexes with bidentate ligands (like en), as they can facilitate such configurations.

Step 3: Analyze Complexes for Optical Isomerism

(a) \([\mathrm{CO}(\mathrm{en})(\mathrm{NH}_{3})_{2}]^{2+}\): Contains en but only one bidentate ligand, low chance of optical isomerism. (b) \([\mathrm{CO}(\mathrm{H}_{2}\mathrm{O})_{4}(\mathrm{en})]^{3+}\): One en among four water molecules, unlikely to form a non-superimposable structure. (c) \([\mathrm{CO}(\mathrm{en})_{2}(\mathrm{NH}_{3})_{2}]^{3+}\): Two en ligands can ensure a chiral arrangement. (d) \([\mathrm{CO}(\mathrm{NH}_{3})_{3} \mathrm{Cl}]^{+}\): No bidentate ligands, unlikely to be chiral.

Step 4: Determine the Optical Isomer

The complex \([\mathrm{CO}(\mathrm{en})_{2}(\mathrm{NH}_{3})_{2}]^{3+}\) (option c) has two bidentate en ligands, making it possible to form optical isomers due to the presence of chirality in its arrangement.

Key Concepts

Chirality in Coordination Compounds

Chirality is an intriguing property of some chemical compounds, where a molecule and its mirror image cannot be superimposed on each other. This is similar to how your left and right hands are mirror images but not identical. In the realm of coordination chemistry, chirality becomes an important factor, especially when discussing optical isomerism. Coordination compounds may display chirality when their spatial arrangement around the central atom gives rise to two non-superimposable mirror images. In these complexes, optical activity is often related to specific geometric arrangements of surrounding atoms or groups, a notable example being complexes that contain bidentate ligands. The presence of such ligands facilitates chirality because they occupy two coordination sites on the central atom, promoting unique spatial orientations that lead to chiral centers. These chiral compounds can rotate plane-polarized light, a property used to distinguish optical isomers. Therefore, in the steps outlined in the solution, identifying structural elements like bidentate ligands and the complex's overall geometry is key to recognizing potential chirality.

Bidentate Ligands

Bidentate ligands are crucial components in many coordination complexes, contributing to the formation of stable and intricate structures. A bidentate ligand, like ethylenediamine (en), binds to a central metal atom through two atoms, forming a five-membered ring. This dual-binding ability enables the ligand to hold onto the central atom more securely compared to monodentate ligands, which attach through only a single atom. Since bidentate ligands occupy more coordination sites, they naturally affect the geometric configuration of the molecule. This altered geometry is crucial for optical isomerism, where two different arrangements create distinct non-superimposable mirror images. Unlike simpler ligands, bidentate ligands can align in several possible configurations, depending on the number of such ligands in the structure and how they are arranged around the central metal ion. Understanding the role of bidentate ligands is essential for predicting and identifying the potential for a compound to have optical isomers, as they directly influence the stereochemistry involved.

Stereochemistry of Transition Metal Complexes

Stereochemistry plays a vital role in the chemical behavior and physical properties of coordination compounds, particularly those formed around transition metals. Transition metals are versatile, offering various coordination numbers and geometric possibilities, which influence how ligands arrange around them. Common geometries include octahedral, tetrahedral, and square planar, with each geometry accommodating different ligand arrangements.In coordination chemistry, especially with transition metals, the stereochemistry often involves considering both the spatial arrangement of ligands and the symmetry of the complex. Complexes like \([\mathrm{CO}(\mathrm{en})_{2}(\mathrm{NH}_{3})_{2}]^{3+}\) exhibit interesting stereochemical properties because of their potential to form chiral structures, resulting from the orientation of bidentate ligands such as ethylenediamine. The arrangement of these ligands creates unique three-dimensional structures, driving the formation of optical isomers. In developing an understanding of such stereochemical concepts, one appreciates how a simple change in ligands or their arrangement can lead to significant changes in the properties of a compound, reflecting the elegant complexity of coordination chemistry.