Problem 15 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A reaction of cobalt(III) chloride and ethylenediamine in a \(1: 2\) mole ratio generates two isomeric products A (violet coloured) and B (green coloured). A can show optical activity, but, \(\mathrm{B}\) is optically inactive. What type of isomers does \(\mathrm{A}\) and \(\mathrm{B}\) represent? (a) Geometrical isomers (b) Coordination isomers (c) Linkage isomers (d) Ionisation isomers

Step-by-Step Solution

Answer

(a) Geometrical isomers

Step 1: Understand Isomer Types

Geometrical isomers arise from different spatial arrangements around a metal center, potentially leading to cis-trans or facial-meridional differences. Coordination isomers involve swapping of ligands between a complex cation and anion. Linkage isomers differ in the connectivity of the ligand to the central metal atom. Ionization isomers result from the exchange of ions in the complex with ions outside the coordination sphere.

Step 2: Analyze the Optically Active Isomer 'A'

Isomer A is violet and optically active, which suggests it can exist in non-superimposable mirror images. This property is typical of complexes with chirality, often associated with geometrical configurations such as cis or facial arrangements in octahedral complexes.

Step 3: Analyze the Optically Inactive Isomer 'B'

Isomer B is green and optically inactive, implying it is symmetrical and does not exist in non-superimposable mirror images. This is often due to a trans or meridional arrangement in geometrical isomerism, where the configuration lacks chirality.

Step 4: Determine the Type of Isomerism

Since isomer A can be optically active while B cannot, and given the descriptions of cis/trans and fac/mer arrangements in a 1:2 cobalt-ethylenediamine complex, they likely represent geometrical isomers.

Key Concepts

Geometrical Isomerism in Coordination Chemistry

Geometrical isomerism in coordination chemistry pertains to the different spatial arrangements of ligands around a metal center. This type of isomerism occurs when the ligands surrounding the metal can occupy different positions, leading to distinct configurations. Commonly, geometrical isomers are cis-trans or facial-meridional varieties in octahedral complexes.

• Cis-trans isomerism: This occurs when two identical or similar ligands are adjacent (cis) or opposite (trans) each other.
• Facial-meridional isomerism: In octahedral complexes, three ligands can be adjacent (facial) or can alternate (meridional) around the central atom.

Geometrical isomers often have different chemical and physical properties, such as color, solubility, and boiling/melting points. This is because their different spatial arrangements affect how they interact with their surroundings.

Understanding Optical Activity

Optical activity refers to a compound's ability to rotate the plane of polarized light. This phenomenon is observed in substances that are chiral, meaning they have non-superimposable mirror images. In coordination chemistry, optical activity is usually linked to certain geometrical arrangements that create a chiral environment.

For cobalt(III) complexes, certain geometries, like the octahedral arrangement of ligands, can lead to optical activity if the compound is chiral. For instance, if two identical ligands form a cis arrangement in such a way that a non-superimposable configuration is achieved, the complex can exhibit optical activity. These chiral isomers are important since they often behave differently in biological systems compared to their mirror images.

• Chiral molecules will rotate plane-polarized light either to the right (dextrorotatory) or left (levorotatory).
• Measuring the degree of rotation can help determine the optical purity and concentration of a chiral compound.

Cobalt(III) Complexes and Isomerism

Cobalt(III) complexes often serve as examples to illustrate various types of isomerism in coordination chemistry. Cobalt, being a transition metal, easily forms complexes with a variety of ligands, leading to interesting structural possibilities.

In a typical setup, cobalt(III) can form octahedral complexes, where different placements of ligands lead to either geometrical or optical isomers. In the exercise discussed, cobalt(III) complexes with ethylenediamine illustrate geometrical isomerism with violet and green isomers.

• Isomer A (Violet): Likely a cis or facial arrangement leading to optical activity.
• Isomer B (Green): Likely a trans or meridional arrangement, making it optically inactive due to symmetry.

Cobalt(III) complexes are popular in studies due to their stability and the vibrant colors they often display, which are a result of the d-d transition of electrons. This results in educationally rich examples for understanding coordination chemistry.