Problem 98 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following will exhibit geometrical isomerism? (M stands for a metal, and a and \(\mathrm{b}\) are achiral ligands, (1) \(\mathrm{Ma}_{2} \mathrm{~b}_{2}\) (2) \(\mathrm{Ma}_{4} \mathrm{~b}_{2}\) (3) \(\mathrm{Ma}_{5} \mathrm{~b}\) (4) \(\mathrm{Ma}_{6}\) (a) 1 and 2 (b) 2 and 3 (c) 1 and 3 (d) 2 and 4

Step-by-Step Solution

Answer

The correct answer is option (a): 1 and 2.

Step 1: Understanding Geometrical Isomerism

Geometrical isomerism occurs in compounds where the same atoms or groups are arranged differently in space, particularly around a central atom, often in coordination complexes. This type of isomerism occurs when different spatial arrangements of ligands around a central atom lead to distinct compounds.

Step 2: Analyzing Compound 1 (\mathrm{Ma}_{2} \mathrm{~b}_{2}\u00b"

Compound (1) \(\mathrm{Ma}_{2} \mathrm{~b}_{2}\) can exhibit geometrical isomerism because, in a square planar or tetrahedral geometry, it can form cis and trans configurations. In the cis configuration, like ligands are adjacent, whereas in the trans configuration, they are opposite each other.

Step 3: Analyzing Compound 2 (\u00b"\mathrm{Ma}_{4} \mathrm{~b}_{2}\u00b"

Compound (2) \(\mathrm{Ma}_{4} \mathrm{~b}_{2}\) also exhibits geometrical isomerism. This can happen when the ligands are arranged in an octahedral structure where the two b ligands can be positioned adjacent (cis) or opposite (trans) to each other.

Step 4: Analyzing Compound 3 (\u00b"\mathrm{Ma}_{5} \mathrm{~b}\u00b"

Compound (3) \(\mathrm{Ma}_{5} \mathrm{~b}\) does not exhibit geometrical isomerism because there is only one \(b\) ligand, leading to no variation in spatial positions for different isomers.

Step 5: Analyzing Compound 4 (\u00b"\mathrm{Ma}_{6}\u00b"

Compound (4) \(\mathrm{Ma}_{6}\) contains only \(a\) ligands, so there is no opportunity for geometrical isomerism, as all ligands are the same and no different spatial arrangements can exist.

Step 6: Selecting the Correct Options

Based on the above analysis, compounds (1) and (2) can exhibit geometrical isomerism due to the possibility of different spatial configurations. Thus, the correct answer is option (a): 1 and 2.

Key Concepts

Coordination Complexes

Coordination complexes are fascinating chemical structures where ligands bind to a central metal atom. These ligands can be ions or molecules, which form coordinate bonds. A coordinate bond is a type of chemical bond where both electrons in the bond originate from the same atom, typically found in the ligand.

• The central metal atom or ion tends to have vacant orbitals capable of accepting electrons from the ligands.
• In coordination complexes, the metal center acts as a Lewis acid, while ligands act as Lewis bases, donating lone pairs of electrons.

This characteristic is pivotal in determining the complex's geometry and properties. Different spatial arrangements can lead to different types of isomerism, impacting the chemical's reactivity and color.

Cis-Trans Isomerism

Cis-trans isomerism, a subset of geometrical isomerism, is where the coordination compounds with the same formula differ in the spatial arrangement of the atoms. Specifically, cis-trans isomerism is concerned with how ligands are placed around a central metal ion. In the cis configuration, similar ligands are adjacent to each other, while in the trans configuration, like ligands are opposite one another.

• This isomerism is often seen in square planar and octahedral complexes.
• It significantly affects the properties of the complex, including color and reactivity.

Understanding cis-trans isomerism is crucial, as these configurations often have different solubilities and boiling points, impacting their practical and industrial applications.

Octahedral Geometry

In octahedral geometry, a central metal atom is surrounded by six ligands symmetrically. This arrangement is typical for coordination numbers of six in transition metal complexes.The octahedral shape results in high symmetry and can allow for various types of isomerism based on the spatial arrangement of different ligands.

• An example of octahedral compounds is \(\mathrm{Ma}_{4} \mathrm{~b}_{2}\).
• The configuration can be either cis, where two identical ligands are adjacent, or trans, where they are opposite each other.

Understanding the octahedral geometry provides insight into the properties of a complex, such as magnetic moment, stability, and the possibility of isomerism.

Square Planar Complexes

Square planar complexes are a distinctive type of coordination complex where four ligands form a flat square around a central metal atom. This geometry is particularly prevalent among complexes of transition metals such as platinum and palladium.These complexes can exhibit cis-trans isomerism, as seen in compounds like \(\mathrm{Ma}_{2} \mathrm{~b}_{2}\). For example:

• In a cis configuration, similar ligands are adjacent.
• In a trans configuration, similar ligands are placed across the metal center from each other.

The arrangement influences the complex’s properties, including catalytic activities and biological activities. Many catalytic processes and metal-based drugs exploit the specific configurations of square planar complexes for desired effects.