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

(a) 1 and 2.

Step 1: Understanding Geometrical Isomerism

Geometrical isomerism occurs in coordination compounds where the spatial arrangement of ligands around the central metal atom can lead to different non-superimposable isomers, typically cis and trans configurations. This is usually possible when there are at least two different types of ligands in certain numbers around the metal.

Step 2: Analyzing Each Option

Examine each compound to see if it can exhibit geometrical isomerism:- (1) \(\mathrm{Ma}_2 \mathrm{b}_2\): This octahedral complex can have the ligands arranged differently (cis or trans), so it exhibits geometrical isomerism.- (2) \(\mathrm{Ma}_4 \mathrm{b}_2\): This octahedral complex allows the two \(\mathrm{b}\) ligands to be either adjacent (cis) or opposite (trans) to each other, indicating geometrical isomerism.- (3) \(\mathrm{Ma}_5 \mathrm{b}\): With only one \(\mathrm{b}\) ligand, there is no possibility for different spatial arrangements leading to geometrical isomers.- (4) \(\mathrm{Ma}_6\): Since all ligands are the same, no positional isomerism is possible.

Step 3: Selecting the Correct Answer

Based on the analysis, both compounds (1) \(\mathrm{Ma}_2 \mathrm{b}_2\) and (2) \(\mathrm{Ma}_4 \mathrm{b}_2\) can exhibit geometrical isomerism. Therefore, the correct choice corresponds to (a) 1 and 2.

Key Concepts

Coordination Compounds

Coordination compounds are fascinating structures consisting of a central metal atom or ion surrounded by a group of molecules or ions known as ligands. These compounds represent a significant area of study in chemistry due to their complex structures and diverse applications, ranging from catalysis to medicine. Coordination compounds are characterized by their coordination number, which refers to the number of ligand atoms bonded directly to the metal center.
The coordination number can vary widely but often includes common numbers like 6 (octahedral complex) or 4 (tetrahedral complex). The unique arrangements of ligands around the metal give rise to different properties, including the ability to exhibit geometrical isomerism. When ligands are arranged in different spatial configurations around the metal, it can lead to multiple isomers, even though the chemical composition remains the same. These isomers can have distinct physical and chemical properties, making them crucial in various applications, such as designing new materials or drugs.

Ligands

Ligands are the vital components in coordination chemistry, acting as the connectors between the central metal atom and the surrounding environment. They are molecules or ions that provide at least one pair of electrons to the metal center, forming a coordinate bond. Ligands can be as simple as a single atom, like in chloride ions, or more complex like organic molecules.
Ligands are classified based on their denticity, which refers to the number of donor atoms they offer to the metal center:

• Monodentate ligands, such as water or ammonia, have a single donor atom.
• Bidentate ligands, like ethylenediamine, offer two sites for binding, which can create chelate rings, providing extra stability to the compound.
• Polydentate ligands can have multiple binding sites, increasing stability and complexity.

The arrangement of these ligands can lead to different spatial configurations, which is central to understanding geometrical isomerism in coordination compounds.

Cis-Trans Isomers

Cis-trans isomerism is a type of geometrical isomerism that arises in coordination compounds, among other types of chemical compounds. It is defined by the relative position of specific ligands attached to the central metal atom. In a cis isomer, two identical or similar ligands are adjacent to each other, while in a trans isomer, they are placed opposite to one another.
For instance, in a compound like \(\mathrm{Ma}_{2} \mathrm{~b}_{2}\), which can adopt an octahedral geometry, ligands ‘a’ can be adjacent, resulting in a cis form, or opposite, resulting in a trans form. This isomerism can significantly affect the properties of the compound, including its physical state, solubility, and even reactivity.
Cis-trans isomerism is crucial in various fields. For example, in medicinal chemistry, the different isomers can lead to dramatically different biological activities; one isomer may be therapeutically active, while the other is inactive or even toxic. This illustrates the importance of understanding and controlling isomerism in the synthesis and application of coordination compounds.