Problem 19 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following does not have an optical isomer? (a) \(\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{3}\right]\) (b) \(\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right] \mathrm{Cl}\) (c) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}_{3}\right]\) (d) \(\left[\mathrm{Co}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right] \mathrm{Cl}\)

Step-by-Step Solution

Answer

Option (c) does not have an optical isomer.

Step 1: Understanding Optical Isomers

Optical isomers, also known as enantiomers, are a type of stereoisomer that are non-superimposable mirror images of each other. This property typically arises due to the presence of a chiral center in the molecule. In coordination compounds, this can occur if the compound has a certain type of geometry and ligand arrangement.

Step 2: Evaluate Each Option

Let's evaluate each complex one by one for potential optical isomers. Optical activity in coordination complexes generally requires a chiral center, usually in a tetrahedral or octahedral complex without any planes of symmetry.

Step 3: Option (a) \\([\mathrm{Co}([\mathrm{en}])_{2} \mathrm{Cl}_{3}]\\)

In this complex, there is no potential for optical isomerism because the complex does not have a chiral arrangement. It likely has a meridional configuration where ligands are symmetric.

Step 4: Option (b) \\([\mathrm{Co}([\mathrm{en}])_{2} \mathrm{Cl}_{2}] \mathrm{Cl}\\)

This complex can have optical isomers because it features a different arrangement of bidentate ethylenediamine ligands that can result in chirality.

Step 5: Option (c) \\([\mathrm{Co}([\mathrm{NH}_{3}])_{3} \mathrm{Cl}_{3}]\\)

This complex is not chiral because it can exist in a fac-mer configuration that leads to a non-chiral plane of symmetry. This arrangement does not result in optical activity.

Step 6: Option (d) \\([\mathrm{Co}([\mathrm{en}])([\mathrm{NH}_{3}])_{2} \mathrm{Cl}_{2}] \mathrm{Cl}\\)

This complex can have optical isomers due to its potential arrangement of the ethylenediamine and ammonia ligands around the metal center, lacking symmetry.

Step 7: Concluding Non-chirality

From the above analysis, option (a) and option (c) are potentially non-chiral, but option (c) definitively lacks any chirality due to its fac-mer symmetrical arrangement.

Key Concepts

Coordination Compounds

Coordination compounds are fascinating structures formed when metal ions bind with molecules or ions, collectively known as ligands. These ligands surround the metal ion, creating a complex structure that can exhibit properties different from the individual components. The metal ion at the center, also referred to as the coordination center, can connect with one or more ligands via coordinate covalent bonds.

Coordination numbers indicate how many ligand atoms are bonded to the central metal ion. Some common coordination numbers are 4 and 6, leading to different geometric shapes for the complexes, such as square planar, tetrahedral, or octahedral configurations.

• The geometry of the complex plays a crucial role in determining its physical and chemical properties.
• Coordination compounds exhibit varied behaviors, such as color changes, magnetism, and catalytic activities, depending on their formation and structure.

Coordination compounds are prevalent in biological systems as well. For example, hemoglobin is a coordination complex where iron acts as the central metal ion, assisting in oxygen transport in the bloodstream.

Chirality in Chemistry

Chirality is a unique concept in chemistry where a molecule is non-superimposable on its mirror image, much like how left and right hands are mirror images but cannot align perfectly. Chirality is vital because it often directly influences the properties of a compound, including its interaction with other chiral molecules.

In the chemical context, for a molecule or complex to be chiral, it often needs to lack a plane of symmetry. In coordination complexes, chirality usually arises when there are specific geometric arrangements without symmetrical elements.

• Chirality in coordination compounds can be determined by looking for an asymmetric arrangement of ligands around the central metal atom.
• Chiral complexes can have optical isomers, which are important in industrial applications, especially in pharmaceuticals where the two enantiomers might have different biological activities.

The complexity and diversity of coordination compounds make chirality a valuable tool for understanding and manipulating chemical processes for targeted applications.

Stereoisomers

Stereoisomers are a class of isomers that have the same molecular formula and sequence of bonded atoms, but different three-dimensional orientations in space. This category of isomers includes geometric isomers (cis-trans) and optical isomers (enantiomers). Stereoisomerism is a critical notion in chemistry, especially in studying molecular properties and reactions.

Optical isomers, a type of stereoisomer, are molecules that can rotate plane-polarized light because of their chiral nature. This ability is due to the molecules' non-superimposable mirror images, known as enantiomers.

• When studying stereoisomerism, it's essential to determine the spatial arrangement of the ligands in coordination compounds.
• Bidentate ligands, such as ethylenediamine (en), often play a significant role in forming stereoisomers due to their ability to form a ring structure around the central metal, providing a chiral environment.

Understanding stereoisomers, especially optical isomers, is crucial in fields like pharmacology because different stereoisomers can have vastly different effects in biological settings. It highlights the essential relationship between molecular structure and function.