Problem 30 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which kind of isomerism is exhibited by octahedral \(\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Br}_{2} \mathrm{Cl} ?\) (a) Geometrical and Ionization (b) Geometrical and Optical (c) Optical and Ionization (d) Geometrical only

Step-by-Step Solution

Answer

The complex exhibits geometrical and ionization isomerism.

Step 1: Understanding the Complex Structure

The given complex is \( \mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Br}_{2} \mathrm{Cl} \). It is an octahedral coordination compound with cobalt as the central metal atom. It is surrounded by four \(\mathrm{NH}_3\) ligands, two bromide ions (\(\mathrm{Br}^-\)), and one chloride ion (\(\mathrm{Cl}^-\)).

Step 2: Identifying Possible Isomers

Due to the presence of different ligands (\(\mathrm{NH}_3\), \(\mathrm{Br}^- ext{ and }\mathrm{Cl}^-\)), this complex can exhibit geometrical isomerism due to different possible arrangements of these ligands around the central metal atom. Also, different ions (\(\mathrm{Br}^-\) and \(\mathrm{Cl}^-\)) provide the possibility for ionization isomerism.

Step 3: Geometrical Isomerism Explanation

Geometrical isomerism is possible in octahedral complexes when there are different ways to arrange the ligands around the metal. In this complex, the \(\mathrm{Br}^- ext{ and }\mathrm{Cl}^-\) ions can be next to each other (cis) or opposite each other (trans).

Step 4: Checking for Optical Isomerism

Optical isomerism typically occurs in chiral complexes that have no plane of symmetry. In octahedral complexes, it arises when the ligands are arranged in a non-superimposable mirror image fashion. However, this specific structure of \( \mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Br}_{2} \mathrm{Cl} \) does not allow for optical activity due to its symmetric substitution pattern.

Step 5: Ionization Isomerism Explanation

Ionization isomerism occurs when the exchange of ligands between the inner coordination sphere and the outer coordination sphere leads to different compounds. The exchange between \(\mathrm{Br}^-\) and \(\mathrm{Cl}^-\) can result in different ionization products, indicating that ionization isomerism is possible. Thus, both geometrical and ionization isomerism are exhibited by this complex.

Key Concepts

Geometrical Isomerism

Geometrical isomerism is a form of stereoisomerism that occurs in coordination compounds where the spatial arrangement of ligands around the central metal atom differ. Imagine a simple model where ligands can be oriented in different positions such as side-by-side or opposite from one another.
In the case of the octahedral complex \( \mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Br}_{2} \mathrm{Cl} \), it can exhibit geometrical isomerism due to the different orientations of the bromide (\(\mathrm{Br}^-\)) and chloride (\(\mathrm{Cl}^-\)) ions compared to each other.

• Cis isomer: Bromide ions are adjacent to each other.
• Trans isomer: Bromide ions are across from each other.

These arrangements do not affect the connectivity of the molecules but affect their physical properties like solubility and color. Geometrical isomerism is common in square planar and octahedral complexes, providing fascinating variation even with the same chemical formula.

Ionization Isomerism

Ionization isomerism occurs when there is an exchange of ligands between the coordination sphere and the outside of the complex. This leads to different isomeric compounds which possess different ions in solution. Each isomer has a distinct identity and chemical formula based on its ion composition.
For the compound \( \mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Br}_{2} \mathrm{Cl} \), ionization isomerism arises because of the switch between \( \mathrm{Br}^- \) and \( \mathrm{Cl}^- \) ions.
The coordination sphere can contain either one of these ligands while the other remains outside.
This creates different compounds which can affect the compound's reactivity, color, and how it conducts electricity when dissolved in water.

• Two isomers: One with bromide inside the coordination complex and chloride outside.
• Another with chloride inside and bromide outside.

Ionization isomers can be identified by exchanging specific ions and observing different chemical properties.

Optical Isomerism

Optical isomerism is a type of stereoisomerism related to the presence of chirality. In simple terms, it occurs when a complex has non-superimposable mirror images, much like a pair of hands.
These isomers, called enantiomers, rotate plane-polarized light in different directions – one clockwise and the other counterclockwise. This property leads us to the "optical" part of its name.
However, not all complexes exhibit optical isomerism. For an octahedral compound like \( \mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Br}_{2} \mathrm{Cl} \), the presence of a symmetric arrangement of ligands means it does not possess optical isomerism.

• The compound must lack a plane of symmetry for optical isomers to exist.
• The specific symmetric layout in this complex disallows optical activity.

Understanding why this specific complex cannot show optical isomerism helps clarify the conditions under which optical activity is possible in coordination chemistry.