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

Question

The pair(s) of coordination complexes/ions exhibiting the same kind of isomerism is(are) (a) \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}\right] \mathrm{Cl}_{2}\) and \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\right] \mathrm{Cl}\) (b) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\right]^{+}\)and \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right) \mathrm{Cl}\right]^{+}\) (c) \(\left[\mathrm{CoBr}_{2} \mathrm{Cl}_{2}\right]^{2-}\) and \(\left[\mathrm{PtBr}_{2} \mathrm{Cl}_{2}\right]^{2}\) (d) \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{NO}_{3}\right)\right] \mathrm{Cl}\) and \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}\right] \mathrm{Br}\)

Step-by-Step Solution

Answer

Pairs (c) and (d) exhibit isomerism: (c) geometric, (d) ionisation.

Step 1: Identify the Type of Isomerism

First, understand the types of isomerism that can occur in coordination complexes, which include structural isomerism, such as linkage isomerism, coordination isomerism, ionisation isomerism, and stereoisomerism, such as geometric or optical isomerism.

Step 2: Analyze Complex (a)

In \([\mathrm{Cr}(\mathrm{NH}_{3})_{5} \mathrm{Cl}] \mathrm{Cl}_{2}\) and \([\mathrm{Cr}(\mathrm{NH}_{3})_{4} \mathrm{Cl}_{2}] \mathrm{Cl}\), the difference is in the ligand arrangement around the central metal ion. This represents ionisation isomerism because counter ions can also be water molecules or other ligands in different coordinations.

Step 3: Analyze Complex (b)

In \([\mathrm{Co}(\mathrm{NH}_{3})_{4} \mathrm{Cl}_{2}]^{+}\) and \([\mathrm{Pt}(\mathrm{NH}_{3})_{2}(\mathrm{H}_{2}\mathrm{O}) \mathrm{Cl}]^{+}\), the coordination number and type of ligands are completely different, so they do not exhibit a common type of isomerism.

Step 4: Analyze Complex (c)

In \([\mathrm{CoBr}_{2} \mathrm{Cl}_{2}]^{2-}\) and \([\mathrm{PtBr}_{2} \mathrm{Cl}_{2}]^{2-}\), both complexes have the same formula but differ in arrangements. These can exhibit geometric, specifically cis-trans isomerism, due to their square planar arrangements and identical sets of ligands.

Step 5: Analyze Complex (d)

In \([\mathrm{Pt}(\mathrm{NH}_{3})_{3}(\mathrm{NO}_{3})] \mathrm{Cl}\) and \([\mathrm{Pt}(\mathrm{NH}_{3})_{3} \mathrm{Cl}] \mathrm{Br}\), ionisation isomerism can occur because exchanging anions such as Cl⁻ and Br⁻ changes their potential ionic state in solution.

Step 6: Conclusion

The pair(s) exhibiting the same kind of isomerism are pairs (c) and (d), as they exhibit geometric and ionisation isomerism, respectively.

Key Concepts

Ionisation Isomerism

In ionisation isomerism, the exchange of ions or ligands within a coordination complex allows compounds to form with the identical chemical formula but varying physical properties. This type of isomerism occurs when an anion that is originally outside the coordination sphere replaces a ligand inside and vice versa.

For example, consider the complexes

• \(\left[\text{Pt}(\text{NH}_3)_3(\text{NO}_3)\right] \text{Cl}\)
• \(\left[\text{Pt}(\text{NH}_3)_3 \text{Cl}\right] \text{Br}\)

Here, the exchange of chloride (Cl⁻) and bromide (Br⁻) ions represents ionisation isomerism. When these compounds dissolve in water, they produce different ions in the solution, affecting properties such as conductivity and reactivity.

This difference stems not from a change in the connectivity of the atoms, but rather from an alteration in which ions are dissociated in the solution, providing unique characteristics to each isomer.

Geometric Isomerism

Geometric isomerism, also known as cis-trans isomerism, occurs when ligands around a central metal ion can be arranged differently in space while maintaining connectivity. Geometric isomers display distinct physical and chemical properties although they share the same molecular formula and sequence of bonded atoms.

A classic example of geometric isomerism is observed in square planar or octahedral complexes, such as

• \(\left[\text{CoBr}_2 \text{Cl}_2\right]^{2-}\)
• \(\left[\text{PtBr}_2 \text{Cl}_2\right]^{2-}\)

In these cases, the positions of the identical ligands (such as bromide (Br⁻) and chloride (Cl⁻)) can vary, leading to either cis configurations, where the same ligands are adjacent, or trans configurations, where they are opposite each other.

This distinction is critical because the different spatial arrangements can have a dramatic impact on the complex's reactivity and interactions with other molecules.

Structural Isomerism

Structural isomerism in coordination chemistry encompasses several types, including coordination isomerism and linkage isomerism. These structural changes mean the connectivity of atoms differs, resulting in compounds with distinct properties despite having the same formula.

Coordination isomerism occurs when ligand exchanges occur between a complex cation and an anion in two distinct salts. An example might be when two different metals exchange places within their coordination spheres or when entire ligands are swapped between them.

Linkage isomerism, on the other hand, happens when a ligand can bind to a central metal in multiple ways. For example, the ligand nitrite can attach via the nitrogen (\(\eta^1-NO_2\)) or via the oxygen (\(\eta^1-ONO\)). This model illustrates how variations in binding sites can give rise to distinct properties and behaviors in structural isomerism.