Problem 23 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An octahedral complex with molecular composition M.5 \(\mathrm{NH}_{3} \cdot \mathrm{Cl} \cdot \mathrm{SO}_{4}\) has two isomers, \(\mathrm{A}\) and \(\mathrm{B}\). The solution of A gives a white precipitate with \(\mathrm{AgNO}_{3}\) solution and the solution of B gives white precipitate with \(\mathrm{BaCl}_{2}\) solution. The type of isomerism exhibited by the complex is: (a) Linkage isomerism (b) Ionisation isomerism (c) Coordinate isomerism (d) Geometrical isomerism

Step-by-Step Solution

Answer

Ionisation isomerism (b).

Step 1: Understand the Problem

The given molecular composition is M.5 \(\mathrm{NH}_{3} \cdot \mathrm{Cl} \cdot \mathrm{SO}_{4}\), and we need to determine the type of isomerism based on the behavior of the two isomers A and B with solutions \(\mathrm{AgNO}_{3}\) and \(\mathrm{BaCl}_{2}\). Isomer A forms a white precipitate with \(\mathrm{AgNO}_{3}\), while isomer B forms a white precipitate with \(\mathrm{BaCl}_{2}\).

Step 2: Identify the Type of Isomerism

Isomers that form different products with the same reagents or different products with two separate reagents typically exhibit ionisation isomerism. This type of isomerism occurs when the compound can exchange ions inside its coordination sphere with those outside it, altering the ion that precipitates with certain reagents.

Step 3: Apply Precipitation Reaction Knowledge

A complex forming a white precipitate with \(\mathrm{AgNO}_{3}\) suggests that \(\mathrm{Cl}^{-}\) ions are free or outside the coordination sphere in isomer A. Similarly, a complex forming a white precipitate with \(\mathrm{BaCl}_{2}\) suggests that \(\mathrm{SO}_{4}^{2-}\) ions are free or outside the coordination sphere in isomer B.

Step 4: Conclusion

Since the different isomers precipitate different ions (\(\mathrm{Cl}^{-}\) in A and \(\mathrm{SO}_{4}^{2-}\) in B), it indicates that the difference in isomers is due to different ions present inside vs. outside the coordination sphere. This confirms that the type of isomerism is ionisation isomerism.

Key Concepts

Ionisation Isomerism

In coordination chemistry, ionisation isomerism occurs when a compound can form isomers through the interchange of anions or neutral molecules between the coordination sphere and the ionization sphere. This means that the ions inside the coordination sphere of a complex are exchanged with those present outside.
For instance, if a compound has an ion, such as chloride, that can move into the position originally occupied by another ion like sulfate, we get different isomers even though the overall composition stays the same. These different arrangements lead to distinct chemical behaviors.

• An isomer of a complex might react with a reagent to form a precipitate because the exchanging ion is no longer bonded within the coordination sphere and remains free in the solution.
• This leads to diverse chemical reactivity even though the primary structure of the compound looks identical.

In our example, isomer A forms a white precipitate with \(\mathrm{AgNO}_3\), suggesting \(\mathrm{Cl}^-\) is free, while isomer B reacts with \(\mathrm{BaCl}_2\), indicating \(\mathrm{SO}_4^{2-}\) is the exchanged ion. This interchange of ions inside versus outside the coordination sphere highlights ionisation isomerism.

Coordination Sphere

The coordination sphere in a coordination complex is the central section that consists of a metal atom or ion and its directly bonded ligands. In general, it dictates the key chemical properties of the entire complex.
These ligands, which can be ions or neutral molecules, surround the metal center and are linked by coordinate covalent bonds. The arrangement of these ligands and their connection to the metal significantly impact the chemical reactivity, solubility, and physical properties of the overall complex.

• The coordination sphere is crucial because it directly influences the geometry and hence the spatial arrangement of the complex. This impacts how the complex will interact with other molecules.
• The nature of the bonds and the positions of ligands around the metal center can also affect the optical and electronic properties of the compound.

In the original exercise, the coordination sphere effectively hides or exposes certain ions (either \(\mathrm{Cl}^-\) or \(\mathrm{SO}_4^{2-}\) in our case), explaining the different reactions in presence of \(\mathrm{AgNO}_3\) or \(\mathrm{BaCl}_2\). Changes in the coordination sphere can therefore result in different isomers with distinct chemical profiles.

Precipitation Reaction

A precipitation reaction occurs when two aqueous solutions combine to form an insoluble solid, known as the precipitate, that separates from the solution. In the context of coordination chemistry, such reactions can be very telling about the nature of a compound.
The reagents used in these reactions interact with specific ions, providing insight into which ions are present freely in the solution versus those part of the coordination sphere. This can help determine the type of isomerism exhibited by a complex.

• In our exercise, the formation of a white precipitate with \(\mathrm{AgNO}_3\) implies the presence of \(\mathrm{Cl}^-\) ions free in the solution, indicating their absence from the coordination sphere.
• Conversely, the formation of a precipitate with \(\mathrm{BaCl}_2\) points towards free \(\mathrm{SO}_4^{2-}\) ions.

Because the precise ions that precipitate change depending on which isomer you're dealing with, this hints at how the ions are distributed within or outside the coordination sphere, confirming ionisation isomerism in the context of this problem.