Problem 28 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The ionisation isomer of \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}\left(\mathrm{NO}_{2}\right)\right] \mathrm{Cl}\) is (a) \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\left(\mathrm{O}_{2} \mathrm{~N}\right)\right] \mathrm{Cl}_{2}\) (b) \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]\left(\mathrm{NO}_{2}\right)\) (c) \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}(\mathrm{ONO})\right] \mathrm{Cl}\) (d) \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\left(\mathrm{NO}_{2}\right)\right] \cdot \mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

The correct ionisation isomer is option (b).

Step 1: Understanding Ionisation Isomers

Ionisation isomers are a type of isomer where ions inside and outside the coordination complex interchange. This means that an ion that is part of the complex is swapped with an ion that is outside the coordination sphere, affecting the compound's overall structure but leaving the central metal and its basic coordination unchanged.

Step 2: Identifying the Original Complex

The given complex is \(\left[\mathrm{Cr}\left(\mathrm{H}_{2}\mathrm{O}\right)_{4} \mathrm{Cl}\left(\mathrm{NO}_{2}\right)\right]\\mathrm{Cl}\). This indicates that the \(\mathrm{Cr}^{3+}\) has a coordination sphere consisting of 4 water molecules, 1 chloride ion, and 1 nitrite ion as ligands.

Step 3: Finding the Ionisation Isomer

The ionisation isomer involves exchanging the nitrite ion, which is inside the coordination sphere, with the chloride ion outside it. Therefore, in the ionisation isomer, the chloride ion becomes part of the coordination sphere while the nitrite ion moves outside.

Step 4: Checking the Options

Let's examine the options: (a) The complex has 2 chloride ions inside and releases \(\mathrm{NO}_{2}\) as counter ion. (b) This option swaps chloride and nitrite correctly, matching the change we need. (c) This option suggests linkage isomerism, changing the bonded atom from \(\mathrm{O}\) to \(\mathrm{N}\), which is incorrect for ionisation isomers.(d) This does not achieve the swapping needed for ionisation isomers and includes a water molecule outside the sphere.

Step 5: Selecting the Correct Answer

Option (b) \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]\left(\mathrm{NO}_{2}\right)\) correctly represents the ionisation isomer, as it swaps the position of chloride and nitrite ions between inside and outside the coordination sphere.

Key Concepts

Coordination Complex

A coordination complex, also known as a coordination compound, is an essential part of coordination chemistry. It's formed when a central metal atom or ion (like chromium in our case) bonds with molecules or ions called ligands. These ligands donate electron pairs to the central metal, creating a stabilized entity. Such complexes have varied applications, including catalysts in industrial processes and in medical therapeutics.

• The central metal, typically a transition metal like chromium, acts as a Lewis acid, accepting electron pairs from ligands.
• Ligands, which can be ions or neutral molecules, behave as Lewis bases, donating electron pairs.
• Coordination numbers and geometries can vary; for instance, in the original complex \(\left[\mathrm{Cr}\left(\mathrm{H}_{2}\mathrm{O}\right)_{4} \mathrm{Cl}\left(\mathrm{NO}_{2}\right)\right]\mathrm{Cl}\), chromium binds with four water molecules, one chloride, and one nitrite ion, indicating a coordination number of six and usually an octahedral geometry.

Understanding these structures is fundamental in studying not only the makeup of materials but also their reactivity and properties.

Isomerism

Isomerism is a key concept in chemistry, referring to compounds that have the same chemical formula but different structures or arrangements. This subtle difference in structure leads to distinct properties, making isomers an intriguing field of study.

• Structural Isomerism: Involves differences in the connectivity of the atoms. This category includes ionisation isomers, where ions switch places between inside and outside the coordination complex.
• Stereoisomerism: Despite having the same connectivity, these isomers differ in the spatial orientation of atoms. In coordination chemistry, common types include geometric and optical isomers.

In the exercise, we focused on ionisation isomerism. Here, a switch occurs between a ligand within the coordination sphere and a counterion outside it, affecting the compound's interaction with its surroundings. This concept is central to recognising why isomers can demonstrate varied chemical behaviour despite having similar compositions.

Chromium Complexes

Chromium, being a transition metal, plays a crucial role in forming coordination complexes due to its ability to adopt various oxidation states and coordination numbers. This attribute makes chromium complexes quite diverse.

• In its complexes, chromium often exhibits an oxidation state of +3 as in \(\left[\mathrm{Cr}\left(\mathrm{H}_{2}\mathrm{O}\right)_{4} \mathrm{Cl}\left(\mathrm{NO}_{2}\right)\right]\mathrm{Cl}\), which is common for many of its stable complexes.
• Chromium complexes frequently display octahedral geometries, as seen in the complex when encircled by six ligands.
• One of the fascinating aspects of chromium complexes is their bright colors, a result of d-d transitions where electrons jump between energy levels within the d-orbitals.

The variations in chromium complexes indicate its significance in both nature and industry, such as in pigments and dyes, despite the toxicity concerns associated with some of its oxidation states.