Problem 74 - A Complete Resource Book in Chemistry for JEE Main

Question

Isomerism shown by octahedral complex \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4}\right.\) \(\left.\mathrm{Br}_{2}\right] \mathrm{Cl}\) are (a) geometrical and ionization (b) optical and ionization (c) geometrical and optical (d) only geometrical

Step-by-Step Solution

Answer

(a) geometrical and ionization

Step 1: Identify Ligands and Coordination

The complex is given as \( [\mathrm{Co}(\mathrm{NH}_3)_4\mathrm{Br}_2] \mathrm{Cl} \). Here, Co (Cobalt) is surrounded by 4 ammonia (\( \mathrm{NH}_3 \)) ligands and 2 bromide (\( \mathrm{Br}^- \)) ligands, forming an octahedral complex. The chloride (\( \mathrm{Cl}^- \)) is the counter ion.

Step 2: Check for Geometrical Isomerism

In octahedral complexes, geometric isomerism can occur when there are different possible arrangements of ligands. With two \( \mathrm{Br}^- \) ligands, we can have cis and trans configurations with respect to these. Thus, \( [\mathrm{Co}(\mathrm{NH}_3)_4\mathrm{Br}_2] \) can show both configurations.

Step 3: Check for Optical Isomerism

Optical isomerism in octahedral complexes typically requires bidentate ligands that can create a non-superimposable mirror image. Since \( [\mathrm{Co}(\mathrm{NH}_3)_4\mathrm{Br}_2] \) does not have bidentate ligands and symmetrically placed ligands, it does not show optical isomerism.

Step 4: Check for Ionization Isomerism

Ionization isomerism occurs when the isomers produce different ions in solution. The complex \( [\mathrm{Co}(\mathrm{NH}_3)_4\mathrm{Br}_2] \mathrm{Cl} \) can have an isomer where a \( \mathrm{Br}^- \) is exchanged with \( \mathrm{Cl}^- \), resulting in \( [\mathrm{Co}(\mathrm{NH}_3)_4\mathrm{Br}\mathrm{Cl}] \mathrm{Br} \).

Key Concepts

Octahedral Complex

In coordination chemistry, an octahedral complex consists of a central metal atom or ion surrounded by six ligands at the corners of an octahedron. The geometry and symmetry of octahedral complexes directly influence their chemical and optical properties. In the case of - \( [\mathrm{Co}(\mathrm{NH}_3)_4\mathrm{Br}_2] \mathrm{Cl} \), cobalt (Co) is the central metal ion bonded with four ammonia (\(\mathrm{NH}_3\)) ligands and two bromide (\(\mathrm{Br}^-\)) ions. The chloride (\(\mathrm{Cl}^-\) ) acts as a counter ion. - The structure allows various isomerism phenomena due to the distinctive spatial arrangements possible in the octahedral geometry. Understanding these structures is crucial to predict the chemical behavior and reactions of the complex.

Geometrical Isomerism

Geometrical isomerism in coordination compounds occurs due to different spatial arrangements of ligands around the central metal ion. An octahedral complex, like - \( [\mathrm{Co}(\mathrm{NH}_3)_4\mathrm{Br}_2] \), can exhibit geometrical isomerism by having either a **cis** or **trans** configuration of ligands. - - **Cis Configuration**: The two bromide ions are adjacent to each other.- **Trans Configuration**: The two bromide ions are opposite each other. - - These different arrangements lead to distinct physical properties despite having the same chemical composition. Geometrical isomerism can significantly affect properties like color, reactivity, and even solubility.

Ionization Isomerism

Ionization isomerism takes place when a complex can yield different ions in solution, which in turn can change its overall properties. In - \( [\mathrm{Co}(\mathrm{NH}_3)_4\mathrm{Br}_2] \mathrm{Cl} \), there exists a possibility to rearrange the outer-sphere and inner-sphere ions. - By exchanging one \(\mathrm{Br}^-\) with \(\mathrm{Cl}^-\), you can form a different isomer: - [\mathrm{Co}(\mathrm{NH}_3)_4\mathrm{Br}\mathrm{Cl}] \mathrm{Br}. - This exchange results in the release of new ions when dissolved in water, showing how the same set of elements can yield varied ionic reactions. This type of isomerism is crucial for applications where specific ionic conductivities or reactions to ions are desired.

Optical Isomerism

Optical isomerism occurs when a molecule cannot be superimposed on its mirror image, similar to how the left and right hands are mirror images but are not identical. Although common in complexes with bidentate ligands, - \( [\mathrm{Co}(\mathrm{NH}_3)_4\mathrm{Br}_2] \) does not show optical isomerism because it consists of monodentate ligands and lacks the necessary asymmetry. - To exhibit optical isomerism:

• Bidentate or polydentate ligands are often needed to create a chiral center.
• There should be an arrangement that makes the complex non-superimposable on its mirror image.

In this complex, the symmetry prevents it from exhibiting optical isomerism, differentiating it from complexes that are optically active and can affect polarized light.