Problem 16 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Consider the following reaction and statements: \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Br}_{2}\right]^{+}+\mathrm{Br}^{-} \rightarrow\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Br}_{3}\right]+\mathrm{NH}_{3}\) (I) Two isomers are produced if the reactant complex ion is a cis-isomer. (II) Two isomers are produced if the reactant complex ion is a trans isomer (III)Only one isomer is produced if the reactant complex ion is a trans isomer (IV) Only one isomer is produced if the reactant complex ion is a cis-isomer. The correct statements are: (a) (I) and (II) (b) (I) and (III) (c) (III) and (IV) (d) (II) and (IV)

Step-by-Step Solution

Answer

Option (b) (I) and (III) are correct.

Step 1: Analyze the cis-isomer of the reactant

For the cis-isomer of the reactant \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Br}_{2}\right]^{+}\), the two bromides are adjacent to each other. During the reaction with \(\mathrm{Br}^{-}\), one of these bromides can be replaced, leading to the formation of \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Br}_{3}\right]\). This replacement can occur at two different locations, generating two possible arrangements in the final complex, meaning two isomers are produced. This supports Statement (I).

Step 2: Analyze the trans-isomer of the reactant

For the trans-isomer, the bromides are opposite each other in the starting complex. During the substitution of \(\mathrm{Br}^{-}\), replacing one bromide results in a unique structure for \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Br}_{3}\right]\), producing only one isomer. Therefore, Statement (III) is correct.

Step 3: Evaluate the incorrect statements

Since we have established that statement (I) is true and leads to two isomers in the cis isomer. Statement (II) is false, as the trans isomer only leads to one product. Likewise, statement (IV) is incorrect, as it suggests the cis-isomer reacts to yield only one isomer, which contradicts the analysis in Step 1.

Step 4: Determine the correct answer based on evaluations

From the analysis, Statement (I) and Statement (III) are correct. Therefore, the combination of correct statements is given by option (b).

Key Concepts

Cis-Trans Isomerism

In coordination chemistry, cis-trans isomerism is an important concept, especially when analyzing complex ions and reactions. This type of isomerism is often encountered in square planar and octahedral complexes where ligands have different spatial arrangements.

For example, consider the complex \([\mathrm{Co}(\mathrm{NH}_3)_4 \mathrm{Br}_2]^{+}\). Its ligands, \(\mathrm{NH}_3\) and \(\mathrm{Br}\), can be arranged differently around the central metal ion, cobalt.

• Cis-isomer: The two bromide ions (\(\mathrm{Br}^-\)) are adjacent to each other. This proximity allows for multiple possible substitutions to occur when reacting, as seen in the transformation to \([\mathrm{Co}(\mathrm{NH}_3)_3 \mathrm{Br}_3]\). This can result in two distinct product isomers.
• Trans-isomer: Here, the bromide ions are opposite each other. This configuration leads to a single possible product after reaction due to uniform substitution patterns.

Recognizing the cis or trans nature of complexes is crucial, as it determines the number and type of isomers formed during chemical reactions.

Complex Ion Reactions

During reactions involving complex ions, like the transformation from \([\mathrm{Co}(\mathrm{NH}_3)_4 \mathrm{Br}_2]^{+}\), understanding how ligands are replaced or reorganized is key. In these reactions, ligands surrounding the central metal ion can be substituted, leading to new coordination geometry and properties.

In the given example, the reactant complex ion engages in a substitution reaction with \(\mathrm{Br}^-\), resulting in the replacement of one ammonia (\(\mathrm{NH}_3\)) ligand with a bromide ion (\(\mathrm{Br}^-\)). This results in the new complex \([\mathrm{Co}(\mathrm{NH}_3)_3 \mathrm{Br}_3]\), highlighting two subtypes of reactions depending on the initial isomer:

• Cis Reactant: The resulting complex may form two distinct isomers because of the flexibility in substituting the adjacent bromide ions.
• Trans Reactant: The symmetry causes only one type of arrangement, hence only one isomer is produced.

Grasping these substitution mechanisms is fundamental for predicting and analyzing the outcomes of complex ion reactions.

Coordination Compounds

Coordination compounds consist of a central metal atom or ion surrounded by molecules or anions, known as ligands. These complexes are pivotal in coordination chemistry and exhibit a range of structures and isomerism, such as cis-trans isomerism.

Their properties depend heavily on the type of ligands, their spatial arrangement, and the metal center, which can affect color, magnetism, and reactivity. Let's break down a few vital points:

• Ligands: Ligands such as \(\mathrm{NH}_3\) and \(\mathrm{Br}^-\) can dictate the properties of the complex. They bind to the coordination center through coordinate covalent bonds.
• Coordination Number: This number represents the count of ligand donor atoms attached to the central atom. For \([\mathrm{Co}(\mathrm{NH}_3)_4 \mathrm{Br}_2]^{+}\), the coordination number is six.
• Isomerism: Apart from cis-trans, other forms of isomerism can occur. These include optical isomerism, which is significant in complex compounds, influencing how they interact with other chemicals.

Understanding these key attributes of coordination compounds is essential to mastering concepts related to their structure and reactivity. These foundations help in predicting the behavior of the compounds in various chemical environments.