Problem 129 - A Complete Resource Book in Chemistry for JEE Main

Question

$$ \begin{aligned} &\text { Match the following }\\\ &\begin{array}{ll} \hline \text { Column-I } & \text { Column-II } \\ \hline \text { (a) }\left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\left(\mathrm{NH}_{3}\right)_{2}\right] & \text { (p) } \text { Number of } \\ \mathrm{NO}_{3} & \text { stereoisomers } \\ & =6 \\ \text { (b) }\left[\mathrm{Ni}(\mathrm{en})_{3}\right] \mathrm{Br}_{3} & \text { (q) } \begin{array}{l} \text { Linkage } \\ \text { isomerism } \end{array} \\ \text { (c) }\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}(\mathrm{py})_{3}\right] \mathrm{Br}_{2} & \text { (r) } \begin{array}{l} \text { Ionization } \\ \text { isomerism } \end{array} \\ \text { (d) }\left[\mathrm{Pt}(\mathrm{en})(\mathrm{SCN})_{2}\right]\left(\mathrm{NO}_{3}\right)_{2} & \text { (s) } \begin{array}{l} \text { Optical } \\ \text { isomerism } \end{array} \\ & \text { (t) } \text { Geometrical } \\ & \text { isomerism } \\ \hline \end{array} \end{aligned} $$

Step-by-Step Solution

Answer

(a) - (q), (b) - (s), (c) - (t), (d) - (q)

Step 1: Analyzing compound (a)

The compound \([ ext{Co(NO}_2)_2( ext{H}_2 ext{O})_2( ext{NH}_3)_2]\) is an octahedral complex with two bidentate ligands. It has nitrite ligands that can exhibit linkage isomerism by binding through either nitrogen or oxygen. This suggests linkage isomerism is possible.

Step 2: Analyzing compound (b)

The compound \([ ext{Ni}( ext{en})_3] ext{Br}_3\) contains a tris-bidentate ethylenediamine (en) ligand with nickel in an octahedral geometry. Such compounds exhibit optical isomerism due to the possibility of non-superimposable mirror images.

Step 3: Analyzing compound (c)

The compound \([ ext{Co}( ext{NH}_3)_3( ext{py})_3] ext{Br}_2\) can exhibit geometrical isomerism. The ligands around the octahedral structure can be arranged such that similar ligands are adjacent or opposite to each other, resulting in isomerism.

Step 4: Analyzing compound (d)

The compound \([ ext{Pt}( ext{en})( ext{SCN})_2]( ext{NO}_3)_2\) contains thiocyanate ion \( ext{SCN}^-\), which can connect via sulfur or nitrogen, producing linkage isomers.

Step 5: Matching Column I with Column II

- Compound (a) matches with (q), as it exhibits linkage isomerism. - Compound (b) matches with (s), as it shows optical isomerism. - Compound (c) matches with (t), as it demonstrates geometrical isomerism. - Compound (d) matches with (q), as it can also show linkage isomerism due to \(SCN^−\).

Key Concepts

Linkage Isomerism

In coordination chemistry, linkage isomerism occurs when a ligand can bind to a metal center in more than one way. For example, nitrite ( ext{NO}_2^-) and thiocyanate ( ext{SCN}^-) ligands can bind through different atoms. Take for instance, the complex \([\text{Co(NO}_2)\text{}_2(\text{H}_2\text{O})_2(\text{NH}_3)_2]\), where nitrite can bind via nitrogen (resulting in nitro) or through oxygen (resulting in nitrito).
Linkage isomers have the same chemical formula but differ in the atom that coordinates to the metal center. This leads to different chemical and physical properties such as color and solubility.

• Ligands capable of linkage isomerism often contain atoms like nitrogen, oxygen, or sulfur.
• Linkage isomerism is significant because it can alter the electronic properties of a compound.

Optical Isomerism

Optical isomerism, a subset of stereoisomerism, occurs in coordination complexes that have non-superimposable mirror images, also called enantiomers. This is analogous to how left and right hands are mirror images but not identical. Consider the complex \([\text{Ni}(\text{en})_3]\text{Br}_3\), where the presence of the tris-bidentate en ligands in an octahedral arrangement leads to chiral structures.
Enantiomers rotate plane-polarized light in different directions, a property used to distinguish them. Though enantiomers have identical physical properties in non-chiral environments, their differing optical activities are critical in fields like pharmaceuticals.

• In optical isomerism, the arrangement of atoms or groups in space results in chiral centers.
• Only complexes without a plane of symmetry can exhibit optical isomerism.

Geometrical Isomerism

Geometrical isomerism appears in coordination complexes where ligands can adopt different spatial arrangements around a central atom. This is common in square planar and octahedral complexes. In compound \([\text{Co}(\text{NH}_3)_3(\text{py})_3]\text{Br}_2\), geometrical isomers arise based on the positions of ammonia and pyridine ligands around the cobalt center.
Consider a complex with two distinct ligands in an octahedral complex. The ligands can be adjacent (cis) or opposite (trans) leading to different forms. These forms can affect reactivity and interactions with other molecules.

• Cis-trans isomerism is a type of geometrical isomerism specific to this type of isomers.
• Geometrical isomers have different properties in terms of density, boiling point, and more.