Problem 4 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The one that is not expected to show isomerism is : (a) \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}\) (b) \(\left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+}\) (c) \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]\) (d) \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]\)

Step-by-Step Solution

Answer

Compound (c) is not expected to show isomerism if tetrahedral.

Step 1: Understand Isomerism in Coordination Complexes

Coordination complexes can exhibit different types of isomerism, such as structural isomerism (including linkage and coordination isomerism) and stereoisomerism (including geometric and optical isomerism). An analysis must be conducted for each given compound to determine the presence of these isomers.

Step 2: Evaluate Compound (a)

The compound \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2}\mathrm{O}\right)_{2}\right]^{2+}\) can show geometric isomerism (cis-trans) due to the presence of two different types of ligands: \(\mathrm{NH}_3\) and \(\mathrm{H}_2\mathrm{O}\).

Step 3: Evaluate Compound (b)

For the compound \(\left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+}\), the ligand \("en"\) is ethylenediamine, a bidentate ligand. This coordination generally leads to optical isomerism when arranged in an octahedral shape.

Step 4: Evaluate Compound (c)

The compound \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]\), can exhibit geometric isomerism (cis-trans) in square planar and possibly tetrahedral configurations.

Step 5: Evaluate Compound (d)

The compound \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]\), is well known for its geometric isomerism, specifically as cisplatin and transplatin in a square planar configuration.

Step 6: Identify the Compound with No Isomerism

The analysis shows that options (a), (b), (c), and (d) can potentially show isomerism. But in typical tetrahedral complexes, if (c) \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]\) is tetrahedral, it will not show cis-trans isomerism, unlike its square planar counterpart.

Key Concepts

Isomerism

In chemistry, particularly coordination chemistry, isomerism is an important concept. It occurs when two or more compounds have the same formula but different arrangements of atoms in space. This leads to different properties and reactivities for each isomer.
There are several types of isomerism:

• Structural Isomerism: This involves differences in basic connectivity. Examples include linkage isomerism, where ligands can attach via different atoms, and coordination isomerism, where ligand and metal arrangement varies.
• Stereoisomerism: This type focuses on different spatial configurations. Two main categories are geometric and optical isomerism.

Understanding isomerism is crucial as it impacts the chemical behavior and utility of coordination complexes in fields like pharmaceuticals and materials science.

Geometric Isomerism

Geometric isomerism, a subset of stereoisomerism, is a fascinating phenomenon in coordination chemistry. It occurs when ligands are arranged differently around a central metal atom, leading to isomers with distinct properties but the same connectivity.
Geometric isomerism frequently appears in:

• Square Planar Complexes: Here, ligands can be positioned in either cis (same side) or trans (opposite side) fashion, such as in cisplatin and transplatin.
• Octahedral Complexes: This also shows cis and trans forms, where two identical ligands are either adjacent or opposite each other.

However, in tetrahedral complexes, geometric isomerism typically does not occur due to the symmetrical nature of a tetrahedron, which lacks distinct "sides" to have cis-trans positions.

Optical Isomerism

Optical isomerism is another form of stereochemical isomerism. It occurs when isomers are non-superimposable mirror images of each other, much like a pair of hands are.
This type of isomerism is often found in:

• Chiral Molecules: These lack an internal plane of symmetry and have a distinct "handedness" or chirality.
• Octahedral Complexes with Bidentate Ligands: When such ligands create chiral environments, optical isomers or enantiomers can form.

Optical isomerism is significant in fields like biochemistry, as different enantiomers can have dramatically different biological effects.

Tetrahedral Complexes

In coordination chemistry, tetrahedral complexes are coordination compounds where four ligands are symmetrically arranged around a central metal atom, forming a tetrahedron. This symmetrical shape leads to unique characteristics compared to their square planar counterparts.
Notable features include:

• Lack of Geometric Isomerism: Due to the evenly distributed tetrahedral shape, cis-trans isomerism is typically not possible. Each ligand in a tetrahedron is equidistant, making orientation less distinct.
• Potential for Optical Isomerism: Although rare in simpler tetrahedral structures, chiral compounds can form optical isomers if complexity or asymmetry is introduced.

Understanding tetrahedral complexes is crucial for appreciating their distinct behavior and stability in chemistry. A classic example is the absence of geometric isomerism, as seen in \([\mathrm{Ni}(\mathrm{NH}_{3})_{2} \mathrm{Cl}_{2}]\) when it forms a tetrahedral structure.