Problem 69 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following will give four isomers? (a) \(\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right] \mathrm{Br}\) (b) \(\left[\mathrm{Co}\right.\) (en) \(\left.\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right] \mathrm{Cl}\) (c) \(\left[\mathrm{Co}\left(\mathrm{PPh}_{3}\right)_{2}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right] \mathrm{Cl}\) (d) \(\left[\mathrm{Co}(\mathrm{en})_{3}\right] \mathrm{Cl}_{3}\)

Step-by-Step Solution

Answer

(b) \(\left[\mathrm{Co}(\mathrm{en})(\mathrm{NH}_{3})_{2} \mathrm{Cl}_{2}\right] \mathrm{Cl}\) forms four isomers.

Step 1: Understand Isomers

Isomers are compounds with the same chemical formula but different arrangements of atoms. In coordination complexes, isomers can arise from different spatial arrangements of the ligands around the central metal atom/ion.

Step 2: Analyze Compound (a)

The compound is \[ \left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right] \mathrm{Br} \]where \(\mathrm{en}\) is ethylenediamine, a bidentate ligand that forms a chelate ring with the metal center. This complex can potentially form geometric isomers depending on the positioning of the two chloride ions and \(\mathrm{en}\) ligands. Test the different spatial configurations to see how many isomers can be formed.

Step 3: Compound (a) Isomer Calculation

For compound (a), there can be only two geometric isomers: cis and trans. Positioning of the \(\mathrm{Cl}\) ions either next to each other (cis) or opposite to each other (trans) will not generate four distinct isomers.

Step 4: Analyze Compound (b)

For compound \[\left[\mathrm{Co}\right. (en) \left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right] \mathrm{Cl} \]this complex can form multiple geometric isomers based on the positions of \(\mathrm{NH}_{3}\) and \(\mathrm{Cl}\) ligands.

Step 5: Compound (b) Isomer Calculation

With two \(\mathrm{NH}_{3}\) groups and two \(\mathrm{Cl}\) ions, you can arrange these ligands into cis-trans configurations. Additionally, due to the presence of ethylenediamine, which is chelating, different rotational configurations also contribute to geometric isomerism. This can lead to four different isomers.

Step 6: Verify Other Compounds

Compounds (c) and (d) were initially considered for isomerism possibilities. However, compound (c) \[\left[\mathrm{Co}\left(\mathrm{PPh}_{3}\right)_{2}\left(\mathrm{NH}_{3}\right)_{2}\mathrm{Cl}_{2}\right] \mathrm{Cl}\]provides configurations resulting in fewer isomers similar to compound (a). Compound (d) \([\mathrm{Co}(\mathrm{en})_{3}] \mathrm{Cl}_{3}\) is saturated concerning its octahedral coordination and strictly forms optical isomers, not geometric.

Step 7: Final Step: Conclusion

Based on the calculations, the compound \[\left[\mathrm{Co}\right. (en) \left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right] \mathrm{Cl} \]produces four distinct isomers, fulfilling the conditions of forming four isomers due to different possible geometric and rotational configurations.

Key Concepts

Isomerism

In coordination chemistry, isomerism is a fascinating phenomenon where molecules or complexes with identical chemical formulas exhibit different arrangements of their atoms. This different arrangement influences their chemical properties and sometimes their color as well. Isomers are classified into several categories:

• Structural Isomers: These have different connections among atoms within the molecule.
• Stereoisomers: These have the same connections but differ in the spatial arrangement of atoms. This category includes both geometric and optical isomers.

In the context of coordination complexes, isomerism primarily occurs as geometric or optical isomerism due to the variety of ways ligands can associate with the central metal atom or ion.

Geometric Isomers

Geometric isomerism, sometimes known as cis-trans isomerism, occurs within coordination complexes when ligands can be arranged in different spatial orientations around a central metal atom. This type of isomerism is highly dependent on the symmetric properties of the central structure and the positioning of specific ligands:

• Cis Form: Ligands are adjacent to each other.
• Trans Form: Ligands are positioned opposite each other.

Geometric isomers arise in coordination compounds when there is restricted rotation around bonds, typically in complexes with square planar or octahedral geometries. These isomers usually have different physical and chemical properties.

Chelating Ligands

Chelating ligands are special types of ligands that can attach to a central metal atom or ion at multiple points, forming a ring structure. This is often referred to as a chelate ring. One common example of a chelating ligand is ethylenediamine (en), which is bidentate, meaning it can form two bonds to the metal.

• Often, chelating ligands increase the stability of a complex.
• The inclusion of such ligands can significantly affect the isomerism of a coordination compound due to their ability to "lock" certain spatial configurations into place.
• Chelates also have implications in biological systems and industrial applications due to their strong bonding nature.

Coordination Complex

A coordination complex consists of a central metal atom or ion surrounded by a set of molecules or ions known as ligands. These complexes are characterized by a specific coordination number, which represents the number of ligand attachment points to the central metal.

• Coordination complexes can exhibit both geometric and optical isomerism due to varying arrangements of ligands.
• The properties of a coordination complex—such as solubility, color, and reactivity—depend greatly on its isomers.
• The study of coordination complexes is crucial, as they have significant applications in fields such as catalysis, medicine, and materials science.

Understanding the intricate details of these complexes allows for the precise manipulation of chemical properties for desired applications.