Problem 35 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The number of geometric isomers possible for the complex \(\left[\mathrm{Co} L_{2} \mathrm{Cl}_{2}\right]^{-}\) \(\left(L=\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{O}^{-}\right)\)is

Step-by-Step Solution

Answer

There are two geometric isomers possible for this complex: cis and trans.

Step 1: Understanding the Complex

The complex \([\mathrm{Co}L_{2} \mathrm{Cl}_{2}]^-\) involves a cobalt atom, two bidentate ligands \(L\), and two chloride ions. The ligand \(L\) is \(\mathrm{H_2NCH_2CH_2O^-}\), a bidentate ligand, meaning it can attach to the metal center at two points.

Step 2: Determine Geometry

Cobalt in this complex typically has a coordination number of 6, leading to an octahedral geometry. Therefore, \([\mathrm{Co}L_{2} \mathrm{Cl}_{2}]^-\) forms an octahedral complex.

Step 3: Identify Possible Arrangements

Within an octahedral geometry, the arrangement of ligands can vary. For example, if two like ligands are adjacent (cis) or opposite each other (trans). The two bidentate ligands \(L\) and two chloride ions can have spatial arrangements that lead to different isomers.

Step 4: Explore Geometric Isomers

In an octahedral complex, the cis arrangement allows two chloride ions to be adjacent, and in the trans arrangement, they are opposite. By systematically arranging the ligands, we can identify possible geometric isomers: one cis and one trans.

Step 5: Count the Isomers

Therefore, there are two geometric isomers possible for this complex: one cis isomer and one trans isomer.

Key Concepts

Octahedral Geometry

An octahedral geometry is a common arrangement in coordination chemistry, characterized by six ligands symmetrically surrounding a central metal atom. This geometry is named for its resemblance to an octahedron, a shape with eight triangular faces. In the case of the complex \([\mathrm{Co}L_{2} \mathrm{Cl}_{2}]^-\), the six coordination sites created by the cobalt center are occupied by two bidentate ligands and two chloride ions.
This results in a typical octahedral configuration where ligands can be systematically arranged to explore different possible spatial arrangements.

• Six coordination sites with 90-degree angles between adjacent sites.
• Allows for several isomeric forms depending on ligand identities.
• Geometric isomers can include cis and trans variants.

Recognizing the octahedral geometry of this cobalt complex is essential for understanding its geometric isomers, which stem from the spatial arrangement of the ligands.

Bidentate Ligands

Bidentate ligands are special types of ligands that can form two bonds with a metal ion through two different atoms.
This creates a chelate ring, which stabilizes the structure of the complex. In this exercise, the ligand \(\mathrm{H_2NCH_2CH_2O^-}\) acts as a bidentate ligand, providing two points of attachment to the cobalt center.

By forming such strong chelate rings, bidentate ligands influence the geometry and isomerism of complexes.
They limit the number of possible isomers by reducing flexibility compared to monodentate ligands.

Key features of bidentate ligands:

• Establish two coordination points with the central metal atom, enhancing stability.
• Form cyclic structures, which can restrict isomer formation.
• Regularly used in maintaining structure integrity in coordination complexes.

In our analysis of the cobalt complex, bidentate ligands help determine which geometric isomers can exist due to their unique binding mode.

Coordination Chemistry

Coordination chemistry focuses on metal complexes formed with ligands, examining how ligands attach to central metal ions to produce stable structures. This field explores the formation, bonding, and behavior of these complexes.
The cobalt complex \([\mathrm{Co}L_{2} \mathrm{Cl}_{2}]^-\) is an excellent example of coordination chemistry in action.

Crucial elements of coordination chemistry include:

• Chemical and spatial properties are determined by the coordination number and geometry.
• The nature of ligands—mono, bi, or polydentate—affects stability and reactivity.
• Understanding the structural arrangements helps predict isomer formation and chemical properties.

In this specific complex, coordination chemistry guides the determination of geometric isomers by revealing how the cobalt, bidentate ligands, and chloride ions arrange spatially within an octahedral framework. As students navigate exercises like these, appreciating the nuances of coordination chemistry is crucial for uncovering the potential isomers that can form.