Problem 181 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following complexe shows optical isomerism (a) \(\mathrm{Cis}\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right] \mathrm{Cl}\) (b) \(\operatorname{trans}\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right] \mathrm{Cl}\) (c) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\right] \mathrm{Cl}\) (d) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}_{3}\right]\)

Step-by-Step Solution

Answer

The complex (a) \( \text{Cis} [\text{Co(en)}_{2} \text{Cl}_{2}] \text{Cl} \) shows optical isomerism.

Step 1: Definition of Optical Isomerism

Optical isomerism occurs when coordination compounds can exist in two forms that are non-superimposable mirror images of each other. This is typical in complexes with a chiral center.

Step 2: Examine Complex (a)

The complex \( \text{Cis} [\text{Co(en)}_{2} \text{Cl}_{2}] \text{Cl} \) contains two \( \text{en} \) (ethylenediamine) bidentate ligands and two chloride ions in a cis configuration. This arrangement allows the complex to be chiral and exhibit optical isomerism because the two \( \text{en} \) ligands and chloride ions create an asymmetric environment.

Step 3: Examine Complex (b)

The complex \( \operatorname{trans} [\text{Co(en)}_{2} \text{Cl}_{2}] \text{Cl} \) has \( \text{en} \) ligands in a trans arrangement. This symmetry causes the complex to be superimposable on its mirror image, meaning it cannot exhibit optical isomerism.

Step 4: Examine Complex (c)

The complex \( [\text{Co} (\text{NH}_{3})_{4} \text{Cl}_{2}] \text{Cl} \) does not contain bidentate ligands or have the asymmetry needed for optical isomerism, given its tetrahedral geometry.

Step 5: Examine Complex (d)

The complex \( [\text{Co} (\text{NH}_{3})_{3} \text{Cl}_{3}] \) is generally arranged in a facial (fac) or meridional (mer) form. Neither form typically supports non-superimposable mirror images due to symmetry, hence it does not show optical isomerism.

Step 6: Conclusion

After analyzing each complex, the only one with the structural prerequisites for optical isomerism is \( \text{Cis} [\text{Co(en)}_{2} \text{Cl}_{2}] \text{Cl} \), as the two \( \text{en} \) ligands provide the necessary chirality.

Key Concepts

Coordination Chemistry

In coordination chemistry, we explore compounds that contain a central metal atom or ion bonded to surrounding ligands. These ligands can be molecules or ions that donate electrons to the metal, forming coordinate covalent bonds. Such compounds are widely studied due to their unique structures and properties.

At the heart of coordination chemistry is the coordinate complex. The central metal ion, like cobalt in our examples, acts as a hub to which various ligands attach. The geometry of this complex can be diverse, including arrangements such as tetrahedral, square planar, and octahedral.

• Complexes and Coordination Sphere: The central metal and its attached ligands are known as the coordination sphere.
• Coordination Number: The number of ligand bonds to the central metal ion, defining the geometry of the complex.
• Stability and Color: The nature of the metal and ligands influences the stability and color of the complexes.

Coordination chemistry not only helps us understand molecular geometry but also Reactivity and the electronic properties of compounds. It plays a crucial role in biological processes, catalysis, and material science.

Chirality in Complexes

Chirality is a fascinating concept within coordination chemistry. It refers to a molecule's ability to exist in two mirror-image forms that are not superimposable, much like our left and right hands. When this occurs in coordination complexes, it results in optical isomerism.

To be chiral, a coordination complex must lack symmetry elements that could negate its non-superimposable nature. For example, the presence of a plane of symmetry or a center of symmetry would prevent chirality. In an optical isomer, although the individual components are the same, their arrangement in space creates two distinct forms.

• Chiral Centers: A central atom bonded in a way that creates non-superimposable mirror images.
• Optical Activity: These isomers can rotate plane-polarized light, a key indicator of chirality.
• Practical Applications: Essential for fields like pharmaceuticals, where optical activity can affect drug efficacy.

In complex (a) from the exercise, the presence of bidentate ligands in a cis arrangement gives it the asymmetry needed for chirality, leading to optical isomerism.

Bidentate Ligands

Bidentate ligands are a core element in coordination chemistry, especially when discussing optical isomerism. Unlike monodentate ligands, which can only donate one pair of electrons to a metal center, bidentate ligands can attach at two points, forming rings known as chelates.

Some essential characteristics of bidentate ligands include their ability to stabilize coordination complexes and their role in chirality. Ethylenediamine (en) is a perfect example. It wraps around a metal ion, using two nitrogen atoms to bond to cobalt in our example complexes. This type of bonding is sometimes referred to as a chelating effect, resulting in stronger and more stable formations.

• Chelate Effect: Bidentate ligands often form more stable complexes than their monodentate counterparts due to multiple attachment points.
• Stability and Geometry: These ligands can influence the 3D shape of the complex, important for optical properties.
• Use in Catalysis: These ligands are particularly valuable in catalytic processes, owing to their stability.

In our featured exercise, the bidentate nature of ethylenediamine helps the complex achieve a chiral, non-superimposable form when arranged in a cis configuration, leading to optical isomerism.