Problem 171 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following has an optical isomer? [2009] (a) \(\left[\mathrm{CO}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2}\right]^{2+}\) (b) \(\left[\mathrm{CO}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}(\mathrm{en})\right]^{3+}\) (c) \(\left[\mathrm{CO}(\mathrm{en})_{2}\left(\mathrm{NH}_{3}\right)_{2}\right]^{3+}\) (d) \(\left[\mathrm{CO}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}\right]^{+}\)

Step-by-Step Solution

Answer

Option (c) has an optical isomer.

Step 1: Understanding Optical Isomers

Optical isomers, also known as enantiomers, are compounds that are non-superimposable mirror images of each other. This occurs when a compound has a chiral center that causes the molecule to lack a plane of symmetry.

Step 2: Identify Coordination Compounds

Observe the given coordination compounds: (a) \([\mathrm{Co}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2}]^{2+}\) (b) \([\mathrm{Co}\left(\mathrm{H}_{2}\mathrm{O}\right)_{4}(\mathrm{en})]^{3+}\) (c) \([\mathrm{Co}(\mathrm{en})_{2}\left(\mathrm{NH}_{3}\right)_{2}]^{3+}\) (d) \([\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}]^{+}\)

Step 3: Analyzing the Coordination Sphere

In order to have optical isomerism, the coordination complex should be such that there is no superimposable mirror image. This generally happens in octahedral complexes where bidentate ligands (like 'en' ethylenediamine) form a chiral center.

Step 4: Check Each Option for Optical Isomerism

- **Option (a)**: It contains \(\mathrm{en}\), a bidentate ligand, but lacks sufficient asymmetry due to other monodentate ligands.- **Option (b)**: Contains one bidentate ligand and is symmetrical due to four identical water ligands; no optical isomers.- **Option (c)**: Features two bidentate \(\mathrm{en}\) ligands creating an unsymmetrical structure capable of optical isomerism.- **Option (d)**: Composed of only monodentate ligands that create no asymmetry.

Step 5: Conclusion

Only option (c) \([\mathrm{Co}(\mathrm{en})_{2}\left(\mathrm{NH}_{3}\right)_{2}]^{3+}\) has the necessary conditions for optical isomerism because it contains two bidentate ligands (\(\mathrm{en}\)) that create a chiral environment.

Key Concepts

Coordination Compounds

Coordination compounds are a fascinating class of substances in chemistry where central metal atoms bind with groups of other atoms called ligands. These metals are usually transition elements, such as cobalt in our exercises. The coordination compounds can take various shapes and configurations, often resulting in unique chemical and physical properties.
These structures show how metal atoms can connect with different ligands, whether they're simple such as water or more complex like ethylenediamine ('en').

• The metal center in coordination compounds is surrounded by ligands, forming what we call a coordination sphere.
• The number of ligands directly bound to the metal is known as the coordination number.
• The entire entity, including the metal and ligands, acts as a singular chemical species, often carrying a charge.

Coordination compounds are vital in various fields, from biochemistry affecting how we understand hemoglobin to industry, where they are used in catalysis and dyes.

Chirality

Chirality is a phenomenon where a structure or molecule cannot be superimposed on its mirror image. It is akin to how your left and right hands are mirror images yet not superimposable. This property is especially important in coordination chemistry because some complex compounds can exhibit optical isomerism due to chirality.
In coordination compounds, chirality arises when there is no plane of symmetry. Typically, this occurs in octahedral complexes when bidentate ligands, such as ethylenediamine, create an asymmetric coordination sphere. This leads to the formation of non-superimposable mirror images or enantiomers.

• Chirality results in optical isomers, which can rotate plane-polarized light in different directions: either to the left (levorotatory) or right (dextrorotatory).
• Among the provided options, only the compound with multiple bidentate ligands, like option (c), achieves the necessary asymmetric arrangement for optical isomerism.

Recognizing chirality in coordination compounds helps in understanding their reactivity and interactions in biological and chemical systems.

Bidentate Ligands

Bidentate ligands are unique in the world of chemistry. They are ligands that can bind to a central metal at two locations. This dual attachment forms more stable complexes due to the chelate effect, which results in enhanced stability.
Bidentate ligands like ethylenediamine ('en') play a crucial role in determining the properties of coordination compounds. Unlike monodentate ligands, which only attach at one site, bidentate ligands create rings within the coordination sphere. This significantly influences the compound's overall geometry and can lead to chirality when two or more bidentate ligands are present.

• 'En', with its two nitrogen atoms ready to bind, heavily influences the geometric and chemical properties of the complexes.
• This binding pattern often results in coordination compounds capable of existing in optical isomers, as in option (c) of our exercise.
• The presence of bidentate ligands enhances the possibility of forming chiral centers, leading to optical activity in the solution.

Bidentate ligands, therefore, are not just active spectators but play a vital role in the configuration and properties of coordination compounds.