Problem 87 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following complex ions will not show optical activity? (a) \(\left[\mathrm{Co}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]^{+}\) (b) \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\right]^{+}\) (c) \(\left[\mathrm{Pt}(\mathrm{Br})(\mathrm{Cl})\right.\) (I) \(\left.\left(\mathrm{NO}_{2}\right)(\mathrm{Py}) \mathrm{NH}_{3}\right]\) (d) \(\operatorname{cis}-\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]^{+}\)

Step-by-Step Solution

Answer

The complex ion \([\mathrm{Cr}(\mathrm{NH}_3)_4\mathrm{Cl}_2]^+\) (Option b) will not show optical activity.

Step 1: Understand Optical Activity

Optical activity is the ability of a compound to rotate the plane of polarized light. For a complex ion to be optically active, it should have a non-superimposable mirror image, i.e., it should be chiral.

Step 2: Analyze Complex Ion - Option (a)

The complex ion \([\mathrm{Co} (\mathrm{en}) (\mathrm{NH}_3)_2 \mathrm{Cl}_2]^+\) contains an ethylenediamine (en) ligand which is bidentate and can form a chiral complex if the positioning allows for non-superimposable mirror images. This complex can exist as optical isomers, hence it is optically active.

Step 3: Analyze Complex Ion - Option (b)

The complex ion \([\mathrm{Cr}(\mathrm{NH}_3)_4\mathrm{Cl}_2]^+\) is not prone to optical isomerism because it does not incorporate any bidentate ligands that could lead to chirality. Hence, this complex ion will not show optical activity.

Step 4: Analyze Complex Ion - Option (c)

The complex \([\mathrm{Pt}(\mathrm{Br})(\mathrm{Cl})(\mathrm{I})(\mathrm{NO}_2)(\mathrm{Py}) \mathrm{NH}_3]\) contains different ligands around the platinum, which may not automatically lead to non-superimposable mirror images. The lack of bidentate ligands typically results in a superimposable structure, making this unlikely to be optically active.

Step 5: Analyze Complex Ion - Option (d)

The \(\operatorname{cis}-[\mathrm{Co}(\mathrm{en})_2\mathrm{Cl}_2]^+\) complex has bidentate ligands that can form non-superimposable mirror images, hence it can exist in optical isomer forms and will show optical activity.

Step 6: Conclusion

After analyzing the options, it is concluded that the complex ion \([\mathrm{Cr}(\mathrm{NH}_3)_4\mathrm{Cl}_2]^+\) (Option b) will not show optical activity as it lacks chiral centers or bidentate ligands to form non-superimposable mirror images.

Key Concepts

Chirality in Coordination Complexes

In chemistry, chirality refers to a property where objects are not identical to their mirror images, similar to how left and right hands are mirror images but cannot be superposed perfectly on each other. In coordination complexes, chirality occurs when the complex is composed in such a way that it cannot overlap perfectly with its mirror image. This is a crucial aspect for a complex to display optical activity as chiral compounds can rotate the plane of polarized light.

Coordination complexes are made up of a central metal atom bonded to surrounding ligands. The spatial arrangement of these ligands can render a complex chiral.

• Chirality arises particularly when the coordination polyhedron (often octahedral) is asymmetric.
• Chiral coordination complexes will possess non-superimposable mirror images, known as enantiomers.
• This chirality is essential for the complex to rotate the plane of polarized light, showing optical activity.

Identifying chirality in coordination compounds involves looking at the geometry and the types of ligands bonded to the metal center. A mix of ligands, especially those forming chelates or involving asymmetry, tends to increase the likelihood of chirality.

Bidentate Ligands

Bidentate ligands are those that can attach to a central metal atom through two separate bonds. This dual attachment allows them to form what is known as a chelate ring around the metal center, significantly affecting the geometry and possibly the optical properties of the complex.

One common example of a bidentate ligand is ethylenediamine (en), which often shows up in coordination complexes.

• Bidentate ligands enhance the stability of complexes due to the chelate effect.
• These ligands can influence the chirality of a complex by enforcing specific spatial arrangements.
• In cases where a molecule would otherwise be non-chiral, the introduction of bidentate ligands can create a situation where optical isomerism is possible.

Their ability to create these structural alterations is why complexes featuring bidentate ligands, such as those mentioned in options (a) and (d) of the original exercise, are often investigated for potential optical activity. These ligands play a crucial role in the overall three-dimensional arrangement, making the complex chiral or achiral.

Optical Isomerism

Optical isomerism is a type of stereoisomerism where isomers have the ability to rotate the plane of polarized light differently. These isomers, known as optical isomers or enantiomers, possess the same chemical formula but differ in the spatial arrangement of atoms, leading to non-superimposable mirror images.

In the context of coordination chemistry, optical isomerism is particularly important:

• It arises from the three-dimensional layout of ligands around a central metal atom, especially when the complex lacks a plane of symmetry.
• Enantiomers will rotate plane-polarized light to the right (dextrorotatory) or left (levorotatory).
• Only chiral complexes can exhibit optical isomerism, as seen in complexes containing asymmetric arrangements, often due to bidentate ligands.

For instance, in the original problem, complexes like \(\operatorname{cis}-[\mathrm{Co}(\mathrm{en})_2\mathrm{Cl}_2]^+\) exhibit optical isomerism because their arrangement allows for non-superimposable mirror images due to bidentate ligands affecting their shape and symmetry. Recognizing whether a coordination compound will exhibit optical isomerism involves examining the geometry and interactions of the ligands involved.