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) \(\left(\mathrm{NO}_{2}\right)\) (Py) \(\left.\mathrm{NH}_{3}\right]\) (d) \(\operatorname{cis}-\left[\mathrm{Co}(\mathrm{en}), \mathrm{Cl}_{2}\right]^{+}\)

Step-by-Step Solution

Answer

Ion (b) \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{4}\mathrm{Cl}_{2}\right]^{+}\) is not optically active.

Step 1: Understand Optical Activity

Optical activity refers to the ability of a molecule or ion to rotate the plane of polarization of polarized light. This phenomenon typically occurs in chiral compounds, which lack an internal plane of symmetry.

Step 2: Analyze Ion (a): \(\left[\mathrm{Co}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2}\mathrm{Cl}_{2}\right]^{+}\)

The complex involves ethylenediamine (en), which is a bidentate ligand, and two \(\mathrm{NH}_{3}\) and two \(\mathrm{Cl}\) ligands. This complex can exist in a chiral form due to the arrangement of these ligands; it is possible to have non-superimposable mirror images, depending on the spatial arrangement of the ligands.

Step 3: Analyze Ion (b): \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{4}\mathrm{Cl}_{2}\right]^{+}\)

This complex is of the form \([\mathrm{MA}_{4}\mathrm{B}_{2}]\). Since it is a coordination number of six with identical ligand pairs, it tends to have a trans configuration, which is symmetrical and non-chiral, thus showing no optical activity.

Step 4: Analyze Ion (c): \(\left[\mathrm{Pt}(\mathrm{Br})(\mathrm{Cl})\right) (\mathrm{I}) (\mathrm{NO}_{2}) (\mathrm{Py}) (\mathrm{NH}_{3}\right]\)

The complex has six different ligands, making it highly asymmetric without any plane of symmetry and essentially chiral. Chiral configurations are optically active.

Step 5: Review Ion (d): \(\operatorname{cis}-\left[\mathrm{Co}(\mathrm{en}),\mathrm{Cl}_{2}\right]^{+}\)

The cis configuration with ethylenediamine and two chlorine ligands can lead to chiral configurations, similar to ion (a). As a result, it can show optical activity.

Key Concepts

Chiral Compounds

A chiral compound is one that cannot be superimposed on its mirror image. It's similar to how your left hand is different from your right hand even though they look similar. Chiral compounds are often optically active, meaning they can rotate the plane of polarized light. This property makes them particularly interesting in fields like chemistry and biology, where molecular orientation can affect functionality.

• Chirality usually means having no plane of symmetry within the molecule.
• Chiral compounds have two enantiomers or mirror-image forms, that are non-superimposable.
• This concept is central in coordination chemistry, where chirality often influences how molecules interact.

Chirality can exist in many substances, including coordination complexes where specific arrangements of ligands around a central atom create this unique property.

Coordination Chemistry

Coordination chemistry deals with the study of complex structures formed between metal ions and ligands. A ligand is a molecule or ion that can donate a pair of electrons to the metal to form coordinate bonds. These complexes have diverse geometrical configurations, like octahedral, tetrahedral, and square planar.

• Coordination chemistry explores the interactions between metal centers and ligands.
• The geometry impacts the electronic, magnetic, and optical properties of the complex.
• The number and types of bonds dictate whether a complex is chiral.

In optical activity analysis, the spatial arrangement in coordination chemistry is vital. Chirality in coordination complexes results when they arrange in such a way that the mirror image cannot superimpose, often leading to optical activity.

Ligand Symmetry

Ligand symmetry refers to the symmetrical or asymmetrical arrangement of ligands around a central metal atom in a coordination complex. Symmetrical arrangements usually imply that the complex will be achiral and not optically active, but asymmetrical arrangements can lead to chirality.

• If a complex has a symmetrical arrangement, its mirror image is superimposable.
• An asymmetrical arrangement often leads to chiral and optically active complexes.
• Ligand placement around the central atom is crucial in determining if a complex is optically active.

Understanding ligand symmetry helps in predicting the optical properties of complex ions and is crucial when analyzing or synthesizing chiral compounds.

Bidentate Ligands

Bidentate ligands are a type of ligand that can form two bonds to a central metal ion. Ethylenediamine (en) is a classic example of a bidentate ligand. This ability to form two bonds with the central metal, instead of just one, often results in more stable complexes.

• Bidentate ligands form chelate rings with metal ions, which can stabilize the complex.
• Their configuration can contribute to the overall chirality of the coordination complex.
• They offer multiple points of attachment, affecting the symmetry and potential optical activity of the molecule.

In coordination complexes, the role of bidentate ligands is crucial in determining the optical activity. Their presence and bonding pattern can lead to the formation of non-superimposable mirror images, resulting in optically active configurations.