Problem 57 - A Complete Resource Book in Chemistry for JEE Main

Question

Optical isomerism is not shown by the complex (a) \(\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]^{+}\)(cis-form) (b) \(\left[\mathrm{Cr}\left(\mathrm{en}_{3}\right)\right]^{3+}\) (c) \(\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]^{+}\)(trans- form) (d) \(\left[\operatorname{Cr}(o x)_{3}\right]^{3-}\)

Step-by-Step Solution

Answer

The complex that does not show optical isomerism is (c) [Co(en)2Cl2]+ (trans-form).

Step 1: Review Optical Isomerism Criteria

Optical isomerism occurs in coordination complexes when they are non-superimposable on their mirror images. This is often due to the presence of chirality, which can stem from asymmetric arrangements of ligands around the central metal atom.

Step 2: Analyze cis-isomer of [Co(en)2Cl2]+

The len (ethylenediamine) in cen complexes acts as a bidentate ligand, coordinating twice to the metal. In the cis form of \([\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}]^{+}\), the arrangement allows for the formation of enantiomers due to the non-superimposable mirror images, hence it can show optical isomerism.

Step 3: Analyze [Cr(en)3]3+

Similarly, in \([\mathrm{Cr}(\mathrm{en})_{3}]^{3+}\),the three len ligands create a non-superimposable structure with its mirror image, qualifying it for optical activity.

Step 4: Analyze trans-isomer of [Co(en)2Cl2]+

In the trans-form of \([\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}]^{+}\), the symmetry is higher due to ligands being positioned on opposite sides, making the molecule superimposable on its mirror image. Thus, it does not exhibit optical isomerism.

Step 5: Analyze [Cr(ox)3]3-

\([\operatorname{Cr}(o x)_{3}]^{3-}\), where \text{ox}\a is oxalate, is a complex with ligands that can create an asymmetric arrangement, allowing for non-superimposable mirror images, thus this complex displays optical isomerism.

Key Concepts

Coordination Complexes

Coordination complexes are fascinating structures formed through the interaction of metal ions and organic or inorganic molecules known as ligands. These complexes are a key area of study in coordination chemistry, a branch that delves into the properties and reactions of metal complexes. In these structures, the central metal atom is bonded to surrounding ligands, creating a specific geometric arrangement.

Understanding coordination complexes involves recognizing the types of bonds formed between the metal and the ligands, which are typically coordinate covalent bonds. These are special types of bonds where both electrons in the bond originate from the same atom, namely the ligand. The geometry of these complexes can vary, with common forms including octahedral, tetrahedral, and square planar geometries. This shape and the configuration of ligands around the metal core significantly influence the complex's chemical and physical properties, including its color, reactivity, and magnetic properties.

Coordination complexes are integral to various biological systems and industrial processes. They appear in natural biological molecules, such as hemoglobin, and catalyze important reactions in industrial chemistry.

Chirality in Chemistry

Chirality is a fundamental concept in chemistry that refers to a property of an object or molecule that makes it distinguishable from its mirror image. This is analogous to left and right hands, which are mirror images but cannot be superimposed on each other. In chemistry, chirality is frequently due to the presence of a chiral center - typically a carbon atom bonded to four different substituents.

In coordination compounds, chirality can arise not only from carbon centers but also from the spatial arrangement of ligands around a central atom. As seen in the exercise, different isomers such as cis and trans forms can show diverse chiral properties. For instance, cis-isomers might be chiral and show optical activity, whereas trans-isomers, due to their symmetrical arrangement, are often achiral.

Chirality has enormous implications in various fields, such as pharmaceuticals. For example, the efficacy and safety of drugs can be chiral-specific, meaning a drug's enantiomer - one of two chiral molecules - can have different biological activity and toxicity.

Stereochemistry

Stereochemistry is the study of the spatial arrangements of atoms in molecules and the impact of this arrangement on the physical and chemical properties of substances. Particularly relevant in coordination complexes, stereochemistry can describe differences between molecules that have the same formula but differ in the three-dimensional placement of their atoms, called stereoisomers.

There are two kinds of stereoisomers: enantiomers, which are non-superimposable mirror images of each other, and diastereomers, which are not mirror images. The spatial orientation of ligands in a coordination complex determines the compound's stereochemistry.

This aspect of chemistry is crucial when discussing optical isomerism, where different isomers can manipulate polarized light differently. Enantiomers, for instance, rotate plane-polarized light in opposite directions. This property is not only essential in analytical chemistry but also in understanding the chiral nature of molecules and their behaviors in biological systems.

Coordination Chemistry

Coordination chemistry focuses on the study of coordination complexes and their formation, stability, reactivity, and applications. It explores how metal ions bind to surrounding ligands, forming complex ions or molecules. This branch of chemistry is crucial for understanding the behavior of metal-based substances in different environments.

One of the central concepts in coordination chemistry is the coordination number, which specifies the number of ligand atoms directly bonded to the central metal ion. For example, in octahedral complexes, the coordination number is six. The nature and type of ligands - whether they are monodentate, bidentate, or polydentate - influence the structure and properties of the coordination complex.

Coordination chemistry applies to various fields such as metallurgy, where it helps in extracting metals from ores, and biochemistry, where it explains the role of metal ions in biological systems. Understanding the principles of coordination chemistry is essential for designing new materials and catalysts, as well as in drug development.

Enantiomers

Enantiomers are a special type of stereoisomer, specifically molecules that are non-superimposable mirror images of each other. These molecules arise when a molecule is chiral, meaning it lacks an internal plane of symmetry and cannot be aligned with its mirror image through translation or rotation. Imagine holding a pair of gloves: each glove is an enantiomer of its pair.

In coordination complexes, enantiomers can appear when ligands create an asymmetric arrangement around a central metal ion, as seen in complexes capable of optical isomerism. Enantiomers are significant because they can have dramatically different effects in chemical reactions and biological environments.

This difference is prominent in pharmacology, where one enantiomer of a drug might be therapeutic, while the other is inactive or even harmful. As such, understanding enantiomers, their properties, and how to separate them is a crucial aspect of modern chemistry.