Problem 178 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following complex species is not expected to exihibit optical isomerism? (a) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}_{3}\right]\) (b) \(\left[\mathrm{Co}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]^{+}\) (c) \(\left[\mathrm{Co}(\mathrm{en})_{3}\right]^{3+}\) (d) \(\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]^{+}\)

Step-by-Step Solution

Answer

Option (a) \(\left[\mathrm{Co(NH_{3})_{3} Cl_{3}\right]\) does not exhibit optical isomerism.

Step 1: Identify Optical Isomerism

Optical isomerism occurs in chiral molecules, which are non-superimposable on their mirror images. Generally, for coordination complexes, this is possible when the geometry of the complex allows for chirality.

Step 2: Analyze Option (a)

The complex \([\mathrm{Co(NH_{3})_{3} Cl_{3}]}\) is a coordination complex with a coordination number of six, having a geometry of an octahedral. In this structure, having all three chloride ions and all three ammonia molecules opposite to each other leads to a plane of symmetry, making it superimposable on its mirror image, hence not optically active.

Step 3: Analyze Option (b)

In \([\mathrm{Co(en)(NH_{3})_{2} Cl_{2}]}^{+}\), there is potential for chirality due to the presence of ethylenediamine (en), which can create an asymmetrical arrangement, but the presence of two identical ligands (ammonia) on opposite sides reduces this asymmetry leading to a plane of symmetry.

Step 4: Analyze Option (c)

The complex \([\mathrm{Co(en)_{3}]}^{3+}\) has a chiral arrangement due to the three bidentate \(en\) ligands. The tris-bidentate complex cannot be superimposed on its mirror image, indicating it exhibits optical isomerism.

Step 5: Analyze Option (d)

In the complex \([\mathrm{Co(en)_{2} Cl_{2}]}^{+}\), the two \(en\) ligands can create an asymmetrical structure, leading to chirality. The two chloride ions being in different positions also supports the lack of a plane of symmetry, making it chiral.

Key Concepts

Chirality in Coordination Compounds

In chemistry, chirality is a property where a molecule or a complex is not superimposable on its mirror image. This characteristic is crucial in coordination compounds as it determines whether a compound can exhibit optical isomerism. For optical isomerism to occur, the molecular arrangement must lack planes of symmetry, making it possible for the compound to have two different forms—one being the mirror image of the other.
In coordination compounds, chirality often arises when complex ligands are involved, or when the ligands themselves possess a specific spatial arrangement, such as in bidentate ligands. The structural geometry plays a big role too. Complexes with geometries like octahedral can potentially exhibit chirality, depending on the types and arrangements of the ligands attached to the central metal ion.

Coordination Chemistry

Coordination chemistry focuses on the study of compounds formed between metal ions and ligands. Ligands are ions or molecules that can donate a pair of electrons to the metal ion, forming a coordinate covalent bond.
These compounds display interesting geometries such as linear, square planar, tetrahedral, and octahedral. The geometry is determined by the metal's coordination number and the ligand's binding characteristics. This determines the three-dimensional arrangement of atoms around the metal, affecting properties like chirality and reactivity.
Coordination compounds are prevalent in nature and technology, with applications ranging from catalysis to biological systems. Understanding the principles of coordination chemistry is essential for predicting how these complexes will behave.

Cobalt Complexes

Cobalt complexes serve as fantastic examples in coordination chemistry due to their versatile coordination number and varied oxidation states. Cobalt can form complexes with coordination numbers ranging from four to six, often resulting in tetrahedral or octahedral shapes.
The complex \([\text{Co(en)_3}]^{3+}\), with a coordination number of six, arranges in an octahedral geometry. It has three bidentate ethylenediamine ligands, demonstrating its capacity to form chiral complexes.
Cobalt complexes often exhibit colorful properties due to electronic transitions, making them useful in spectroscopic and catalytic applications.

Ethylenediamine Ligands

Ethylenediamine, often abbreviated as "en", is a popular bidentate ligand in coordination chemistry. It contains two nitrogen atoms which can each donate a pair of electrons to form stable chelates when coordinated to a central metal ion.
Being a bidentate ligand, ethylenediamine wraps around the metal ion, occupying two coordination sites. This behavior can significantly impact the structural arrangement of complexes, often enhancing the potential for chirality.
In cobalt complexes, ethylenediamine helps create chiral centers, especially when used in multiples, such as in the complex \([\text{Co(en)_3}]^{3+}\), which features a non-superimposable mirror image contributing to optical isomerism. Ethylenediamine’s ability to stabilize metals also makes it favored in forming durable and manipulable metallic structures.