Problem 100 - A Complete Resource Book in Chemistry for JEE Main

Question

When concentrated HCl is added to a solution of \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2^{+}}\)ion, an intense blue colour develops due to the formation of which one of the following? (a) \(\left[\mathrm{CoCl}_{4}\right]^{2}\) (b) \(\left[\mathrm{CoCl}_{6}\right]^{-}\) (c) \(\left[\mathrm{CoCl}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3}\right]^{+}\) (d) \(\left[\mathrm{CoCl}_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]\)

Step-by-Step Solution

Answer

The blue color is due to the formation of \\( [\text{CoCl}_4]^{2-} \\).

Step 1: Identify the Reaction Type

The addition of concentrated HCl to the solution indicates a ligand substitution reaction where chloride ions (Cl⁻) replace water molecules in the coordination complex.

Step 2: Determine the Coordination Complex

When chloride ions replace water molecules in the coordination sphere of cobalt, the blue color suggests the formation of a tetrahedral complex. This often involves four chloride ions coordinating with cobalt.

Step 3: Analyze the Coordination and Oxidation State

In tetrahedral complexes, cobalt typically forms \( [\text{CoCl}_4]^{2-} \) with a coordination number of four. The 2- charge indicates it’s a common stable state for this configuration.

Step 4: Verify the Option Based on Color and Complex

Among the given options, \( [\text{CoCl}_4]^{2-} \) is known to exhibit a blue color when formed. It's consistent with the description when concentrated HCl is added to `\([\text{Co(H}_{2}\text{O})_{6}]^{2+} \) ion.

Key Concepts

Ligand Substitution

Ligand substitution is a fascinating process in coordination chemistry where one or more ligands in a coordination complex are replaced by different ligands. This process plays a crucial role in defining the properties and reactivity of coordination complexes.
For instance, when you add concentrated hydrochloric acid (HCl) to a coordination complex such as \([\text{Co(H}_{2}\text{O})_{6}]^{2+} \), the chloride ions (Cl⁻) act to displace the water molecules in the complex. This changes the overall configuration and properties of the metal center.

• Driving Forces: Thermodynamics and kinetics play a role in ligand substitution. Key factors include the stability of the new complex and the leaving group ability of the original ligands.
• Mechanism Types: Substitution can occur either by associative or dissociative pathways, depending on the metal center and the ligands involved.

In the case of the cobalt complex, the substitution of water ligands with chloride results in an observable color change, highlighting the dynamic nature of the substitution process.

Coordination Complexes

Understanding coordination complexes is central to exploring the beauty of coordination chemistry. A coordination complex is basically a molecule that includes a central metal ion bonded to surrounding molecules or anions, known as ligands.
Coordination complexes exhibit a variety of geometries, electronic properties, and reactivities.

• Central Metal Ion: Usually a transition metal, like cobalt, which can form multiple bonds with ligands.
• Ligands: These molecules or ions coordinate with the metal ion. They can range from water and ammonia to more complex organic molecules.

The coordination number indicates how many ligand bonds are formed with the metal ion. For example, \([\text{Co(H}_{2}\text{O})_{6}]^{2+} \) has a coordination number of six due to six water molecules being bonded to the cobalt ion. This coordination creates distinct colors depending on the ligands involved, often shifting due to ligand substitution.

Tetrahedral Complexes

Tetrahedral complexes are one of the many shapes coordination complexes can take. In a tetrahedral complex, four ligands are symmetrically arranged around a central metal ion. This shape leads to unique chemical and physical properties compared to other geometries like octahedral.
Tetrahedral complexes are often less common than their octahedral counterparts for transition metals partly due to electronic factors.

• Geometry: Four atoms or groups create a three-dimensional shape where the angle between any two ligands is about 109.5 degrees.
• Example: The formation of \([\text{CoCl}_4]^{2-} \) from \([\text{Co(H}_{2}\text{O})_{6}]^{2+} \) represents a classic conversion to a tetrahedral complex upon ligand substitution.

This particular transformation is also reflected in the striking color change, with the blue hue being characteristic of tetrahedral cobalt complexes.