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

Question

When concentrated HCl is added to a solution of \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)\right]^{2^{+}}\)ion, an intense blue colour develops due to the formation of which one of the following? (a) \(\left[\mathrm{CoCl}_{4}\right]^{2-}\) (b) \(\left[\mathrm{CoCl}_{6}\right]^{4-}\) (c) \(\left[\mathrm{CoCl}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\right]^{+}\) (d) \(\left[\mathrm{CoCl}_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]\)

Step-by-Step Solution

Answer

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

Step 1: Identifying the Problem

We are given a chemical reaction involving the complex ion \( [\mathrm{Co(H_2O)_6}]^{2+} \) with concentrated hydrochloric acid (HCl), resulting in a change of color to blue. Our task is to identify which complex ion is responsible for this color change.

Step 2: Understanding the Chemical Reaction

Adding concentrated HCl to \( [\mathrm{Co(H_2O)_6}]^{2+} \) introduces a high concentration of chloride ions (\( \mathrm{Cl}^- \)). According to coordination chemistry principles, chloride ions can replace water molecules in the coordination sphere around the cobalt ion, forming different complexes.

Step 3: Analyzing the Effect of Chloride Ligands

Chloride ions have the ability to create tetrahedral complexes, especially with cobalt(II), which often exhibit a blue color. Common tetrahedral cobalt(II) chloride complexes include \( [\mathrm{CoCl}_4]^{2-} \), which is intensely blue.

Step 4: Selecting the Correct Option

Evaluating the options given, only option (a), \( [\mathrm{CoCl}_4]^{2-} \), is a tetrahedral complex capable of explaining the intense blue color observed in the reaction. The other complexes (options b, c, and d) involve higher coordination numbers or water molecules, which are less likely to be blue.

Step 5: Conclusion

After reviewing the potential cobalt-chloride complexes and their associated colors, option (a) \( [\mathrm{CoCl}_4]^{2-} \) is the correct complex responsible for the blue color observed when concentrated HCl is added to \( [\mathrm{Co(H_2O)_6}]^{2+} \).

Key Concepts

Cobalt Complexes

Cobalt, a transition metal, forms a variety of coordination complexes. These complexes consist of cobalt ions bonded to various ligands, which are molecules or ions that can donate electron pairs. The cobalt ion is typically found in two oxidation states — +2 and +3. The oxidation state greatly influences the geometric shape, stability, and color of the complex.

• **Octahedral Complexes:** These are common for cobalt(III), where cobalt is coordinated to six ligands, often leading to a range of vibrant colors depending on the ligands.
• **Tetrahedral Complexes:** More typical for cobalt(II) ions, involving four ligands. These complexes often show different spectral properties compared to octahedral equivalents.

The coordination environment around the cobalt ion impacts the energy levels of the d-orbitals, resulting in different absorption and, thus, observable colors. Understanding these nuances is essential when studying the reactivity and applications of cobalt complexes in chemistry and various industries.

Ligand Exchange

Ligand exchange is a fundamental concept in coordination chemistry where ligands attached to a central metal ion are replaced by other ligands. This can appear with cobalt complexes when reacting with various agents, such as acids or bases.

The key to understanding ligand exchange is knowing the stability of complexes and the strength of the new ligand compared to the existing one. Chloride ions from concentrated HCl, for instance, can replace water molecules in a cobalt water complex, forming a new cobalt chloride complex.

• A reaction with a ligand that forms a stronger bond to the metal ion can drive the exchange.
• The nature of both the leaving and incoming ligands, whether they are strong field or weak field ligands, influences the exchange kinetics and thermodynamics.

This process can lead to significant changes in physical properties, such as the color of the complex, due to changes in coordination geometry and electronic properties of the central metal ion.

Color Change in Complexes

The color change in coordination complexes, like those containing cobalt, is often an indicator of a change in the chemical environment around the central metal ion. This is largely influenced by the type of ligands present and the geometry of the coordination complex.

When you add concentrated HCl to a \( [\mathrm{Co(H_2O)_6}]^{2+} \) solution, it turns blue due to the formation of the \( [\mathrm{CoCl}_4]^{2-} \) complex. Chloride ions replace water molecules around the cobalt ion, changing its coordination number and electronic structure.

• **Tetrahedral vs. Octahedral:** Tetrahedral complexes, like \( [\mathrm{CoCl}_4]^{2-} \), tend to absorb light at different wavelengths compared to octahedral complexes, causing a color shift.
• **Spectrochemical Series:** The position of ligands like chloride and water in this series helps predict color changes. Chloride is a weaker field ligand than water, affecting the splittings of d-orbital energies and ultimately the color.

These color changes are not just visually striking but also provide insights into the structural and electronic transitions happening within the complex. This is why color changes are often used as a diagnostic tool in chemistry to infer the kind of reactions occurring within a system.