Problem 29 - Chemistry in 30 days

Question

\(\mathrm{CuSO}_{4}\) reacts with KCN solution and forms (a) \(\mathrm{Cu}(\mathrm{CN})\) (b) \(\mathrm{Cu}(\mathrm{CN})_{2}\) (c) \(\mathrm{K}_{3}\left[\mathrm{Cu}(\mathrm{CN})_{4} \mid\right.\) (d) \(\mathrm{K}_{4}\left[\mathrm{Cu}(\mathrm{CN})_{6}\right]\)

Step-by-Step Solution

Answer

The correct product is (c) \( \mathrm{K}_{3}\left[\mathrm{Cu}(\mathrm{CN})_{4}\right].\)

Step 1: Analyzing the Reactants

Copper sulfate, \( \mathrm{CuSO}_4 \), reacts with potassium cyanide, \( \mathrm{KCN} \). We start by considering the nature of these reactants. Copper sulfate is an ionic compound that dissociates into \( \mathrm{Cu}^{2+} \) and \( \mathrm{SO}_4^{2-} \) ions in solution. Potassium cyanide also dissociates into \( \mathrm{K}^+ \) and \( \mathrm{CN}^- \) ions in solution.

Step 2: Identifying the Intermediate Complex

When \( \mathrm{Cu}^{2+} \) ions are in presence of \( \mathrm{CN}^- \) ions, cyanide can act as a ligand forming complexes with copper due to its ability to donate electrons. The cyanide ion \( \mathrm{CN}^- \) is a strong field ligand, which typically forms stable complexes with transition metals like copper.

Step 3: Determining Possible Complexes

The most common cyanide copper complex with copper in +2 oxidation state is not stable. However, reduction often occurs, enabling the formation of complexes with copper in +1 oxidation state. One stable complex that can form is \( \mathrm{K}_3[\mathrm{Cu}(\mathrm{CN})_4] \), where \( \mathrm{Cu} \) has an oxidation state of +1 and is surrounded by four \( \mathrm{CN}^- \) ligands, resulting in a tetrahedral geometry.

Step 4: Selecting the Correct Answer

Considering the known chemistry of copper and cyanide, the stable and likely product due to the reduction to +1 oxidation state is the tetrahedral complex \( \mathrm{K}_3[\mathrm{Cu}(\mathrm{CN})_4] \). Hence the correct answer is option (c).

Key Concepts

Copper-Cyanide Complex

When copper sulfate dissociates in solution, it releases copper ions (\( \mathrm{Cu}^{2+} \) ions). These copper ions play a crucial role in complex formation reactions with cyanide ions (\( \mathrm{CN}^- \)). Cyanide ions, acting as ligands, possess a lone pair of electrons. This allows them to donate these electrons to copper, resulting in the formation of complex ions. Copper has the ability to form complexes of varying coordination numbers, but with cyanide, it commonly results in a tetrahedral or planar configuration. It's important to note that although potential complexes like \( \mathrm{Cu} \) ( +2) with cyanide can initially form, they are generally unstable. This instability is due to copper's tendency to undergo reduction, leading us to more stable products like \( \mathrm{K}_3[\mathrm{Cu}(\mathrm{CN})_4] \). The copper-cyanide complex is a great example of how reduction can influence the stability and formation of complexes in aqueous solutions.

Ligand Stability

Ligands are crucial components in coordination chemistry. They determine the stability of the metal complex. Cyanide (\( \mathrm{CN}^- \)), as a ligand, is known for its strength and high field property. What makes it a strong ligand is its capacity to form multiple bonds and its ability to induce stabilization in transition metal complexes.In coordination chemistry, the stronger the ligand, the higher the energy gap it can enforce in the electron orbitals of a metal ion. This influences the electron configuration and ultimately the stability of complexes. Cyanide’s role as a strong field ligand makes it particularly influential in stabilizing complexes involving copper. Due to its high ability to donate electrons and form \( \pi \)-bonds with copper, a stable electron configuration is achieved in complexes like \( \mathrm{K}_3[\mathrm{Cu}(\mathrm{CN})_4] \), leading to reduced potential energy and increased complex stability.

Oxidation State Reduction

Copper can exist in various oxidation states, most commonly +1 and +2. When forming complexes, its oxidation state can impact the type of complex formed and its stability. In cases involving strong field ligands like cyanide, the copper typically undergoes reduction from +2 to +1.This reduction improves the overall stable state of the complex. For instance, starting as \( \mathrm{Cu}^{2+} \), copper usually forms the unstable \( \mathrm{Cu}(\mathrm{CN})_2 \). However, when cyanide ions are present, they can facilitate this reduction.Reduction to +1 copper allows the formation of more stable complexes such as \( \mathrm{K}_3[\mathrm{Cu}(\mathrm{CN})_4] \). This is due to the linear coordination and tetrahedral arrangement these ions naturally favor when copper is in its reduced state. This fundamental process highlights the interplay between ionic interactions, electron configuration, and overall compound stability in coordination chemistry.