Problem 90 - A Complete Resource Book in Chemistry for JEE Main

Question

Which compound is formed when excess of \(\mathrm{KCN}\) is added to an aqueous solution of copper sulphate? (a) \(\mathrm{Cu}(\mathrm{CN})_{2}\) (b) \(\mathrm{K}_{2}[\mathrm{Cu}(\mathrm{CN})]\) (c) \(\mathrm{K}\left[\mathrm{Cu}(\mathrm{CN})_{2}\right]\) (d) \(\mathrm{K}_{3}\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]\)

Step-by-Step Solution

Answer

The compound formed is (d) \( K_3[Cu(CN)_4] \).

Step 1: Understand the Reaction Between KCN and CuSO4

When KCN d s added to a solution containing CuSO_4 , a complex ion forms. CN^- ions from KCN act as ligands for Cu^{2+} ions. We need to determine what complex ion forms when KCN is in excess.

Step 2: Assess Ligand Formation Capacity

KCN d nsufficient CN^- ions to coordinate with Cu^{2+} . The copper ion tends to form more stable and higher coordination complexes in the presence of excess ligands. Cu^{2+} commonly forms a tetrahedral or square planar complex with four ligands when in excess CN^- .

Step 3: Determine the Complex Formation

With excess KCN , Cu^{2+} forms a complex with 4 CN^- ions, resulting in the formation of the complex ion [Cu(CN)_4]^{3-} . To balance this, 3 K^+ ions should be coordinated to maintain ionic balance, forming K_3[Cu(CN)_4].

Step 4: Identify the Correct Compound

Analyze the options given: (a) Cu(CN)_2 , (b) K_2[Cu(CN)] , (c) K[Cu(CN)_2] , (d) K_3[Cu(CN)_4]. The correct answer is based on the complex formed from the previous step: K_3[Cu(CN)_4], which corresponds to option (d).

Key Concepts

Ligand Formation

Ligands are molecules or ions that can donate a pair of electrons to a central metal atom to form a coordination complex. In coordination chemistry, ligand formation is vital because it determines the structure and stability of the resulting complex.
When potassium cyanide (KCN) is added to a copper(II) sulfate (CuSO₄) solution, the cyanide ions (CN⁻) act as ligands. Each cyanide ion donates a pair of electrons to the copper ion (Cu²⁺), which is the central metal atom in this scenario.
The nature and number of ligands that can attach to a central metal ion depend on several factors:

• The oxidation state of the central metal: Copper, in its Cu²⁺ form, generally prefers a coordination number of four when forming complexes with cyanide ions.
• The type of ligand: Cyanide is a strong field ligand capable of forming stable complexes with metals.
• The availability of the ligands: In excess cyanide conditions, copper ions will coordinate with the maximum number of cyanide ions possible, often forming tetrahedral or square planar complexes.

This formation is key to determining which compounds will form from a given reaction.

Copper Complexes

Copper complexes are unique structures where copper ions interact with various ligands. In coordination chemistry, copper can form complexes with different geometries, including linear, tetrahedral, and square planar configurations.
Specifically, when dealing with copper(II) ions and cyanide ligands, a high level of coordination is typical. The copper ion prefers to bind with four cyanide ions in a tetrahedral geometry in many scenarios when cyanide is abundant.

• Tetrahedral complexes form when the steric and electronic properties of the ligands favor this spatial arrangement.
• Square planar complexes might also form, particularly when factors such as ligand field strength influence the spatial coordination.

In our particular reaction with potassium cyanide, the copper ion, Cu²⁺, coordinates with four cyanide ligands to form [Cu(CN)₄]³⁻. These complexes are often very stable due to the ability of cyanide to act as a strong ligand, ensuring the stability of the complex solution.

Coordination Chemistry

Coordination chemistry is the study of the structures and behaviors of complex compounds formed between metal ions and ligands. It is a vital field in understanding how metals interact chemically with other substances.
This discipline involves:

• The understanding of ligand types and their bonding capabilities.
• The electronic configurations of the metal centers, such as copper, dictating how many and what type of ligands can bond to it.
• Exploring the geometry and stability of the resulting complexes, such as tetrahedral [Cu(CN)₄]³⁻ that forms when copper ions meet excess cyanide ions.

For example, in our exercise where KCN is introduced to CuSO₄, coordination chemistry principles are at play to identify the resulting product. A combination of ligand field theory and knowledge of metal-ligand interactions helps predict that K₃[Cu(CN)₄] will form in excess cyanide environment, illustrating the powerful interplay between these foundational concepts.