6.4 Ligand Substitution Reactions

In the previous sections, we explored the formation of complex compounds, their naming, and their geometries. We also learned that the color of a complex depends on the ligands attached to the central metal ion. This leads to an important type of reaction in coordination chemistry: the ligand substitution reaction. In these reactions, one ligand in a complex is replaced by another ligand, often resulting in a change in color or other properties.

Definition

A ligand substitution reaction (also called a ligand exchange reaction) is a reaction in which one ligand in a complex is substituted by another ligand. The reaction takes place if the complex formed is more stable than the reacting complex. They involve either complete or partial exchange of ligands.

Simple Explanation: It is a reaction where one "guest" (ligand) attached to the central metal ion is replaced by a different "guest."

Core Concept

The core concept is that a more stable complex is formed when a stronger-field ligand replaces a weaker-field ligand. The stability of a complex is determined by factors such as the charge density of the metal ion, the basicity of the ligand, and the chelate effect (for polydentate ligands). The reaction is driven by the formation of a more stable complex, which often has a larger ligand field splitting and a different color.

Detailed Explanation

The textbook focuses on the ligand substitution reactions of copper(II) complexes and cobalt(II) complexes with the ligands water, ammonia, hydroxide, and chloride ions.

i. Copper(II) Complexes

The textbook provides several examples of ligand substitution reactions for copper(II) complexes.

1. Reaction with Hydroxide Ions (Strong Base)

When a strong base, like sodium hydroxide (NaOH), is added to a blue solution of copper sulphate (CuSO_4), a pale blue precipitate of copper(II) hydroxide is formed. In this reaction, two hydroxide ligands (OH^-) replace two water ligands.

[Cu(H_2O)_6]²⁺_(aq) + 2OH^-_(aq) ⟶ [Cu(H_2O)_4(OH)_2]_(s) + 2H_2O_(l)

- The hydroxide ions do not substitute water ligands directly. They act as a base and take protons from water molecules, forming water and leaving hydroxide ions behind. The hydroxide ions thus made act as ligands and exchange with water molecules.

2. Reaction with Excess Ammonia (Ligand Exchange)

Adding an excess of ammonia solution dissolves the pale blue precipitate, giving a deep blue solution. In this reaction, four ammonia molecules replace two water ligands and two hydroxide ligands.

[Cu(H_2O)_4(OH)_2]_(s) + 4NH_3_(aq) ⟶ [Cu(NH_3)_4(H_2O)_2]²⁺_(aq) + 2H_2O_(l) + 2OH^-_(aq)

- Pale blue precipitate → deep blue solution.

3. Reaction with Aqueous Ammonia (Deprotonation and Ligand Exchange)

If we add aqueous ammonia instead of aqueous sodium hydroxide, we get the same observations. In this reaction, ammonia acts as a base and removes protons from water, leaving behind hydroxide ions which substitute water as ligands. This reaction is specifically called a deprotonation reaction.

[Cu(H_2O)_6]²⁺_(aq) + 2NH_3_(aq) ⟶ [Cu(H_2O)_4(OH)_2]_(s) + 2NH_4^+_(aq)

- Blue solution → pale blue precipitate.

This is a ligand exchange reaction in which four ammonia molecules replaced two water ligands and two hydroxide ligands.

ii. Cobalt(II) Complexes

The textbook also discusses cobalt(II) complexes, particularly the formation of the tetrachlorocobaltate(II) ion.

- Formation of [CoCl_4]^2-: When concentrated hydrochloric acid is added to a solution of cobalt(II) chloride, the blue complex ion [CoCl_4]^2- is formed. The reaction can be written as: [Co(H_2O)_6]²⁺_(aq) + 4Cl^-_(aq) ⇌ [CoCl_4]^2-_(aq) + 6H_2O_(l) The pink octahedral [Co(H_2O)_6]²⁺ ion reacts with chloride ions to form the blue tetrahedral [CoCl_4]^2- ion. This is a ligand substitution reaction where water ligands are replaced by chloride ligands.

Intermediate Level

Ligand substitution reactions are an important class of reactions in coordination chemistry. They involve the replacement of one ligand by another. The driving force is the formation of a more stable complex, often due to the chelate effect (for polydentate ligands) or a stronger ligand field. For copper(II), adding hydroxide ions precipitates copper(II) hydroxide, and adding excess ammonia dissolves the precipitate to form a deep blue solution of [Cu(NH_3)_4(H_2O)_2]²⁺. The reaction with ammonia also involves deprotonation of coordinated water. For cobalt(II), the pink octahedral [Co(H_2O)_6]²⁺ complex can be converted to the blue tetrahedral [CoCl_4]^2- complex by adding concentrated hydrochloric acid. These reactions are often accompanied by a change in color, which is a useful indicator of the reaction.

Important Points

• - Ligand Substitution: Replacement of one ligand by another.
• - Driving Force: Formation of a more stable complex.
• - Copper(II) Reactions:
• - With OH^-: Forms pale blue precipitate of Cu(OH)_2.
• - With excess NH_3: Forms deep blue [Cu(NH_3)_4(H_2O)_2]²⁺.
• - Cobalt(II) Reactions:
• - With concentrated HCl: Forms blue [CoCl_4]^2-.
• - Color Change: Often accompanies ligand substitution due to change in Δ.
• - Deprotonation: Ammonia can act as a base, removing protons from coordinated water, facilitating ligand exchange.

Remember

• - Ligand substitution = ligand exchange.
• - More stable complex is formed.
• - Cu(II) + OH^- → pale blue precipitate.
• - Cu(II) + excess NH_3 → deep blue solution.
• - Co(II) + concentrated HCl → blue [CoCl_4]^2-.
• - Color change is due to a change in the ligand field.
• - Ammonia can act as a base (deprotonation) as well as a ligand.

Common Mistakes

• - Thinking that the reaction is just a simple mixing. Ligand substitution involves the breaking of metal-ligand bonds and the formation of new ones.
• - Forgetting that the stability of the complex is the driving force. The reaction proceeds if the product is more stable than the reactant.
• - Not recognizing the color changes. The color changes are a key observation in these reactions.
• - Confusing the role of ammonia. Ammonia can act both as a ligand and as a base (removing protons from water).
• - Assuming all ligand substitutions are fast. Some are slow (inert complexes), while others are fast (labile complexes).

Quick Revision

Ligand substitution reactions involve the replacement of one ligand in a complex by another. The reaction is driven by the formation of a more stable complex. For copper(II), adding hydroxide ions forms a pale blue precipitate, while adding excess ammonia forms a deep blue solution of [Cu(NH_3)_4(H_2O)_2]²⁺. For cobalt(II), adding concentrated hydrochloric acid converts the pink [Co(H_2O)_6]²⁺ complex to the blue [CoCl_4]^2- complex. These reactions are often accompanied by color changes, which are due to changes in the ligand field splitting.

60-Second Revision

Ligand substitution is the replacement of one ligand by another in a complex, driven by the formation of a more stable complex. For Cu(II), OH^- gives a pale blue precipitate, and excess NH_3 gives a deep blue solution. For Co(II), concentrated HCl gives the blue [CoCl_4]^2- complex. Color changes accompany these reactions due to changes in the ligand field.

Frequently Asked Questions (FAQs)

Q1. What is a ligand substitution reaction? Answer: A ligand substitution reaction (or ligand exchange reaction) is a reaction in which one ligand in a complex is replaced by another ligand. The reaction takes place if the complex formed is more stable than the reacting complex.

Q2. What happens when sodium hydroxide is added to a copper(II) sulphate solution? Answer: When sodium hydroxide (a strong base) is added to a blue solution of copper(II) sulphate, a pale blue precipitate of copper(II) hydroxide, [Cu(H_2O)_4(OH)_2], is formed. Two hydroxide ligands replace two water ligands: [Cu(H_2O)_6]²⁺ + 2OH^- → [Cu(H_2O)_4(OH)_2]_(s) + 2H_2O

Q3. What happens when excess ammonia is added to the pale blue precipitate of copper(II) hydroxide? Answer: When excess ammonia is added, the pale blue precipitate dissolves to give a deep blue solution of [Cu(NH_3)_4(H_2O)_2]²⁺. In this reaction, four ammonia molecules replace two water ligands and two hydroxide ligands: [Cu(H_2O)_4(OH)_2]_(s) + 4NH_3 → [Cu(NH_3)_4(H_2O)_2]²⁺ + 2H_2O + 2OH^-

Q4. What is the role of ammonia in the reaction with copper(II) solution? Answer: Ammonia can act as both a base and a ligand. As a base, it removes protons from coordinated water molecules, forming hydroxide ions. As a ligand, it replaces water molecules in the complex. For example: [Cu(H_2O)_6]²⁺ + 2NH_3 → [Cu(H_2O)_4(OH)_2]_(s) + 2NH_4^+ This is called a deprotonation reaction. Then, excess ammonia can replace the hydroxide ligands to form the deep blue ammine complex.

Q5. What is the reaction between cobalt(II) chloride and concentrated hydrochloric acid? Answer: When concentrated hydrochloric acid is added to a solution of cobalt(II) chloride, the pink octahedral [Co(H_2O)_6]²⁺ ion reacts with chloride ions to form the blue tetrahedral [CoCl_4]^2- ion. This is a ligand substitution reaction: [Co(H_2O)_6]²⁺ + 4Cl^- ⇌ [CoCl_4]^2- + 6H_2O

Q6. Why do these ligand substitution reactions often involve a color change? Answer: The color of a complex depends on the energy gap (Δ E) between the split d orbitals, which is determined by the ligands. When one ligand is replaced by another, the ligand field strength changes, which changes Δ E. This changes the wavelength of light absorbed and, consequently, the color observed.

Q7. What is the driving force for a ligand substitution reaction? Answer: The driving force is the formation of a more stable complex. The stability of a complex is determined by factors such as the charge density of the metal ion, the basicity of the ligand, and the chelate effect (for polydentate ligands). The reaction proceeds in the direction that forms the more stable complex.

Q8. What is a deprotonation reaction in the context of ligand substitution? Answer: A deprotonation reaction is a reaction in which a base (like ammonia) removes a proton from a coordinated water molecule, converting it into a hydroxide ion. This hydroxide ion can then act as a ligand. For example: [Cu(H_2O)_6]²⁺ + 2NH_3 → [Cu(H_2O)_4(OH)_2]_(s) + 2NH_4^+

• Q9. What is the color of the [Cu(H_2O)_6]²⁺ complex and the [Cu(NH_3)_4(H_2O)_2]²⁺ complex?
• Answer: The [Cu(H_2O)_6]²⁺ complex is blue. The [Cu(NH_3)_4(H_2O)_2]²⁺ complex is deep blue (violet-blue).

Multiple Choice Questions (MCQs)

• Q1. What is a ligand substitution reaction?
• A. A reaction where a ligand is added to a complex.
• B. A reaction where a ligand is removed from a complex.
• C. A reaction where one ligand is replaced by another.
• D. A reaction where a metal ion is replaced.
• Correct Answer: C
• Explanation: A ligand substitution reaction is a reaction in which one ligand in a complex is replaced by another ligand.

• Q2. When sodium hydroxide is added to a copper(II) sulphate solution, what is observed?
• A. A deep blue solution.
• B. A pale blue precipitate.
• C. A green precipitate.
• D. No change.
• Correct Answer: B
• Explanation: A pale blue precipitate of copper(II) hydroxide is formed.

• Q3. When excess ammonia is added to the pale blue precipitate of copper(II) hydroxide, what is observed?
• A. A pale blue precipitate.
• B. A deep blue solution.
• C. A green precipitate.
• D. No change.
• Correct Answer: B
• Explanation: The precipitate dissolves to give a deep blue solution of [Cu(NH_3)_4(H_2O)_2]²⁺.

• Q4. What is the color of the [CoCl_4]^2- complex?
• A. Pink
• B. Blue
• C. Green
• D. Colorless
• Correct Answer: B
• Explanation: The [CoCl_4]^2- complex is blue.

• Q5. What is the color of the [Co(H_2O)_6]²⁺ complex?
• A. Pink
• B. Blue
• C. Green
• D. Colorless
• Correct Answer: A
• Explanation: The [Co(H_2O)_6]²⁺ complex is pink.

• Q6. What is the driving force for a ligand substitution reaction?
• A. The formation of a less stable complex.
• B. The formation of a more stable complex.
• C. The absorption of light.
• D. The release of heat.
• Correct Answer: B
• Explanation: The reaction is driven by the formation of a more stable complex.

• Q7. In the reaction of copper(II) with hydroxide ions, what is the role of the hydroxide ion?
• A. It acts as a ligand.
• B. It acts as a base.
• C. It acts as an acid.
• D. It acts as a catalyst.
• Correct Answer: A
• Explanation: The hydroxide ion acts as a ligand, replacing water molecules in the complex.

• Q8. In the reaction of copper(II) with ammonia, what is the role of ammonia?
• A. It acts only as a ligand.
• B. It acts only as a base.
• C. It acts as both a base and a ligand.
• D. It acts as an acid.
• Correct Answer: C
• Explanation: Ammonia acts as a base (removing protons from water) and as a ligand (replacing water molecules).

Q9. Why do ligand substitution reactions often involve a color change? A. Because the metal ion changes. B. Because the ligand field strength changes, altering the energy gap. C. Because the temperature changes. D. Because the concentration changes. Correct Answer: B Explanation: The color change is due to the change in the ligand field strength, which alters the energy gap (Δ E) and the wavelength of light absorbed.

• Q10. What is the color change when [Co(H_2O)_6]²⁺ is converted to [CoCl_4]^2-?
• A. Pink to blue.
• B. Blue to pink.
• C. Green to yellow.
• D. No color change.
• Correct Answer: A
• Explanation: The pink octahedral [Co(H_2O)_6]²⁺ complex changes to the blue tetrahedral [CoCl_4]^2- complex.

Multiple Choice Questions (MCQs)

• Q1. What is a ligand substitution reaction?
• A. A reaction where a ligand is added to a complex.
• B. A reaction where a ligand is removed from a complex.
• C. A reaction where one ligand is replaced by another.
• D. A reaction where a metal ion is replaced.
• Correct Answer: C
• Explanation: A ligand substitution reaction is a reaction in which one ligand in a complex is replaced by another ligand.

• Q2. When sodium hydroxide is added to a copper(II) sulphate solution, what is observed?
• A. A deep blue solution.
• B. A pale blue precipitate.
• C. A green precipitate.
• D. No change.
• Correct Answer: B
• Explanation: A pale blue precipitate of copper(II) hydroxide is formed.

• Q3. When excess ammonia is added to the pale blue precipitate of copper(II) hydroxide, what is observed?
• A. A pale blue precipitate.
• B. A deep blue solution.
• C. A green precipitate.
• D. No change.
• Correct Answer: B
• Explanation: The precipitate dissolves to give a deep blue solution of [Cu(NH_3)_4(H_2O)_2]²⁺.

• Q4. What is the color of the [CoCl_4]^2- complex?
• A. Pink
• B. Blue
• C. Green
• D. Colorless
• Correct Answer: B
• Explanation: The [CoCl_4]^2- complex is blue.

• Q5. What is the color of the [Co(H_2O)_6]²⁺ complex?
• A. Pink
• B. Blue
• C. Green
• D. Colorless
• Correct Answer: A
• Explanation: The [Co(H_2O)_6]²⁺ complex is pink.

• Q6. What is the driving force for a ligand substitution reaction?
• A. The formation of a less stable complex.
• B. The formation of a more stable complex.
• C. The absorption of light.
• D. The release of heat.
• Correct Answer: B
• Explanation: The reaction is driven by the formation of a more stable complex.

• Q7. In the reaction of copper(II) with hydroxide ions, what is the role of the hydroxide ion?
• A. It acts as a ligand.
• B. It acts as a base.
• C. It acts as an acid.
• D. It acts as a catalyst.
• Correct Answer: A
• Explanation: The hydroxide ion acts as a ligand, replacing water molecules in the complex.

• Q8. In the reaction of copper(II) with ammonia, what is the role of ammonia?
• A. It acts only as a ligand.
• B. It acts only as a base.
• C. It acts as both a base and a ligand.
• D. It acts as an acid.
• Correct Answer: C
• Explanation: Ammonia acts as a base (removing protons from water) and as a ligand (replacing water molecules).

Q9. Why do ligand substitution reactions often involve a color change? A. Because the metal ion changes. B. Because the ligand field strength changes, altering the energy gap. C. Because the temperature changes. D. Because the concentration changes. Correct Answer: B Explanation: The color change is due to the change in the ligand field strength, which alters the energy gap (Δ E) and the wavelength of light absorbed.

• Q10. What is the color change when [Co(H_2O)_6]²⁺ is converted to [CoCl_4]^2-?
• A. Pink to blue.
• B. Blue to pink.
• C. Green to yellow.
• D. No color change.
• Correct Answer: A
• Explanation: The pink octahedral [Co(H_2O)_6]²⁺ complex changes to the blue tetrahedral [CoCl_4]^2- complex.