6.2.2 Ligand Exchange

What is Ligand Exchange?

Transition metals form complex ions with ligands—molecules or ions that donate a lone pair of electrons to form a coordinate (dative covalent) bond with the metal ion. Ligands can be monodentate (binding through one atom), bidentate (binding through two atoms), or multidentate (binding through multiple atoms), impacting the properties and stability of the resulting complex.

Ligand exchange occurs when one ligand in a complex ion is replaced by another. This substitution can alter properties like colour, coordination number, and geometry, depending on the size and charge of the ligands involved.

Monodentate Ligands

Common Monodentate Ligands:

• Water ($\text{H}_2\text{O}$)
• Ammonia ($\text{NH}_3$)
• Chloride ion ($\text{Cl}^-$)

Size and Charge Considerations:

$\text{H}_2\text{O}$ and $\text{NH}_3$ are uncharged and similar in size, allowing for easy exchange without altering the coordination number.

Chloride Ligand and Coordination Number Changes

The chloride ion, $\text{Cl}^-$, is larger than $\text{H}_2\text{O}$ or $\text{NH}_3$, often leading to a change in coordination number when it substitutes in a complex.

Bidentate and Multidentate Ligands

Bidentate Ligands:

These ligands bind through two donor atoms and form more stable complexes.

Multidentate Ligands:

These ligands can form multiple bonds to a metal ion, providing high stability.

Biological Example: Haemoglobin

• Haem: A complex of iron(II) with a multidentate ligand called porphyrin.
• Oxygen Transport: Oxygen binds to the $Fe$(II) in haemoglobin, allowing oxygen transport in blood.
• Toxicity of Carbon Monoxide: Carbon monoxide ($CO$) is dangerous because it binds more strongly than oxygen to $Fe$(II) in haemoglobin, preventing oxygen transport.

Chelate Effect

• The chelate effect is the increased stability of complexes with bidentate or multidentate ligands over those with monodentate ligands. This occurs due to both enthalpy and entropy changes:
  • Enthalpy: There's often little change because the number of bonds to the metal stays similar.
  • Entropy: When multidentate ligands replace monodentate ligands, the reaction releases free ligands, increasing disorder and favouring the formation of the chelate complex.

Practical Applications of Ligand Exchange Reactions

Here are two practicals to explore ligand exchange reactions. Each example includes a method, what to observe, and explanations of what happens during the reaction. These will help you understand how different ligands affect transition metal complexes.

Example 1: Ligand Exchange with Ammonia – Copper(II) Complex

Aim:

To observe the ligand exchange reaction of $[ \text{Cu}(\text{H}_2\text{O})_6 ]^{2+}$ with ammonia and examine the resulting colour changes.

Materials Needed:

• 0.1 M Copper(II) sulfate solution, $\text{CuSO}_4$
• Ammonia solution (concentrated)
• Test tubes
• Dropping pipette Method:

1. Add about 2 cm$^3$ of copper(II) sulfate solution to a test tube.
2. This solution contains the $[ \text{Cu}(\text{H}_2\text{O})_6 ]^{2+}$ complex ion.
3. Using a dropping pipette, add ammonia solution dropwise to the test tube.
4. After each drop, gently swirl the solution and observe any colour changes.
5. Continue adding ammonia until no further colour change occurs. Expected Results:

• Initially, the solution is pale blue due to the $[ \text{Cu}(\text{H}_2\text{O})_6 ]^{2+}$ complex.
• Upon adding a few drops of ammonia, a pale blue precipitate of copper(II) hydroxide, $\text{Cu(OH)}_2$, forms as $\text{NH}_3$ acts as a base.
• As more ammonia is added, the precipitate dissolves, and the solution turns a deep royal blue.
• This indicates the formation of the $[ \text{Cu}(\text{NH}_3)_4(\text{H}_2\text{O})_2 ]^{2+}$complex. Explanation:

The reaction begins with the formation of $\text{Cu(OH)}_2$, but as ammonia concentration increases, it acts as a ligand and replaces water in the coordination sphere of copper.

The deeper blue colour is characteristic of the new $[ \text{Cu}(\text{NH}_3)_4(\text{H}_2\text{O})_2 ]^{2+}$ complex, showing the completion of ligand exchange.

Example 2: Ligand Exchange with Chloride Ions – Cobalt(II) Complex

Aim:

To observe how chloride ions replace water ligands in $[ \text{Co}(\text{H}_2\text{O})_6 ]^{2+}$, affecting colour and coordination number.

Materials Needed:

• 0.1 M Cobalt(II) chloride solution, $\text{CoCl}_2$
• Concentrated hydrochloric acid
• Test tubes
• Dropping pipette Method:

6. Add about 2 cm$^3$ of cobalt(II) chloride solution to a test tube.
7. This solution contains $[ \text{Co}(\text{H}_2\text{O})_6 ]^{2+}$
8. Add concentrated hydrochloric acid dropwise to the solution, swirling gently after each addition.
9. Observe any colour changes.
10. Continue adding hydrochloric acid until no further colour change occurs. Expected Results:

• The initial pink colour is due to $[ \text{Co}(\text{H}_2\text{O})_6 ]^{2+}$
• As $\text{Cl}^-$ ions are added, the solution turns deep blue.
• This colour change shows the formation of $[\text{CoCl}_4 ]^{2-}$, where chloride ions have replaced water in the coordination sphere. Explanation:

The chloride ion ($\text{Cl}^-$) is larger than water, and as it replaces water molecules, the coordination number changes from 6 to 4, resulting in a $[\text{CoCl}_4 ]^{2-}$ tetrahedral complex.

This change in coordination geometry leads to the observed colour shift from pink to blue.