Ligand Substitution Reactions

In a aqueous solution, transition metal ions exist as hexaaqua complexes, where six water molecules act as monodentate ligands. These water ligands can be replaced by other molecules or ions in a process known as ligand substitution. These reactions are often accompanied by changes in coordination number, shape, and colour.

Substitution by Neutral Ligands of Similar Size

When ligands are of a similar size and carry no charge, such as water (\(\text{H}_2\text{O}\)) and ammonia (\(\text{NH}_3\)), substitution can occur without a change in the coordination number or the overall shape of the complex.

The Reaction of Copper(II) Complexes with Ammonia

When excess ammonia is added to a solution containing hexaaquacopper(II) ions, a ligand substitution reaction takes place. However, only four of the six water ligands are replaced:

During this reaction, the following changes occur:

• Colour change: The pale blue solution of \([\text{Cu}(\text{H}_2\text{O})_6]^{2+}\) turns into a deep blue solution of \([\text{Cu}(\text{NH}_3)_4(\text{H}_2\text{O})_2]^{2+}\).
• Coordination number: Remains 6.
• Shape: Remains octahedral (though it is a distorted octahedron because the copper to nitrogen bond lengths differ from the copper to oxygen bond lengths).

Substitution by Larger, Charged Ligands

When small, neutral ligands (like \(\text{H}_2\text{O}\)) are replaced by larger, negatively charged ligands (like chloride ions, \(\text{Cl}^-\)), both the coordination number and the shape of the complex typically change. This is due to electrostatic repulsion and steric crowding around the central metal ion.

The Reaction of Copper(II) Complexes with Chloride

Adding concentrated hydrochloric acid (a source of \(\text{Cl}^-\) ions) to a solution of hexaaquacopper(II) ions results in the following reversible reaction:

This reaction displays distinct characteristics:

• Colour change: The solution changes from pale blue to a yellow-green colour. The \([\text{CuCl}_4]^{2-}\) complex itself is yellow, but the presence of unreacted blue aqua complexes in the equilibrium mixture makes the solution appear green.
• Coordination number: Decreases from 6 to 4.
• Shape: Changes from octahedral to tetrahedral.

The Reaction of Cobalt(II) Complexes with Chloride

Similarly, adding concentrated hydrochloric acid to a solution containing pink hexaaquacobalt(II) ions drives a substitution reaction:

• Colour change: The solution turns from pink to a distinct deep blue.
• Coordination number: Decreases from 6 to 4.
• Shape: Changes from octahedral to tetrahedral.

The Chelate Effect and Entropy

Complexes containing multidentate ligands (such as bidentate 1,2-diaminoethane or hexadentate \(\text{EDTA}^{4-}\)) are significantly more stable than complexes containing monodentate ligands. This phenomenon is known as the chelate effect.

The chelate effect is explained by the thermodynamic equation for Gibbs free energy change:

When a multidentate ligand replaces monodentate ligands, the reaction proceeds with a very small enthalpy change (\(\Delta H \approx 0\)) because the coordinate bonds being broken are very similar in strength to the new coordinate bonds being formed (e.g. oxygen-metal bonds replaced by nitrogen-metal bonds).

However, there is a large, positive entropy change (\(\Delta S > 0\)) because the number of free particles in solution increases:

Haemoglobin and Carbon Monoxide Poisoning

Haemoglobin is an iron-containing protein found in red blood cells that is responsible for transporting oxygen around the body. The structure of haemoglobin features a central iron(II) ion (\(\text{Fe}^{2+}\)) situated in an octahedral complex:

• Four of the coordinate bonds are formed by a flat, nitrogen-containing tetradentate ligand called a porphyrin ring (or haem group).
• A fifth coordinate bond is formed with a nitrogen atom on a protein called globin.
• The sixth coordinate site is vacant, allowing a water molecule or a diatomic molecule like oxygen (\(\text{O}_2\)) to bind reversibly.

Oxygen Transport Mechanism

In the lungs, where the oxygen concentration is high, water at the sixth coordinate site is replaced by an oxygen molecule via ligand substitution:

This oxyhaemoglobin is carried through the bloodstream to tissues that require oxygen. In the tissues, where oxygen concentration is low, the equilibrium shifts to the left, and the oxygen ligand is released, returning to haemoglobin.

Carbon Monoxide Poisoning

Carbon monoxide (\(\text{CO}\)) can also act as a ligand, coordinating to the \(\text{Fe}^{2+}\) ion at the sixth site. However, carbon monoxide forms a far stronger coordinate bond with the iron(II) ion than oxygen does. This results in a ligand substitution reaction that is essentially irreversible under normal physiological conditions:

Because the coordinate bond in carboxyhaemoglobin is so strong, the carbon monoxide ligand does not readily dissociate. This permanently blocks the oxygen-binding site, preventing haemoglobin from transporting oxygen, which leads to oxygen starvation in vital organs (carbon monoxide poisoning).