Ligand substitution reactions explain colour changes in transition metal complexes and underpin biological systems such as haemoglobin, where ligand exchange enables oxygen transport and poisoning by carbon monoxide.

Ligand Substitution Reactions

What is Ligand Substitution?

Ligand substitution reactions are central to the chemistry of transition metal complexes and arise from the ability of metal ions to accept different ligands into their coordination sphere.

These reactions typically occur in aqueous solution and are often reversible, establishing equilibria rather than proceeding to completion. The metal–ligand bonds broken and formed are coordinate (dative covalent) bonds.

Conditions Affecting Ligand Substitution

The extent and speed of ligand substitution depend on several factors:

• Nature of the ligand
  Stronger ligands form more stable coordinate bonds and can displace weaker ligands.

• Concentration of incoming ligand
  Excess ligand shifts the equilibrium towards substitution.

• Charge and size of the metal ion
  Higher charge density strengthens metal–ligand attraction.

• Temperature
  Increased temperature generally increases reaction rate.

These reactions are commonly observed through distinct colour changes, making them useful in qualitative analysis.

Colour Changes in Ligand Substitution

Origin of Colour Changes

Transition metal complexes are coloured due to d–d electronic transitions, where electrons absorb visible light to move between split d-orbitals. Changing the ligand alters the ligand field strength, modifying the energy gap between d-orbitals.

Stronger field ligands cause a larger splitting, changing the wavelength of light absorbed and therefore the observed colour. Ligand substitution reactions are therefore often identified visually.

Example: Copper(II) Complexes

Copper(II) ions in water exist as the hexaaquacopper(II) complex, which is blue. Substitution of water ligands by ammonia produces a deep blue complex.

This occurs in stages:

• Initial addition of ammonia forms a pale blue precipitate due to hydroxide formation.

• Excess ammonia dissolves the precipitate, forming a deep blue tetraammine complex.

The observed colour change reflects the altered ligand environment around the Cu²⁺ ion.

When aqueous ammonia is added to a pale blue copper(II) aqua solution, ligand substitution produces an intensely deep blue copper–ammine complex.

This image shows the colour change when ammonia substitutes for water ligands around Cu²⁺. The deeper blue colour reflects formation of a copper(II) ammine complex with a different ligand environment. Source

Reversibility and Equilibria

Ligand substitution reactions are often reversible and governed by equilibrium principles. For example, increasing the concentration of one ligand shifts the equilibrium towards the complex containing that ligand.

This reversibility is crucial in biological systems, where rapid ligand exchange must occur under controlled conditions.

A clear understanding of equilibrium behaviour helps explain why some ligands bind preferentially but can still be displaced.

Haemoglobin as a Transition Metal Complex

Structure and Role of Haemoglobin

Haemoglobin is a biological coordination complex found in red blood cells. Each haemoglobin molecule contains four haem groups, each centred around an Fe²⁺ ion.

The iron(II) ion:

• Is coordinated to a porphyrin ring (four nitrogen atoms)

• Has one coordination position occupied by a nitrogen from a protein chain

• Has one remaining site available for ligand binding

This sixth coordination site is crucial for oxygen transport.

Oxygen Binding and Ligand Exchange

Haemoglobin contains Fe²⁺ in a haem group, and oxygen binds reversibly at the metal centre to form oxyhaemoglobin.

This diagram illustrates haemoglobin’s structure and highlights haem groups containing Fe²⁺ ions. Each iron centre can reversibly bind one oxygen molecule, acting as a coordination site for ligand exchange. Source

This equilibrium must be finely balanced:

• Oxygen must bind strongly enough for transport

• Oxygen must be released readily to respiring tissues

The reversibility of ligand substitution allows haemoglobin to function efficiently under varying oxygen concentrations.

Carbon Monoxide Poisoning

Competitive Ligand Binding

Carbon monoxide (CO) competes with oxygen for the same binding site on the iron(II) ion. However, CO acts as a much stronger ligand.

Carbon monoxide binds approximately 200 times more strongly than oxygen. This forms a very stable complex that prevents oxygen binding.

Consequences of CO Binding

• Haemoglobin becomes unavailable for oxygen transport

• Even small amounts of CO significantly reduce oxygen delivery

• This explains the high toxicity of carbon monoxide

Carbon monoxide forms carboxyhaemoglobin because CO binds strongly to the haem iron and can replace O₂ at the binding site.

This structure shows carbon monoxide coordinated directly to the haem iron as a ligand. It demonstrates how CO occupies the same binding site as oxygen, preventing oxygen transport in the blood. Source

The effect is a direct consequence of ligand substitution principles applied to a biological system.

Ligand Substitution and Biological Control

Haemoglobin demonstrates how ligand substitution reactions can be precisely regulated in living organisms. Protein structure influences ligand access, binding strength, and release.

Key features include:

• Reversible ligand substitution

• Selectivity for specific ligands

• Sensitivity to ligand concentration

These same principles underpin many transition metal reactions studied in inorganic chemistry, highlighting the importance of ligand substitution beyond the laboratory.

Summary of Key Concepts

• Ligand substitution involves replacement of ligands without changing oxidation state.

• Colour changes arise from altered d-orbital splitting.

• Reversible equilibria control substitution reactions.

• Haemoglobin functions through controlled ligand exchange with O₂.

• Carbon monoxide poisoning is explained by stronger ligand binding.

This subsubtopic links coordination chemistry directly to real-world biological and health-related applications, reinforcing the relevance of transition metal chemistry.