Ligand exchange is a key idea in IB Chemistry Topic 13 (HL), especially within complex ion chemistry. It explains how transition metal complexes react with incoming ligands, why color changes occur, and how stability depends on ligand type and bonding strength. Understanding ligand exchange helps students describe reactions, predict products, and explain observations seen in qualitative analysis and laboratory investigations.
What Is Ligand Exchange?
Ligand exchange is the process where one or more ligands in a transition metal complex are replaced by new ligands.
A ligand is any species that donates a lone pair of electrons to a central metal ion to form a coordinate bond.
In ligand exchange:
- The metal ion stays the same
- The oxidation state usually stays the same
- Only the surrounding ligands change
This process often causes visible color changes and differences in complex stability.
Why Ligand Exchange Happens
Ligand exchange occurs when:
- The incoming ligand forms stronger coordinate bonds
- The new complex is more stable
- The incoming ligand has higher charge density or stronger field strength
Stronger ligands displace weaker ligands due to better metal–ligand interactions.
General Example of Ligand Exchange
A typical ligand exchange reaction can be written as:
[Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O
Here:
- Water ligands are replaced by ammonia
- The complex changes color (blue → deep royal blue)
- The new complex is more stable
This type of color change is a common exam point.
Types of Ligand Exchange
1. Partial Ligand Exchange
Only some ligands are replaced.
Example:
[Cu(H₂O)₆]²⁺ → [Cu(NH₃)₄(H₂O)₂]²⁺
Two water ligands remain.
2. Complete Ligand Exchange
All ligands are replaced.
Example:
[Fe(H₂O)₆]³⁺ + 6CN⁻ → [Fe(CN)₆]³⁻ + 6H₂O
This completely changes the complex’s structure and color.
3. Chelate Exchange
A monodentate ligand is replaced by a multidentate (chelating) ligand.
Example:
[Cu(H₂O)₆]²⁺ + EDTA⁴⁻ → [Cu(EDTA)]²⁻ + 6H₂O
Chelating ligands form more stable complexes due to the chelate effect.
Factors That Influence Ligand Exchange
1. Ligand Strength (Spectrochemical Series)
Ligands differ in field strength.
Weak field ligands: H₂O < NH₃ < CN⁻ : Strong field ligands
Stronger ligands more readily replace weaker ones.
2. Charge Density
Small, highly charged ligands form stronger bonds and displace weaker ligands.
Example: OH⁻ often replaces H₂O.
3. Chelate Effect
Chelating ligands bind through multiple sites, creating ring structures.
This increases stability and drives ligand exchange forward.
4. Steric Factors
Bulky ligands may slow down or prevent exchange due to limited space around the metal ion.
Examples of Ligand Exchange in IB Chemistry
1. Copper(II) complexes
Most tested in IB exams.
a. With chloride ions:
[Cu(H₂O)₆]²⁺ + 4Cl⁻ → [CuCl₄]²⁻ + 6H₂O
Blue → yellow–green
b. With ammonia:
[Cu(H₂O)₆]²⁺ → [Cu(NH₃)₄(H₂O)₂]²⁺
Blue → deep blue
2. Iron(III) complexes
[Fe(H₂O)₆]³⁺ + SCN⁻ → [Fe(SCN)(H₂O)₅]²⁺
Pale yellow → blood red
3. Cobalt complexes
[Co(H₂O)₆]²⁺ + 4Cl⁻ → [CoCl₄]²⁻ + 6H₂O
Pink → blue
These color changes are classic experimental observations.
Why Ligand Exchange Causes Color Changes
Color arises from:
- Splitting of d-orbitals
- Different ligand field strengths
- Changes in ΔE (energy gap)
New ligands change the extent of d-orbital splitting, altering the wavelength of light absorbed.
This produces a new observable color.
Importance of Ligand Exchange
Ligand exchange explains:
- Color changes in qualitative analysis
- Catalytic behavior of transition metals
- Biological processes (e.g., hemoglobin binding O₂ and CO)
- Stability of coordination complexes
- Properties of metal–ligand bonding
Understanding it gives insight into both inorganic chemistry and biochemistry.
Common IB Misunderstandings
“Ligand exchange changes the metal ion.”
No—the metal ion stays the same; only ligands change.
“Oxidation state always changes.”
Usually it does not change.
“All ligands bind with the same strength.”
Ligands vary widely in binding strength and field strength.
“Color changes mean oxidation has occurred.”
Color changes often come from ligand exchange, not oxidation.
FAQs
Why do strong ligands replace weak ones?
Stronger ligands form more stable, lower-energy complexes.
Why do some exchanges happen partially?
Ligands compete, and some positions are more stable to occupy.
Does ligand exchange occur in biological systems?
Yes—metal centers in enzymes often undergo ligand substitution.
Conclusion
Ligand exchange is the process where ligands around a transition metal ion are replaced by new ones. The process depends on ligand strength, chelation, charge density, and geometry. It often results in dramatic color changes and altered stability. Understanding ligand exchange is essential for mastering transition metal chemistry in the IB curriculum.
