Crystal field splitting is one of the most important ideas in IB Chemistry Topic 13 (HL). It explains why transition metal complexes form colors, how ligand strength affects energy levels, and why certain complexes are high spin or low spin. Understanding crystal field splitting allows students to interpret visible absorption, magnetic properties, and stability of transition metal complexes.
What Is Crystal Field Splitting?
Crystal field splitting is the separation of the five d-orbitals of a transition metal ion into groups of different energies when ligands approach and form a complex.
In a free metal ion, all five d-orbitals have equal energy.
When ligands surround the metal and form coordinate bonds, their negative charge creates an electric field.
This field repels the d-electrons unevenly, causing the orbitals to split into two energy levels.
This process is the foundation of the color and magnetism of transition metal complexes.
Why d-Orbitals Split
Ligands donate lone pairs of electrons to the metal.
These electrons:
- Repel the electrons already in the d-orbitals
- Cause some d-orbitals to rise in energy
- Cause others to drop in energy
The exact pattern of splitting depends on:
- The geometry of the complex
- The type of ligands
- The oxidation state of the metal
This splitting creates an energy gap called Δ (delta).
Crystal Field Splitting in Octahedral Complexes
Octahedral complexes are the most common in IB Chemistry.
In an octahedral field:
- Six ligands surround the metal ion
- Two d-orbitals point directly at ligands (higher energy)
- Three d-orbitals point between ligands (lower energy)
