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)
They split into:
- t₂g (three lower energy orbitals)
- eₙ (two higher energy orbitals)
The energy gap is Δ₀ (octahedral splitting energy).
This is the key to understanding color in complexes.
Crystal Field Splitting in Tetrahedral Complexes
In a tetrahedral complex:
- Four ligands surround the metal
- d-orbitals split into the opposite arrangement of octahedral complexes
- Tetrahedral splitting is smaller because ligands do not point directly at orbitals
They split into:
- e (lower energy)
- t₂ (higher energy)
Tetrahedral complexes often appear in different colors than their octahedral counterparts because Δₜ is smaller than Δ₀.
The Link Between Splitting and Color
The size of the splitting determines which wavelengths of visible light are absorbed.
How color appears:
- Electrons absorb energy equal to Δ
- They jump from lower d-orbitals to higher d-orbitals
- The complex absorbs a specific wavelength
- The color observed is the complementary color of the absorbed wavelength
Example:
If Δ corresponds to absorbing red light, the complex appears green.
This explains why different ligands cause different colors.
The Spectrochemical Series
Ligands are ranked by how much they split the d-orbitals.
Weak field ligands (small Δ):
- I⁻ < Br⁻ < S²⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O
Strong field ligands (large Δ):
- NH₃ < en < CN⁻ < CO
Larger Δ → absorbs higher-energy light → different color.
High-Spin vs Low-Spin Complexes
Crystal field splitting also determines whether electrons pair or remain unpaired.
High-spin complexes
- Weak field ligands
- Small Δ
- Electrons spread out first
- More unpaired electrons
- Higher magnetism
Low-spin complexes
- Strong field ligands
- Large Δ
- Electrons pair before occupying higher orbitals
- Fewer unpaired electrons
- Lower magnetism
This explains magnetic behavior seen in experiments.
Factors Affecting Crystal Field Splitting
- Ligand type (spectrochemical series)
- Metal ion identity
- Oxidation state—higher oxidation → larger Δ
- Geometry (octahedral > square planar > tetrahedral)
- Number of ligands
These factors help predict color and magnetic properties.
Common IB Misunderstandings
“All transition metal complexes are colored.”
Not true—d⁰ and d¹⁰ ions do not undergo splitting with available electrons.
“Ligands change the color by changing oxidation state.”
Colors change due to splitting, not oxidation changes.
“All complexes have the same splitting pattern.”
Geometry strongly affects splitting.
“Color comes from electrons jumping shells.”
Color comes from d–d transitions, not n-level transitions.
FAQs
Why do strong ligands split d-orbitals more?
They donate electron density more strongly, increasing repulsion in certain orbitals.
Why do Zn²⁺ complexes have no color?
Zn²⁺ is d¹⁰ — no electron transitions possible.
Do all colored complexes use visible light?
Most IB examples do, but some absorb UV or IR.
Conclusion
Crystal field splitting is the separation of d-orbitals into different energy levels when ligands interact with a transition metal ion. The size of this splitting determines the color, magnetism, and spin state of the complex. It is a central concept in understanding transition metal chemistry in IB and explains many observable phenomena in the laboratory.
