d-Orbital Splitting
In an isolated gaseous transition metal ion, the five d-orbitals are degenerate (equal energy). When ligands bond to the metal, their lone pairs repel the d-electrons unequally, causing the d-orbitals to split into two energy levels.
The energy gap between the two levels is called ΔE
d-Orbital Splitting in an Octahedral Field
How Colour Arises
- White light passes through the solution
- A d-electron absorbs a photon with the exact energy matching ΔE
- The electron is promoted from the lower to the upper d-orbital level
- The remaining wavelengths are transmitted; the observed colour is the complementary colour of the absorbed light
The Colour Wheel
Factors Affecting ΔE and Colour
| Factor | Effect on ΔE | Effect on Colour |
|---|---|---|
| Nature of ligand | Strong field ligands (CN⁻) → large ΔE; Weak field (Cl⁻, H₂O) → small ΔE | Different ligands cause different absorption wavelengths |
| Oxidation state | Higher charge → ligands pulled closer → larger ΔE | Colour changes with oxidation state |
| Identity of metal | Different nuclear charges and configs → unique splitting | Each metal produces characteristic colours |
| Coordination geometry | Octahedral and tetrahedral fields split differently | Same metal/ligand but different geometry → different colour |
Why d0 and d10 Complexes Are Colourless
Ions with \(\text{d}^0\) (e.g. \(\text{Sc}^{3+}\), \(\text{Ti}^{4+}\)) or \(\text{d}^{10}\) (e.g. \(\text{Zn}^{2+}\), \(\text{Cu}^{+}\)) configurations cannot undergo \(\text{d}\text{--}\text{d}\) transitions: \(\text{d}^0\) has no d-electrons to promote, while \(\text{d}^{10}\) has completely filled d-orbitals with no vacant higher states. These complexes are completely colourless.
3-Step Formula: "Why Is a Complex Coloured?"
When asked to explain the origin of colour in a transition metal complex, always state the 3 essential marking points:
- Ligands cause d-orbitals to split into two sets of non-degenerate energy levels (\(\Delta E\)).
- A d-electron absorbs visible light photons matching \(\Delta E = hf = \frac{hc}{\lambda}\) and is promoted to a higher d-orbital (\(\text{d}\text{--}\text{d}\) transition).
- The transmitted / observed colour is the complementary colour to the absorbed wavelength (deduced from the colour wheel).
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