Use the color wheel by identifying the color of light absorbed by the transition metal complex, then selecting the color directly opposite it. This complementary color is the color observed.
Why the Complementary Color Is Observed
In a transition metal complex, ligands cause the five d orbitals of the metal ion to split into different energy levels. This is called d-orbital splitting.
An electron can absorb a photon and move from a lower-energy d orbital to a higher-energy d orbital. The energy absorbed, called the splitting energy, satisfies:
Therefore, the size of the energy gap determines the wavelength, and hence the color, absorbed. The remaining wavelengths are transmitted or reflected into the eye, so the complex appears the complementary color of the absorbed light.
| Color absorbed | Approximate complementary color observed |
|---|---|
| Violet | Yellow-green |
| Blue | Orange |
| Green | Red or purple |
| Yellow | Blue or violet |
| Orange | Blue |
| Red | Blue-green |
For example, if a complex absorbs orange light, locate orange on the color wheel. The opposite region is blue, so the complex appears blue.
If a question gives an absorption wavelength, first use the electromagnetic spectrum or data provided to identify its color. For example, absorption near is in the red region, so the complex appears blue-green.
The common misconception is that a complex appears the same color as the light it absorbs. This is incorrect: the absorbed color is removed from the visible light reaching the observer.
IB Exam Technique
This is Additional Higher Level content from Structure 3.1.10. For an explain question, state that ligands split the d orbitals, light is absorbed to promote an electron between them, and the complementary color is observed. If a color wheel is supplied, read across its center rather than choosing a neighboring color.
