How do unpaired electrons in transition-metal compounds produce color?

The color of transition-metal compounds arises from the presence of unpaired electrons in their partially filled d orbitals. In these compounds, the surrounding atoms or ligands create an electric field—known as a ligand field—that causes the five d orbitals to split into subsets of different energy levels.123

Because these energy levels are separated, the unpaired electrons can absorb photons of visible light to transition from a lower-energy orbital to a higher-energy one. The specific energy required for this transition corresponds to the energy difference between the split d orbitals. When a compound absorbs certain wavelengths of visible light, the remaining transmitted or reflected light produces the characteristic colors observed in transition-metal salts and complexes.314

The magnitude of this orbital splitting, and thus the resulting color, depends on the nature of the metal ion and the specific ligands attached to it. For example, different ligands can shift these energy levels, leading to the distinct colors seen in various coordination compounds.341