Most complexes are brightly colored. Crystal field theory can be used to figure out the electron arrangement in a complex, which determines the color of the complex. (The spectrochemical series shows which ligands tend to create the largest values of D.)

The energy difference D in many complexes is about equal to the energy of a photon of visible light. When a complex is struck by a photon of the correct energy, it is absorbed and the electron jumps from the lower energy d orbitals to the higher ones. Since the complex absorbs light of that frequency and reflects the rest, we see the complementary color of light. To get the complement of a color, look at the color wheel below. Find the color that is absorbed, then move directly across the wheel to the other side to get the complement.

For example, the complex [Co(NH₃]₆]⁺³ absorbs light with a wavelength of 437 nm. This is in the blue-violet region of the spectrum. If we look directly across the color wheel from blue-violet, we see yellow: this complex appears yellow.

The magnitude of the splitting energy D can be gotten from the color of light absorbed. Remember that the energy of a photon of light is E = hc/l, where h is Planck's constant, c is the speed of light and l is the wavelength of light.

Example: The complex [Co(NH₃]₆]⁺³ absorbs light with a wavelength of 437 nm. What is the splitting energy D?

Solution: The complex absorbs light at this frequency, so D must be equal to this energy. We know that E = hc/l. h = 6.626*10^-34 J*s and c = 3.00*10⁸ m/s. The light wavelength is 437 nm, or 4.37*10^-7 m. Thus, the energy of the light is

E = hc/l

E = (6.626*10^-34 J*s * 3.00*10⁸ m/s) /4.37*10^-7 m

E = 4.55*10^{-19 J}

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