Most coordination complexes are brightly colored. The introduction of the electron pairs from approaching ligands splits the d subshell electrons of the transition metal into two subsets, one whose energy is raised by the new electrons, the other set that is commensurably lowered in energy. Crystal field splitting explains the colors of transition metal complexes, the energy of excitation between the two sets corresponding to photon energy in the visible range. When the complex absorbs a visible photon an electron transitions from the lower energy d orbitals to an excited state. Because only certain wavelengths are absorbed, the substance appears the complementary colour (if red is absorbed, green is observed).

Whether from crystalized coordination complexes or coordinated impurities, this is why many minerals are brightly colored such as malachite, turquoise, citrine, emerald, ruby or amethyst. Now that's just interesting.

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