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Why do nickel complexes with water and ethylenediamine ligands have different colors?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — transition metal Modern version of the periodic table of the elements (printable).

The difference in color between the hexaquonickel ion, [Ni(H₂O)₆]²⁺ (green), and the tris(ethylenediamine)nickel ion, [Ni(H₂NCH₂CH₂NH₂)₃]²⁺ (purple), is explained by the crystal-field theory.

In transition-metal complexes, the d orbitals of the central metal ion split into different energy levels due to the electrical field generated by the surrounding ligands. The color of these complexes arises because the energy difference between these split orbitals corresponds to the absorption of light in the visible spectrum.

The specific color observed depends on the magnitude of this splitting, which varies based on the type of ligand attached to the metal. According to the spectrochemical series, ligands have different abilities to split these orbitals. In the case of nickel, the six nitrogen atoms in the ethylenediamine ligands cause a greater splitting of the d orbitals than the six oxygen atoms in the water molecules, resulting in the different observed colors.1