A complex ion consists most commonly of a metal atom or ion, such as Co3+, and attached groups called ligands. Ligands may be neutral molecules, such as NH3, or anions, such as Cl-. Furthermore, the ligands may be of all one type, as in [Co(NH3)6]3+ or of different types, as in [CoCl(NH3)5]2+.

The region surrounding the central atom or ion and its ligands is called the coordination sphere. The coordination number is the total number of points at which the central atom or ion attaches to its ligands. Both [Co(NH3)6]3+ and [CoCl(NH3)5]2+ have coordination numbers of 6. The most common coordination numbers observed in complex ions are 2, 4, and 6. If the complex carries a net electric charge, as the two examples given here do, it is called a complex ion. If it is electrically neutral, it is referred to as a coordination compound. An example of a coordination compound is [Co(NH3)6]Cl3.

Many complex ions are colored because the energy differences in the d orbitals match the components of visible light. Substituting one ligand for another produces subtle changes in the energy levels of the d orbitals and striking changes in the colors of complex ions. For example, a solution of [Cr(H2O)6 ]3+ is violet, while a solution of [Cr(NH3)6 ]3+ is yellow.

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