Metal and metalloids are bound to ligands in virtually all circumstances. A ligand is an atom, ion, or molecule that bonds to a central metal, generally involving formal donation of one or more of its electrons. Ligands in a complex dictate the reactivity of the central atom, including ligand substitution rates, the reactivity of the ligands themselves, and redox. Ligand selection is a critical consideration in many practical areas.
A metal ion in solution does not exist in isolation, but in combination with ligands or chelating groups, giving rise to complex ions or coordination compounds. The term “chelating ligands” refer to more stable ligands that are attached to a central metal ion by bonds from two or more donor atoms such that they can form a ring including the metal atom. Many of the practical properties of metal complexes (such as color, magnetism, reactivity) are dictated by their electronic structures. Metal complexes often have spectacular colors due to electronic transitions by the absorption of light. The color of a complex depends on the nature of the metal ion as well as the strength and the arrangement of the ligands around the metal ion. Metal complexation is of widespread interest for various applications in areas such as light emitting or light absorbing materials, sensors and catalysts.