A coordination complex is an inorganic chemical compound where a central atom usually a metallic atom, known as the coordination centre, has an electron shell which permits it to combine with one or more molecules to form a surrounding array of bound molecules. These molecules are called ligands or complexing agents that have at least one pair of free electrons for binding. Many metal-containing compounds, especially those of transition metals, are coordination complexes. A coordination complex whose centre is a metal atom is called a metal complex.
The central atom or ion, together with all ligands, comprise the coordination sphere. The central atoms or ion and the donor atoms comprise the first coordination sphere. Coordination refers to the "coordinate covalent bonds" (dipolar bonds) between the ligands and the central atom. Ligands are generally bound to the central atom by a coordinate covalent bond that is, donating electrons from a lone electron pair into an empty metal orbital and are said to be coordinated to the atom. Originally, a complex implied a reversible association of molecules, atoms, or ions through such weak chemical bond.
A coordinate covalent bond, also known as a dative bond or coordinate bond is a kind of 2-center, 2-electron covalent bond in which the two electrons derive from the same atom. The bonding of metal ions to ligands involves this kind of interaction. This type of interaction is central to Lewis theory.
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