Coordination complexes are so pervasive that the structure and reactions are defined in many ways, sometimes confusingly. The atom within a ligand that is bonded to the central atom or ion is known as the donor atom. A typical complex is bound to various donor atoms, which can be the similar or different. Polydentate (multiple bonded) ligands consist of various donor atoms, several of which are bound to the central atom or ion. These complexes are known as chelate complexes, the formation of such complexes is known as chelation, complexation, and coordination.
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) among the ligands and the central atom. Originally, a complex implied a reversible association of molecules, atoms, or ions through such weak chemical bonds. As applied to coordination chemistry, this meaning has evolved. Some metal complexes are formed virtually irreversibly and many are bound together by bonds that are quite strong.