Coordination Compounds and Complexes

A coordination complex is an inorganic compound made up of a central atom or ion—usually a metal—called a coordination center, bound by coordinate covalent bonds to an array of neutral molecules or nonmetal atoms (or anions) called ligands. It is a distinct chemical species, exhibiting properties and behaviors different from the metal atom and ligands of which it is composed. Many metal-containing compounds, such as transition metals or d-block elements, are coordination complexes. They include naturally occurring compounds such as chlorophyll (a magnesium-porphyrin complex), vitamin B12 (a complex of cobalt with a macrocyclic ligand known as corrin), hemoglobin (contains an iron-porphyrin complex), and various enzymes (generically known as metalloenzymes), as well as substances such as dyes, pigments, and catalysts.