Certain organic compounds are capable of forming coordinate bonds with metals through two or more atoms of the organic compound; such organic compounds are called chelating agents. For example, nitrilotriacetic acid is a polyamino carboxylic acid and is used as a chelating agent which forms coordination compounds with metal ions (chelates) such as Ca2+, Cu2+ or Fe3+. A ligand is an atom, ion, or molecule that binds to a central metal-atom to form a coordination complex.