The term Organometallic refers to compounds containing organic moieties bonded to a metal, often covalently. Many organometallic complexes feature coordination bonds between a metal and an organic ligand. The organic ligand often binds the metal through a heteroatom such as oxygen or nitrogen, in which case such compounds are considered coordination compounds. However, if any of the ligands form a direct metal–carbon bond, then the complex is usually considered organometallic. There are many naturally occurring organic coordination compounds. For example, the protein hemoglobin contains four heme groups in which an iron ion is coordinated to the nitrogen atoms of the porphyrin rings. Similarly, magnesium is coordinated via nitrogen atoms at the center of chlorophyll. In contrast, the coenzyme methylcobalamin, which has a cobalt–methyl bond, is a true organometallic complex.