A coordination compound, also called a complex compound, is a type of chemical compound in which one or more groups or molecules each form a coordinate bond (two atoms sharing two electrons from one of the atoms), usually a transition element. The complex may be either a complex ion or a neutral molecule, as in nickel carbonyl (Ni(CO)4). The coordinating species are known as ligands. Some coordination compounds, such as heme and chlorophyll, have biological importance. See also chelation; coordination compound.