A compound in which molecules or ions form coordinate bonds to a metal atom or ion. Often complexes occur as complex ions, such as [Cu(H2O)6]2+ or Fe[(CN)6]3−. A complex may also be a neutral molecule (e.g. PtCl2(NH3)2). The formation of such coordination complexes is typical behaviour of transition metals. The complexes formed are often coloured and have unpaired electrons (i.e. are paramagnetic). See also ligand; chelate.