| Also used in organometallic and inorganic chemistry to define the molecules attached or surrounding a metal ion. In solution any metal will be cloaked in solvent (water molecules, for example). However, there are also organic species that can chelate a metal, providing some of its ligands. Haem molecules occupy four ligand positions around the central iron atom, in a plane. When bound to Haemoglobin, the other two (above and below the haem plane) are occupied by a sidechain atom (a histidine) and the oxygen molecule. Therefore, iron-haem complexes are ligands in the biochemical sense for the globin; and the oxygen is a ligand in the chemical sense for the iron! Clear :) | 
