For doubly bridging (μ-) ligands, two limiting representation are 4-electron and 2-electron bonding interactions.  These cases are illustrated in main group chemistry by  and . Complicating this analysis is the possibility of metal–metal bonding.  Computational studies suggest that metal-metal bonding is absent in many compounds where the metals are separated by bridging ligands.  For example, calculations suggest that  lacks an iron–iron bond by virtue of a 3-center 2-electron bond involving one of three bridging CO ligands.