text: Complexes with η-allyl ligands (classified as X-type ligands) are also known. One example is CpFe(CO)(η-CH), in which only the methylene group is attached to the Fe centre (i.e., it has the connectivity [Fe]–CH–CH=CH).  As is the case for many other η-allyl complexes, the monohapticity of the allyl ligand in this species is enforced by the 18-electron rule, since CpFe(CO)(η-CH) is already an 18-electron complex, while an η-allyl ligand would result in an electron count of 20 and violate the 18-electron rule. Such complexes can convert to the η-allyl derivatives by dissociation of a neutral (two-electron) ligand L.  For CpFe(CO)(η-CH), dissociation of L = CO occurs under photochemical conditions:
  : CpFe(CO)(η-CH) → CpFe(CO)(η-CH) + CO