π allyl complexes
If an allyl group, CH2=CH-CH2-, is bonded to a metal via a carbon atom, it is a 1-electron ligand. like an alkyl group. If the double bond delocalizes, three carbon atoms bond to the metal simu ltaneously as a 3-electron ligand. This is also an odd electron and formal ly anionic ligand and is stailized by being coordinated to t metal. (PPh3 Co(C3H5 etc. are well -known examples. Since η1 , η2 , and η3 coordinated modes are possible in the cataly reactions of unsaturated hydrocarbon various reactions occur.