Allyl ligand
= Allyl ligand
{title2=$\eta^3\text{-}\mathrm{C_3H_5^-}$}
An <allyl ligand> can bind as $\eta^3$ $\mathrm{C_3H_5^-}$, donating four <electrons> in ionic <electron counting>. Protonation can release an alkene and make coordination sites available.