In special situations, some ligands participate in substitution reactions leading to associative pathways. These ligands can adopt multiple motifs for binding to the metal, each of which involves a different number of electrons "donated." A classic case is the indenyl effect in which an indenyl ligand reversibly "slips' from pentahapto (η) coordination to trihapto (η). Other pi-ligands behave in this way, e.g. allyl (η to η) and naphthalene (η to η). Nitric oxide typically binds to metals to make a linear MNO arrangement, wherein the nitrogen oxide is said to donate 3e linear NO ligand to a 1e bent NO ligand.