Associative Substitution - Special Ligand Effects

Special Ligand Effects

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 (η5) coordination to trihapto (η3). Other pi-ligands behave in this way, e.g. allyl (η3 to η1) and naphthalene (η6 to η4). Nitric oxide typically binds to metals to make a linear MNO arrangement, wherein the nitrogen oxide is said to donate 3e- to the metal. In the course of substitution reactions, the MNO unit can bend, converting the 3e- linear NO ligand to a 1e- bent NO ligand.