Backbonding

Backbonding, also called back-donation, is a concept in coordination chemistry describing the transfer of electron density from a filled metal d orbital into an antibonding π* orbital of a bound ligand. It is a key component of synergic metal–ligand bonding described by the Dewar-Chatt-Duncanson model, which also includes σ-type donation from the ligand to the metal.

In typical cases, a ligand such as carbon monoxide, cyanide, or nitrosyl acts as a π-acceptor. The ligand donates electron density to the metal to form a sigma bond, while the metal simultaneously donates electron density back into the ligand’s π* antibonding orbital. This backbonding strengthens the metal–ligand interaction and weakens the internal bond of the ligand, producing characteristic changes in bond properties.

The extent of backbonding depends on factors such as the metal’s electron richness, oxidation state, and the ligand’s ability to accept π electrons. Strong π-acceptor ligands and electron-rich metals favor more pronounced back-donation. Experimental evidence includes changes in vibrational spectra, notably a lowering of the CO stretching frequency in metal carbonyls, and observable modifications in bond lengths and strengths by spectroscopic and crystallographic methods.

Backbonding has important implications for reactivity and catalysis. It influences ligand binding properties, helps explain variations in bond strengths within a complex, and is central to understanding the behavior of many organometallic systems, including metal carbonyls, metal-olefin complexes, and NO-containing species.