π backbonding, also called π backdonation, is a concept from chemistry, in which electrons move from an atomic orbital on one atom to a π* anti-bonding orbital on another atom or ligand. It is especially common in the organometallic chemistry of transition metals with multi-atomic ligands such as carbon monoxide, ethylene or the nitrosonium cation. Electrons from the metal are used to bond to the ligand, in the process relieving the metal of excess negative charge.
The electrons are taken from a d-orbital of the metal, and usually placed into an anti-bonding molecular orbital of the ligand, which causes the bond order of that ligand to decrease. This leads to a decrease in the vibrational frequency of the bond, which can be measured using IR spectroscopy. Although the bond-order within the ligand is decreased by this process, the metal - ligand bond-order is increased.