π backbonding, also called π backdonation, is a concept from chemistry, that refers to the flow of electrons from an atomic orbital on one atom to a π anti-bonding orbital that belongs to other atoms. It is especially common in the coordination of multi-atomic ligands to metal centres, where electrons from the metal flow towards the bond between the ligand atoms, thereby relieving the metal from excess negative charge and allowing it to bond with the ligands.
Because π backbonding places electrons in the anti-bonding molecular orbital of the ligand, it causes the respective bond order to decrease. This leads to an increase in the vibrational energy of the bond, which can be measured using IR spectroscopy.