π backbonding, also called π backdonation,..............
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.............
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.
Although the bond-order within the ligand is decreased by this process, the metal - ligand bond-order is increased.
Compounds where π backbonding occurs include Ni(CO)4 and Zeise's salt.