Back Bonding
The donation of electron density from a filled orbital on one atom into an empty orbital on its bonded partner, in the direction opposite to the sigma bond.
Formula / equation
M(dπ) → L(π*)
Unit
kJ/mol
In depth
Also called pi back-donation, it strengthens the metal–ligand bond while weakening bonds within the ligand, because the electrons enter an antibonding orbital. It is the reason carbon monoxide binds so strongly to transition metals and is central to organometallic catalysis.
Examples in the real world
In Ni(CO)₄ back-donation lowers the C–O stretching frequency from 2143 to about 2060 cm⁻¹; p–d back-bonding shortens the B–F bonds in BF₃.