Pi backbonding is donation from occupied metal orbitals into ligand acceptor orbitals with pi symmetry about the metal–ligand axis. In a carbonyl ligand, occupation of CO weakens C–O while strengthening metal–carbon bonding. Lower CO stretching spectroscopic wavenumber is therefore a useful comparative probe, with electrostatic and mode-coupling qualifications.
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Pi backbonding, often referred to in the context of chemistry, particularly in coordination chemistry and organometallic chemistry, is a type of bonding interaction between a metal center and a ligand. It typically involves the donation of electron density from a filled metal d-orbital to an empty orbital of a ligand, usually a π* (pi-star) orbital, which is the antibonding orbital associated with pi bonds.