Pi backbonding
= Pi backbonding
<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 $\pi^*$ 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.