π backbonding

π backbonding, also called π backdonation, is a concept from chemistry in which electrons move from an atomic orbital on one atom to a π* anti-bonding orbital on another atom or ligand.[1][2] It is especially common in the organometallic chemistry of transition metals with multi-atomic ligands such as carbon monoxide, ethylene or the nitrosonium cation. Electrons from the metal are used to bond to the ligand, in the process relieving the metal of excess negative charge. IUPAC offers the following definition for backbonding:

A description of the bonding of π-conjugated ligands to a transition metal which involves a synergic process with donation of electrons from the filled π-orbital or lone electron pair orbital of the ligand into an empty orbital of the metal (donor–acceptor bond), together with release (back donation) of electrons from an nd orbital of the metal (which is of π-symmetry with respect to the metal–ligand axis) into the empty π*-antibonding orbital of the ligand.[3]

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. This leads to a decrease in the vibrational frequency of the bond, which can be measured using IR spectroscopy. 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.