Pi backbonding

It is common in transition metals with low oxidation states that have ligands such as carbon monoxide, olefins, or phosphines.

The ligands involved in π backbonding can be broken into three groups: carbonyls and nitrogen analogs, alkenes and alkynes, and phosphines.

Compounds where π backbonding is prominent include Ni(CO)4, Zeise's salt, and molybdenum and iron dinitrogen complexes.

In contrast with CO, the σ-donor lone pair on the C atom of isocyanides is antibonding in nature and upon complexation the CN bond is strengthened and the νCN increased.

[7] In the case of metal-alkenes and alkynes, the strengthening of the M–C2R4 and M–C2R2 bond is reflected in bending of the C–C–R angles which assume greater sp3 and sp2 character, respectively.

[11] In early work, phosphine ligands were thought to utilize 3d orbitals to form M–P pi-bonding, but it is now accepted that d-orbitals on phosphorus are not involved in bonding as they are too high in energy.