what are pi acid ligands? give some examples of pi acid ligand. is C2H4 a pi acid ligand? if yes, how?

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* antibonding orbital on a π-acceptor ligand.

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.

A example of a pi acid ligand is carbon monoxide(CO). CO is a good pi acceptor (lewis acid) due to empty pi orbitals and a good sigma donor (lewis acid).

When bonding to a metal the ligand (in this case CO) sigma donates to an empty d-orbital and the filled d-orbitals of the metal donates to the empty pi orbitals of CO, back donation.

This only occurs when the metal has an oxidation state <3+ as higher oxidative states cause electron density to contract towards the metal.

eg:Ni(CO)4