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