Question

What is pi – acid ligand?

Answer 1

A ligand is a substance that has the ability to get attached to a metal to form complexes. A ligand is a Lewis base that can donate electrons. It may be negative and neutral. A carbonyl ligand (CO) when attached with the central metal acts as a pi – acid ligand. This ligand contains a lone pair of electrons. Back bonding is their feature.

Complete answer:
When a carbon atom is attached directly with the central metal atom, then metal carbonyls are formed. In these metals – carbonyls, the metal and the carbon bond M – C bond contains both sigma$\sigma$and pi$\pi$characters. As we know a sigma bond is the basis for a pi – bond.
The metal and the carbon sigma bond is formed by the donation of the lone pair of electrons of the carbonyl (CO) ligand into the vacant d – orbital of the metal. Now, the back bonding occurs and a pi – bond is formed as, this lone pair of electron from filled d – orbital of the metal – carbon bond goes into the vacant anti – bonding ${{\pi }^{*}}$ orbital of the carbonyl ligand. This interaction creates a synergic effect that creates a strong bond. Also, the intensity of the crystal field splitting also increases due to the strengthened interactions.
So, as the electron is donated into the ${{\pi }^{*}}$ and $\pi$orbitals, and then accepted by the same (CO) ligand, therefore it is called as pi – acid ligands.
Hence, carbon monoxide (CO) is a pi – acid ligand as it donates and then accepts the lone pair of electrons from the d – orbital of the metal.

Note:
The elements of the transition metal series are taken to form these types of complexes as they contain a vacant d – orbital. Some examples of pi – acid ligand complexes are, tetra carbonyl nickel $Ni{{(CO)}_{4}}$ , penta carbonyl iron $Fe{{(CO)}_{5}}$ and hexa carbonyl chromium $Cr{{(CO)}_{6}}$. Pi – acid ligands are also called pi – acceptor ligands.