Question

whats a pi acid ligand?

Answer 1

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

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Pi-acidic ligands are ligands that have a relatively low-lying LUMO that has the appropriate symmetry to interact with a d-orbtal on the metal centre and the resultant molecular orbital formed will have pi-symmetry.

For instance with CO (a classic pi-acidic ligand).

One of the d-orbitals on the metal can interact with a pi*-LUMO (see figure) to give a new molecular orbital with bonding character between the metal and the carbon (note the antibonding character between the C and O); population of this orbital by electrons from the metal will lead to an increase in the bond strength between M and C, but a weakening of the C-O bond strength.

Answer 2

Answer:-

$\pi$ acid ligands are one which are capable of accepting an appreciable amount of electron density from the metal atom into empty $\pi$ or $pi^*$ orbital of their own are called as $\pi$ acid or $\pi$ acceptor ligands.

This type of interaction increases the value of crystal field splitting energy and thus are classified as strong filed ligands.

Example of pi acid ligands are $CO$, $NO^+$ and $CN^-$