Which have more π-acceptor strength among the CO,NO+ and CN ligand ?

Which have more π-acceptor strength among the CO, NO ⁺ and CN ^–ligand ?

The three given species CO, NO ⁺ and CN ^– are isoelectronic and isostructural .
Now , which π-acceptor ligands have more strength , can be explained with the help of molecular orbital theory .

According to molecular orbital theory the electron distribution in molecular orbital in each case is ,

Now,bond order = ( bonding electron – anti bonding electron ) /2 = ( 10 – 4 ) /2 =3. So the bond order is 3.

Each of them can behave as ligands . Since in each case there is a suitable lone pair of electrons for donation to metal.

Again each possess vacant anti bonding π* orbital and each can receive back-donated electrons from metal atom or ion in its vacant such orbital.

So they can behave as a Lewis base , that is, as σ-donors as well as they can behave as Lewis acid , that is, as π-acceptors. Bonding of such complexes can be represented as shown below,

Donor and acceptor ability of these species depends on the virtual change on them. Donor ability increases with decrease in oxidation number. Hence the donating ability order is NO ⁺ < CO < CN ^- .

Again π-acceptor ability increases with increase in the oxidation number and hence such ability , that is π-acceptor ability order is CN ^-< CO < NO ⁺ .