Question

The $\pi -$ acid ligand that uses its d-orbital during synergic bonding in its complex compound.
(A) $C{N^ - }$
(B) $P{R_3}$
(C) $NO$
(D) ${N_2}$

Answer 1

Ligands are neutral molecule, cations or anions that are directly linked to the metal atom by co-ordinate or dative bond. Ligands donate lone pair of electrons to the metal atom. Ligands are lewis base while the metal atom is lewis acid.

Complete answer: In a coordination compound there is a central metal atom or ion. To this metal atom or ion, ligands are attached. These ligands can be neutral, cationic or ionic in nature. Ligands donate electrons to the metal. In some cases the filled orbitals of metal donate electrons to the empty antibonding orbital of the ligand. This is known as synergic bonding. Synergic bond means a self-strengthening bond. This kind of bonding is commonly seen in metal carbonyls (compounds having carbonyl group as a ligand).
The bond formed by the back donation of a metal to the ligand has a character of pi-bond. For synergic bonding or back bonding the ligand must have vacant anti-bonding orbital.
A $\pi -$ acid ligand is a ligand that donates electron to the d-orbital of the metal while the metal donates electron to the empty ${\pi ^ * }$ orbital of ligand. For example, carbon-monoxide $(CO)$ .
In the given ligands, $C{N^ - }$ is attached to the central metal atom by carbon. Carbon does not have empty d-orbital that can be used for back-bonding. In case of $NO$ and ${N_2}$ , nitrogen also does not have empty d-orbital that can be used for back-bonding. But in the case of $P{R_3}$ , phosphorus has empty d-orbital. It is an empty orbital that can be used for back-bonding in synergic complexes. So $P{R_3}$ is a $\pi -$ acid ligand that uses its d-orbital during synergic bonding in complex compounds.

The correct option is (B).

Note: Some examples of pi-acceptor ligands are $P{R_3}$ , $CO$ , ethylene, cyclopentadiene.Synergic bonding is observed in organo-metallic complexes. Such complexes find application in catalysis, synthesis etc.The bond formed by ligands with the metal is a dative or co-ordinate bond.