Question

The $\pi$-complexes are known for transition metals only. Why?

Answer 1

A class of ligands, generally the organometallic ligands, that includes linear molecules like ethylene, allyl, and cyclic molecules such as cyclopentadiene, benzene, etc. are known as the$\pi$- ligands.

Complete answer:
The transition metals have a general configuration of$\left( \text{n-1} \right){{\text{d}}^{\text{1-10}}}\text{n}{{\text{s}}^{\text{1-2}}}$. The d-orbitals have a capacity to accommodate upto ten electrons. Hence they readily accept electrons from the pi-orbitals of the ligands containing pi-electrons such as ethylene, benzene, cyclopentadiene, etc. These complexes are known for transition metals only because the energies of the d-orbitals and the pi-orbitals are very close to each other and hence the pi-electrons are easily accepted by the d-orbitals to form the $\text{d }\pi\text{ -p }\pi\text{ }$bonds.

Note:
The linear$\pi$- systems such as alkenes, alkynes, and other unsaturated compounds containing pi-bonds donates the electron density from its pi-bonding orbitals to the d-sigma-orbitals in the transition metals. Here the Highest Occupied Molecular Orbital (HOMO) of the ligand interacts with the Lowest Unoccupied Occupied Molecular Orbital (LUMO) of the ligand. In this way, the ligand acts as the donor and the metal ion acts as the acceptor.
These types of complexes are synthesized through ligand substitution reactions in which an existing ligand is replaced by the pi-ligand.
The pi-acidic ligands are those one in which there is a back donation of electrons from the metallic orbitals to the pi-antibonding orbitals of the ligands. Common examples of such ligands include ethylene, carbon monoxide, and nitric oxide, etc.