Question

What is back bonding? Explain in detail with suitable examples. Also, mention the necessary conditions for back bonding with their significance.

Answer 1

Hint: In some compounds, the stability is not acquired by normal bonding types, so to gain stability back bonding can occur. This occurs when one atom in the bonding has a lone pair of electrons and the other has vacant orbitals in which the lone pairs can be accommodated.

Complete answer:
We know that elements combine with each other to form compounds so, that the octet can be completed and stability can be acquired. These are formed by different types of bonding. In some compounds the stability is not acquired by normal bonding types, so to gain the stability back bonding can occur. This occurs when one atom in the bonding has a lone pair of electrons and the other has vacant orbitals in which the lone pairs can be accommodated and this is known as pi-back bonding or pi-back donation.
This mostly occurs in transition metal complexes. In this electrons from the atomic orbital of the metal or one atom move towards the pi-antibonding orbital of ligand or another atom which leads to the formation of a sigma bond. So, this is formed in metal organic compounds, in which the metal is from transition metals and ligands are carbon monoxide, ethylene, etc.
This occurs because sometimes due to excess of gaining electrons, the metal atom acquires a negative charge, so to decrease the negative charge, back bonding occurs. Some examples are $Ni{(CO)}_4$, Zeise’s salt.

Note:
The bond order and bond length of the compounds are affected by the back bonding, as back bonding decreases the bond length of the compound and it increases the bond order of the compound.