Question

Why ${\mathrm{AlCl}}_3$ does not show back bonding?

Answer 1

Back bonding

• The exchange of electrons between an atomic orbital on one atom and an anti-bonding orbital on another atom is known as back bonding.
• Back bonding takes place when one atom in a compound has a lone pair of electrons and the other has vacant orbitals close to each other.

${\mathrm{AlCl}}_3$ does not show back bonding

• The electronic configuration of aluminum is $1s^{2}2s^{2}2p^{6}3s^{2}3p^1$, and the electronic configuration of chlorine is $1s^{2}2s^{2}2p^{6}3s^{2}3p^5$
• 3p orbitals of $\text{Al}$ and $\mathrm{Cl}$ are comparatively larger. The bond length is longer and back bonding cannot provide the double bond character.
• So, back bonding doesn't occur in ${\mathrm{AlCl}}_3$
• Dimerization completes the octet of $\text{Al}$ in this case.