Chemistry - CHEMICAL BONDING Question with Solution | TestHub

In BF₃, back-bonding occurs between the vacant $2p$ orbital of Boron and the filled $2p$ orbital of Fluorine ($2p\pi -2p\pi$), which is highly effective due to their similar sizes. When F is replaced by Cl to form BCl₃, the back-bonding involves the larger, more diffuse $3p$ orbital of Chlorine ($2p\pi -3p\pi$). This poorer orbital overlap significantly weakens the back-bonding interaction in BCl₃. Consequently, the back bond strength decreases, which also explains why BCl₃ is a stronger Lewis acid than BF₃.