What is the correct descending order of Lewis acid strength among the boron trihalides? — Chemical Bonding Chemistry Question

💡 Solution & Explanation

Boron's Lewis acidity is mitigated by $p\pi-p\pi$ back bonding from the halogen to the vacant $2p$ orbital of Boron. Fluorine ($2p$) matches Boron's $2p$ size perfectly, creating strong back bonding that quenches the acidity. Larger halogens (Cl $3p$, Br $4p$, I $5p$) overlap poorly, leading to weak back bonding and higher Lewis acidity.