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QUESTION

# Which is considered a stronger lewis acid, BF3 or BCl3?

"BCl"_3 is the stronger Lewis acid.

We would expect "BF"_3 to be stronger, because "F" is more electronegative than "Cl".

Chemists explain this unexpected result by an electronic argument and a steric argument.

The electronic argument — backbonding

The boron atom in "BF"_3 is sp^2 hybridized, with a vacant 2p orbital.

The "F" atoms can also be sp^2 hybridised, with lone pairs in their 2p orbitals.

These "F" orbitals can overlap with the orbital on "B", thereby increasing the electron density on the boron atom and making it less acidic.

This effect is called backbonding, because electron density is leaving the more electronegative atom.

In "BCl"_3, the 3p orbitals on "Cl" are bigger than the 2p orbital on "B", so orbital overlap is less efficient, and backbonding is less important.

Hence, the greater backbonding in "BF"_3 makes it a weaker Lewis acid.

The steric argument — ligand close-packing (LCP)

The LCP model is based on the observation that the "X" atoms (ligands) in "AX"_n systems are always the same distance from each other.

For example, the distance between the "F" atoms in "BF"_3 and "BF"_4^- is 226 pm, despite the longer "B-F" distance in the tetrahedral structure.

(from alpha.chem.umb.edu)

It is as if the "F" atoms are closest-packed (like in a crystal), with the "F" atoms having a ligand radius of 113 pm.

When the "BF"_3 forms a Lewis complex, the "F" atoms remain close-packed, but the "B-F" bonds must become longer in the new tetrahedral geometry.

It takes more energy to lengthen the short, strong "B-F" bonds than the longer, weaker "B-Cl" bonds.

Hence "BF"_3 is a weaker Lewis acid than "BCl"_3.