Question

In the following molecules:
$B{F}_3,BC{l}_3,BB{r}_3,B{I}_{3}I,II,III,IV$
The correct order of Lewis acidic strength will be:

• A. I > II > III > IV
  
• B. IV > III > II > I
  
• C. IV > II > I > III
  
• D. III > I > II > IV

Answer 1

The correct option is B

IV > III > II > I

These are all Lewis acids, and the halogen atoms - with their lone pairs - can partake in back-bonding into the pure, unhybridized 2p-orbital or the central Boron atom in each compound. As the size of halogen atom increases, effective overlapping of orbitals (via back-bonding) decreases. This means that the extent of back-bonding is the least in $B{I}_3$ and that will be the most reluctant to accept an e–pair among the given boron trihalides.