Question

Among the hydrides diborane is an important compound. Which of the following halides is the strongest Lewis acid?

• A. $B{F}_3$
• B. $BC{l}_3$
• C. $BB{r}_3$
• D. $B{I}_3$

Answer 1

Answer: (D)

$B{I}_3$

$B{I}_3>BB{r}_3>BC{l}_3>B{F}_3$
This order can be explained on the basis of the tendency of halogen atom to back donate its lone pair of electrons to the empty p-orbital of $B$ atom through $p\pi -p\pi$ bonding. As this tendency is higher in case of $F$ (due to identical size of 2p-orbitals of $B$ and $F$).
The electron deficiency of $B$ decreases and thus, $B{F}_3$ behaves as the weakest lewis acid. As the size of halogen atom increases from $Cl$ to $I$, the tendency for back donation decreases and consequently the electron deficiency of $B$ increases and the lewis acid character increases accordingly from $B{F}_3$ to $B{I}_3$.