Question

The main factor responsible for the weak acidic nature of $B-F$ bonds in $B{F}_3$ is:

• A. Large electronegativity of F
• B. $p\pi -p\pi$ back bonding
• C. $p\pi -d\pi$ back bonding
• D. Three centred two electron bonds in $B{F}_3$

Answer 1

The correct option is B $p\pi -p\pi$ back bonding

The strong $2p\pi -2p\pi$ back bonding between $B$ and $F$ atoms is responsible for the less acidic nature of boron fluoride. As we move down the group, the size of $p$ orbitals increases and thus the strength of $p\pi -p\pi$ back bonding decreases.
In $BC{l}_3$ there will be $2p\pi -3p\pi$ back bonding.
In $BB{r}_3$ there will be $2p\pi -4p\pi$ back bonding.
In $B{I}_3$ there will be $2p\pi -5p\pi$ back bonding.

In the last three cases, the strength of $p\pi -p\pi$ back bonding is less.

The acidic strength increases in the following order,
$B{F}_3<BC{l}_3<BB{r}_3<B{I}_3$