Question

Which of the following molecules has the largest extent of back bonding?

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

Answer 1

The correct option is A $B{F}_3$

There is a stronger $2p\pi -2p\pi$ back bonding in $B{F}_3$ (lone pair of $F$ donated into vacant orbital of $B$) and weakest in $B{I}_3$ $(2p\pi —5p\pi )$. This is due to the large size of $5p$ orbital which results in lesser overlapping. As the size of the orbital increases, the extent of back bonding decreases. Hence, $B{F}_3$ has the strongest partial double bond character.
Extent of back Bonding order is:
$B{F}_3>BC{l}_3>BB{r}_3>B{I}_3$