Question

In $B{F}_3$, the $B—F$ bond length is $1.30\mathring{A}$, when $B{F}_3$ is allowed to be treated with $M{e}_{3}N$, it forms an adduct, $M{e}_{3}N\to B{F}_3$, the bond length of $B—F$ in the adduct is

• A. greater than $1.30\mathring{A}$
• B. smaller than $1.30\mathring{A}$
• C. equal to $1.30\mathring{A}$
• D. none of these

Answer 1

Answer: (A)

In $B{F}^3$, there is back bonding in between fluorine and boron due to presence of p-orbital in boron.

back bonding imparts double bond characteristics.

As $B{F}^3$ forms adduct the back bonding is no longer present and thus double bond characteristic disappears. bond becomes a bit longer than earlier (1.30 $\mathring{A}$).