Question

In $BF_3$, the $B— F$ bond length is $1.30\, \mathring{A}$, when $BF_3$ is allowed to be treated with $Me_3N$, it forms an adduct, $Me_3N \to BF_3$, 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 $BF^3$, there is back bonding in between fluorine and boron due to presence of p-orbital in boron.

back bonding imparts double bond characteristics.

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