AIIMS 2013 Chemistry Question Paper for free online practice with solutions

In BF3, there is back bonding in between fluorine and boron due to presence of p-orbital in boron.

F |

B

| F

−F, back bonding imparts double bond characteristics.
As BF3 forms adduct the back bonding is no longer present and thus double bond characteristic disappears. Hence, bond becomes a bit longer than earlier (1.30 Å).