Question

Question 11.14 Suggest reasons why the B–F bond lengths in BF3 (130 pm) and (143 pm) differ.

Class XI The p-Block Elements Page 324

Answer 1

In BF3, Boron is sp² - Hybridized. it has a vacant 2p- orbital. Each fluorine in BF3 has completely filled unutilized 2p- orbitals. since , both of these orbitals belong to same energy level so Pπ - Pπ back bonding occurs in which a lone pair of electrons is transferred from unutilized completely filled 2p- orbital of F to vacant 2p- orbital of B. This type of bond formation is known as back binding. therefore B-F bond has some double bond character. that's why all the three B-F bonds are shorter than the usual single B-F bond.

In [BF4]– ion, Boron is sp³ - Hybridized .it doesn't empty 2p - orbital, so there is no back bonding. in [BF4]– ion all the four B-F bonds are purely single bonds . Double bonds are shorter than single bond .
therefore B-F bond length ( 130 pm) in BF3 is shorter than B-F bond length(143 pm) in [BF4]– ion.

Answer 2

Answer:

In BF3 ‘B’ is sp2 hybridised and in BF4– ‘B’ is sp3 hybridised. Thus, the difference in bond length is due to the state of hybridisation

Hope this helps you !!!!