GIVE REASON
c] the B-F BOND LENGTH IN BF4 is longer than that in BF3
Answers
In BF3, the B-F "single bond" has some double bond character, not present in BF4-. If you write a conventional valence bond structure for BF3, with single B-F bonds, there are only six electrons on B and a vacant p-orbital at right angles to the plane of the molecule. So you can give B an octet by writing a formal negative charge on B, a formal positive charge on one of the F, and a double bond between B and that F. In molecular orbital terms, you can say that the appropriate p orbitals on F overlap this vacant orbital on B.
❤.BF3 is a planar molecule in which B is sp2 hybridised.
❤.It has an empty 2P -orbital .
❤.F-Atom has three lone pairs of electrons in the 2p-orbitals.
❤. Because of its small size and strong inter electronic repulsions, pTT -pTT back bonding or back donation occurs in which a lone pair is transfered from F to B .
❤. As a result of this back bonding , B-F bond acquries some double bond characters .
❤. In contrast,in [BF-4] ion, B is sp3 hybridised and hence does not have an empty p-orbital available to accept the electrons donated by the F -atom.
❤. Consequently, in B-F is a purely single bond.
❤. Since double bonds are shorter than single bonds, the B-F bond length in BF3:is shorter (130pm) than B-F bond length (143pm) in [BF-4]
