Suggest reasons why the B–F bond lengths in BF_(3) (130 pm) and BF_(4 )^(– )(143 pm) differ.

Suggest reasons why the B–F bond lengths in BF_(3) (130 pm) and BF_(

1.	Suggest reasons why the B–F bond lengths in BF_(3) (130 pm) and BF_(4 )^(– )(143 pm) differ.
Answer» Solution :DUE to `P pi-P pi` back bonding in `BF_3` molecule (for details, (3), Figs.B-F BOND acquires some double bond character. This shortens the B-F bond LENGTH. In `BF_4^-`, the B-F bond is a pure single bond and has a LARGER bond length. This is why B-F bond length in `BF_3` is shorter as compared to that in `BF_4^-` .