All the boron trihalides except `Bl_3`, may be prepared by direct reaction between the elements. Boron trihalides consistoftrigonal-planar `BX_3` molecules. Unlike the halides of the other elements in the group they are monomeric in the gas, liquid and solid states, `BF_3 and BCI_3` are gases, `BBr_3` is a volatile liquid and `BI_3` is a solid. Boron trihalides are Lewis acids because they form Lewis complexes with suitable bases.
`BF_(3(g)) + : NH_(3(g)) to F_3B - NH_(3(g))`
However, boron chlorides, bromides and iodides are susceptible (sensitive) to protolysis by mild proton sources such as water, alcohols and even amines for example `BCl_3` undergoes rapid hydrolysis.
`BCl_(3(g)) + 3H_2O((l)) to3(OH)_(3(aq.)) + 3HCl_((aq.))`
It is supposed that the first step in the above reaction is the formation of the complex `Cl_3B larr OH_2` which then eliminates HCl and reacts further with water.
Which of the following is the correct prediction about observed B-X bond length, in `BX_3` molecule?

Options

A. B-F bond length in `BF_3` is found to be less than theoretical value because the electronegativety values of B (2.04) and F(4.0) suggest the bond to be ionic and hence the attraction between oppositely charged ions must decrease the bond length.
B. `BF_3 and [BF_4]^(-)` have equal B - F bond length.
C. The decrease in the B - F bond length in`BF_3`, is due to delocalised `p pi - p pi` bonding between valcant 2p - orbital of B and filled 2p - orbital of F.
D. The correct B - X bond length order is `B - F gt B - Cl gt B - Br gt B - I`

Answer

C

Explanation

The decrease in the B - F bond length in`BF_3`, is due to delocalised `p pi - p pi` bonding between valcant 2p - orbital of B and filled 2p - orbital of F.