Which of the following is the correct prediction about observed B - F bond length in `BF_3` molecule ?

Options

A. B - F bond length in `BF_3` is found to be less than theoretical value because the electronegativity values of B (2.04) and F (4.0) suggest the bond length 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 `ppi - ppi` bonding between vacant 2p orbital of B and filled 2p orbital of F
D. The correct B - X bond length order is `B - F gtBClgtB-BrgtB-I`

Answer

To determine the correct prediction about the observed B-F bond length in the BF3 molecule, we can analyze the factors affecting bond lengths and the specific characteristics of the BF3 molecule.

### Step-by-Step Solution:

1. **Understanding the Structure of BF3:**
- BF3 (Boron Trifluoride) has a trigonal planar structure with boron at the center and three fluorine atoms surrounding it.
- Boron has an electronic configuration of 1s² 2s² 2p¹, while fluorine has an electronic configuration of 1s² 2s² 2p⁵.

2. **Bond Formation in BF3:**
- The B-F bond in BF3 is primarily a sigma bond formed by the overlap of the boron’s 2p orbital and the fluorine’s 2p orbital.
- Due to the high electronegativity of fluorine, there is a significant electron density around the fluorine atoms.

3. **Back Donation and Delocalization:**
- Fluorine atoms can back donate electrons to the empty 2p orbital of boron, leading to the formation of a π bond.
- This back donation results in a delocalization of electrons, giving the B-F bond a partial double bond character.

4. **Effect on Bond Length:**
- The presence of partial double bond character due to delocalization shortens the B-F bond length compared to what would be expected if the bond were purely ionic or single covalent.
- Therefore, the observed B-F bond length in BF3 is less than the theoretical value predicted based solely on ionic character.

5. **Comparison with BF4-:**
- In BF4- (Tetrafluoroborate ion), all bonds are single sigma bonds without the partial double bond character found in BF3.
- Consequently, the B-F bond length in BF4- is longer than that in BF3.

6. **Conclusion:**
- The correct prediction about the observed B-F bond length in BF3 is that it is shorter than expected due to the delocalized p-π bonding between the vacant 2p orbital of boron and the filled 2p orbitals of fluorine.

### Summary of Statements:
- The statement that the decrease in B-F bond length in BF3 is due to delocalized p-π bonding is correct.
- The bond length in BF3 is less than the theoretical value due to this delocalization and partial double bond character.