Correct among the follownig

Options

A. `BF_(3)` is a better Lewis base than `BCl_(3)`
B. `BF_(3)` does not form dimer as B-F bond is less polar.
C. Hydrides are the only species which may form 3 centre 2 `e^(-)` bond
D. All the hydrides form 3 centre i.e., bonds.

Answer

To solve the question, we need to evaluate the correctness of the given statements regarding BF3, BCl3, and hydrides. Let's analyze each statement step by step.

### Step 1: Evaluate Statement A
**Statement A:** BF3 is a better Lewis base than BCl3.

- **Analysis:**
- BF3 has an incomplete octet and can accept electrons, but it also engages in back bonding with fluorine, which reduces its ability to donate electrons.
- BCl3, while also a Lewis acid, does not have as strong back bonding as BF3 due to the lower electronegativity of chlorine compared to fluorine.

- **Conclusion:** This statement is incorrect because BF3 is not a better Lewis base than BCl3 due to back bonding.

### Step 2: Evaluate Statement B
**Statement B:** BF3 does not form a dimer as BF bond is less polar.

- **Analysis:**
- BF3 does not form a dimer primarily because boron is small and cannot accommodate another fluorine atom.
- The back bonding in BF3 stabilizes it and reduces the polarity of the BF bond, making dimerization unfavorable.

- **Conclusion:** This statement is correct.

### Step 3: Evaluate Statement C
**Statement C:** Hydrides are the only species which may form 3 center 2 electron bonds.

- **Analysis:**
- While some hydrides can form 3 center 2 electron bonds, this is not exclusive to hydrides. For example, AlCl3 can also form a dimer (Al2Cl6) which involves 3 center 2 electron bonding.

- **Conclusion:** This statement is incorrect.

### Step 4: Evaluate Statement D
**Statement D:** All hydrides form 3 center 2 electron bonds.

- **Analysis:**
- Not all hydrides form 3 center 2 electron bonds. For instance, sodium hydride (NaH) does not form such bonds.
- This statement generalizes the behavior of hydrides incorrectly.

- **Conclusion:** This statement is incorrect.

### Final Conclusion
Based on the evaluations:
- Statement A: Incorrect
- Statement B: Correct
- Statement C: Incorrect
- Statement D: Incorrect

Thus, the only correct statement is **Statement B**.