Which of the following statement is correct ?

Options

A. Boron and aluminium halides behave as Lewis acids
B. Al forms `[Al F_(6)]^(3-)` ion but B does not form `[BF_(6)]^(3-)` ion
C. The `p pi - p pi` back bonding occurs in the halides of boron and not in those of aluminium
D. All of these

Answer

To determine which of the statements regarding boron and aluminum is correct, we will analyze each statement step by step.

### Step 1: Analyze the Lewis Acid Behavior of Boron and Aluminum
- **Boron (BH3)**: Boron has only 6 electrons in its valence shell (2 from 2s and 4 from 2p). It is electron-deficient and can accept a pair of electrons to complete its octet, thus behaving as a Lewis acid.
- **Aluminum (AlH3)**: Similarly, aluminum also has only 6 valence electrons (3 from 3s and 3 from 3p). It can also accept a pair of electrons to complete its octet, behaving as a Lewis acid as well.

**Conclusion**: Both boron and aluminum behave as Lewis acids.

### Step 2: Analyze the Formation of AlF6^3- and BF6^3-
- **Aluminum**: Aluminum can expand its octet due to the presence of vacant d-orbitals in the third period. Therefore, it can form AlF6^3-.
- **Boron**: Boron does not have vacant d-orbitals (it only has 1s, 2s, and 2p orbitals). Thus, it cannot expand its octet and cannot form BF6^3-.

**Conclusion**: The statement that aluminum forms AlF6^3- but boron does not form BF6^3- is correct.

### Step 3: Analyze p-pi p-pi Back Bonding
- **Boron Halides**: Boron has a smaller atomic size and can effectively engage in p-pi p-pi back bonding due to its ability to overlap with p orbitals of halogens.
- **Aluminum Halides**: Aluminum, being larger, has less effective p-pi p-pi back bonding due to the larger size of the aluminum atom compared to boron.

**Conclusion**: The statement that p-pi p-pi back bonding occurs in the halides of boron but not in those of aluminum is also correct.

### Final Conclusion
After analyzing all the statements:
1. Both boron and aluminum behave as Lewis acids.
2. Aluminum can form AlF6^3- while boron cannot form BF6^3-.
3. p-pi p-pi back bonding occurs in boron halides but not in aluminum halides.

Thus, the correct answer is that all of the statements provided are correct.