Which of the following involves `ppi-ppi` back bonding ?

Options

A. `:C Cl_2`
B. `BCl_3`
C. `B_2H_6`
D. `Na_2B_4O_7. 10H_2O`

Explanation

To determine which of the given compounds involves p-π-p-π back bonding, we will analyze each compound step by step.

### Step 1: Analyze CCl2 (Dichlorocarbene)
- **Structure**: CCl2 has a carbon atom bonded to two chlorine atoms and has a lone pair of electrons on carbon.
- **Hybridization**: The hybridization of carbon in CCl2 is sp². This means there is one vacant p orbital on carbon.
- **Back Bonding**: Chlorine has three lone pairs of electrons. The lone pairs from chlorine can overlap with the vacant p orbital of carbon, leading to p-π-p-π back bonding.
- **Conclusion**: CCl2 involves p-π-p-π back bonding.

### Step 2: Analyze BCl3 (Boron Trichloride)
- **Structure**: BCl3 consists of a boron atom bonded to three chlorine atoms.
- **Hybridization**: The hybridization of boron in BCl3 is sp², which means there is one vacant p orbital on boron.
- **Back Bonding**: Chlorine has lone pairs of electrons that can donate to the vacant p orbital of boron, resulting in p-π-p-π back bonding.
- **Conclusion**: BCl3 also involves p-π-p-π back bonding.

### Step 3: Analyze B2H6 (Diborane)
- **Structure**: B2H6 has a structure where boron atoms are bonded to hydrogen atoms.
- **Hybridization**: The hybridization of boron in B2H6 is sp³ due to four hydrogen atoms bonded to boron.
- **Back Bonding**: Since boron is sp³ hybridized, there are no vacant p orbitals available for back bonding.
- **Conclusion**: B2H6 does not involve p-π-p-π back bonding.

### Step 4: Analyze Na2B4O7 (Borax)
- **Structure**: Na2B4O7 consists of boron atoms bridged by oxygen atoms.
- **Hybridization**: The hybridization of boron in Na2B4O7 is sp², which means there are vacant p orbitals on boron.
- **Back Bonding**: The oxygen atoms have lone pairs that can donate to the vacant p orbitals of boron, leading to p-π-p-π back bonding.
- **Conclusion**: Na2B4O7 involves p-π-p-π back bonding.

### Final Conclusion
The compounds that involve p-π-p-π back bonding are:
- CCl2
- BCl3
- Na2B4O7

Thus, the correct options are A (CCl2), B (BCl3), and D (Na2B4O7). Option C (B2H6) is incorrect.