Maximum `p pi-p pi` back bonding exists in

Options

A. `BCl_(3)`
B. `BI_(3)`
C. `BBr_(3)`
D. `BF_(3)`

Explanation

To solve the question regarding which compound exhibits maximum pπ-pπ backbonding, we will analyze the bonding characteristics of the given compounds, particularly focusing on their atomic sizes and the ability to form effective backbonding.

### Step-by-Step Solution:

1. **Understanding pπ-pπ Backbonding:**
- pπ-pπ backbonding occurs when there is overlap between the p orbitals of two atoms, allowing for the donation of electron density from a filled p orbital (donor) to an empty p orbital (acceptor). This is particularly effective when the sizes of the atoms involved are similar.

2. **Analyzing the Compounds:**
- The question implies that we need to compare various compounds, likely including BF3, BCl3, BBr3, and BI3. We will focus on BF3 as it is commonly known for its backbonding characteristics.

3. **Examining BF3:**
- Boron (B) has an atomic number of 5 with the electronic configuration of 1s² 2s² 2p¹. It has one electron in its p orbital, which can accept electron density.
- Fluorine (F) has an atomic number of 9 with the configuration of 1s² 2s² 2p⁵, meaning it has five electrons in its p orbitals, including three lone pairs.
- The similar sizes of boron and fluorine facilitate effective pπ-pπ backbonding, as the overlap between their p orbitals is significant.

4. **Comparing with Other Compounds:**
- **BCl3:** Chlorine is larger than fluorine, leading to less effective backbonding due to the increased distance and decreased overlap between the p orbitals.
- **BBr3:** Bromine is even larger than chlorine, further reducing the effectiveness of backbonding.
- **BI3:** Iodine is the largest of the halogens, and the size difference with boron results in very weak backbonding.

5. **Conclusion:**
- Among the compounds analyzed, BF3 exhibits the maximum pπ-pπ backbonding due to the similar sizes of boron and fluorine, which allows for effective overlap of their p orbitals.

### Final Answer:
The compound that exhibits maximum pπ-pπ backbonding is **BF3**.