In which of the following compounds , back bonding is possible

Options

A. `PF_(3)`
B. `PH_(3)`
C. `NH_(3)`
D. `NF_(3)`

Answer

To determine which of the given compounds allows for back bonding, we need to analyze the electronic structure of each compound and the ability of the surrounding atoms to donate electron density to the central atom.

### Step-by-Step Solution:

1. **Understanding Back Bonding:**
Back bonding occurs when a central atom has vacant orbitals that can accept electron density from surrounding atoms that have lone pairs of electrons. For back bonding to be possible, the following conditions must be met:
- The central atom must have vacant orbitals.
- The surrounding atom must have lone pairs of electrons that can be donated.

2. **Analyzing PF3:**
- **Central Atom:** Phosphorus (P)
- **Lone Pairs:** Phosphorus has one lone pair and three bond pairs with fluorine.
- **Vacant Orbitals:** Phosphorus, being in the third period, has vacant 3d orbitals.
- **Surrounding Atom:** Fluorine has lone pairs that can donate electrons.
- **Conclusion:** Back bonding is possible in PF3 due to the p-pi to d-pi interaction.

3. **Analyzing PH3:**
- **Central Atom:** Phosphorus (P)
- **Lone Pairs:** Phosphorus has one lone pair and three bond pairs with hydrogen.
- **Vacant Orbitals:** Phosphorus has vacant 3d orbitals.
- **Surrounding Atom:** Hydrogen does not have lone pairs to donate.
- **Conclusion:** Back bonding is not possible in PH3 because hydrogen cannot donate electrons.

4. **Analyzing NH3:**
- **Central Atom:** Nitrogen (N)
- **Lone Pairs:** Nitrogen has one lone pair and three bond pairs with hydrogen.
- **Vacant Orbitals:** Nitrogen belongs to the second period and does not have vacant d orbitals.
- **Surrounding Atom:** Hydrogen does not have lone pairs to donate.
- **Conclusion:** Back bonding is not possible in NH3 due to the lack of vacant orbitals and the inability of hydrogen to donate electrons.

5. **Analyzing NF3:**
- **Central Atom:** Nitrogen (N)
- **Lone Pairs:** Nitrogen has one lone pair and three bond pairs with fluorine.
- **Vacant Orbitals:** Nitrogen does not have vacant d orbitals as it is in the second period.
- **Surrounding Atom:** Fluorine has lone pairs, but nitrogen cannot accept them due to lack of vacant orbitals.
- **Conclusion:** Back bonding is not possible in NF3.

### Final Conclusion:
Back bonding is only possible in **PF3**. Therefore, the answer is that back bonding is possible in **PF3** only.