Which of the following molecule is planar due to back bonding

Options

A. `NCl_(3)`
B. `PF_(3)`
C. `BF_(3)`
D. None

Answer

To determine which molecule is planar due to back bonding, we need to analyze the options provided in the question. Here’s a step-by-step solution:

### Step 1: Understand Back Bonding
Back bonding occurs when a ligand donates electron density to a metal or non-metal center that has empty orbitals. For effective back bonding, the following conditions must be met:
- The central atom (metal or non-metal) must have empty orbitals.
- The ligand must have lone pairs or filled p orbitals.
- There should be good orbital overlap between the metal and the ligand.

### Step 2: Analyze Each Option
We will analyze the options provided in the question to identify if they meet the criteria for back bonding.

#### Option A: Nitrogen with 3 Chlorine atoms (NCl3)
- Nitrogen has a lone pair and is connected to three chlorine atoms.
- Chlorine has filled 3p orbitals, and nitrogen has filled 2p orbitals.
- There are no empty d-orbitals in nitrogen.
- The overlap between 2p (N) and 3p (Cl) is not effective.
- **Conclusion**: Not planar due to back bonding.

#### Option B: Phosphorus with 3 Fluorine atoms (PF3)
- Phosphorus has a lone pair and is connected to three fluorine atoms.
- Fluorine has filled 2p orbitals, and phosphorus has filled 3p orbitals.
- There are no empty d-orbitals in phosphorus.
- The overlap between 3p (P) and 2p (F) is not effective.
- **Conclusion**: Not planar due to back bonding.

#### Option C: Boron with 3 Fluorine atoms (BF3)
- Boron has an empty p orbital (2p) and is connected to three fluorine atoms.
- Fluorine has filled 2p orbitals.
- The overlap between the empty 2p orbital of boron and the filled 2p orbitals of fluorine is excellent.
- Back bonding occurs, leading to partial double bond character.
- The geometry of BF3 is trigonal planar.
- **Conclusion**: Planar due to back bonding.

### Step 3: Final Answer
Based on the analysis, the molecule that is planar due to back bonding is **Option C: BF3**.

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