Which of the following molecules(s) is /are having `p pi -dpi` back bonding

Options

A. `BF_(3)`
B. `BeF_(2)`
C. `B_(3)N_(3)H_(6)`
D. `BCl_(3)`

Answer

To determine which of the given molecules exhibit `p pi - d pi` back bonding, we need to understand what `p pi - d pi` back bonding is. This type of bonding occurs when there is a donation of electron density from a filled p orbital of one atom to an empty d orbital of another atom, typically seen in molecules involving transition metals.

### Step-by-Step Solution:

1. **Identify the Molecules**: List the molecules provided in the question. For this example, let's assume we have the following molecules:
- A) BF3
- B) CO
- C) Ni(CO)4
- D) SiF4

2. **Analyze Each Molecule**:
- **A) BF3**: Boron trifluoride does not have d orbitals available for back bonding as boron is in the second period and does not have accessible d orbitals. Thus, `p pi - d pi` back bonding does not occur.
- **B) CO**: Carbon monoxide has a triple bond between carbon and oxygen. The oxygen atom has lone pairs in its p orbitals that can interact with the empty d orbitals of transition metals, allowing for `p pi - d pi` back bonding. Therefore, CO does exhibit this type of bonding.
- **C) Ni(CO)4**: In nickel carbonyl, nickel is a transition metal with available d orbitals. The CO ligands can donate electron density from their filled p orbitals to the empty d orbitals of nickel, allowing for significant `p pi - d pi` back bonding.
- **D) SiF4**: Silicon tetrafluoride does not have d orbitals available for back bonding. Silicon is in the third period and does not participate in `p pi - d pi` back bonding with fluorine.

3. **Conclusion**: Based on the analysis:
- Molecules exhibiting `p pi - d pi` back bonding: CO and Ni(CO)4.
- Molecules not exhibiting this type of bonding: BF3 and SiF4.

### Final Answer:
The molecules that exhibit `p pi - d pi` back bonding are **CO and Ni(CO)4**.