In which of the following molecular species both `sigma`-dative and `pi`- dative bonds are present?

Options

A. `BF_4^-`
B. `Be_2Cl_4`
C. `NH_4^+`
D. `[BeF_4]^(2-)`

Answer

To determine which molecular species contains both sigma-dative and pi-dative bonds, we need to analyze the bonding in each option provided. Let's break down the process step by step.

### Step-by-Step Solution:

1. **Understanding Sigma and Pi Bonds**:
- **Sigma Bonds**: Formed by the head-on overlap of atomic orbitals. They can be single bonds or part of double/triple bonds.
- **Pi Bonds**: Formed by the side-to-side overlap of p orbitals. They are present in double and triple bonds along with sigma bonds.

2. **Identifying Dative Bonds**:
- **Dative Bonds**: A type of covalent bond where one atom provides both electrons for the bond. This can be a sigma dative bond or a pi dative bond.
- **Sigma Dative Bond**: A dative bond that also acts as a sigma bond.
- **Pi Dative Bond**: A dative bond that contributes to a pi bond.

3. **Analyzing Each Molecular Species**:
- **Option A**: Check if it contains both types of bonds. If only sigma dative is present, it does not meet the criteria.
- **Option B**: Analyze the bonding in this species. If beryllium (Be) forms bonds with chlorine (Cl) and receives lone pairs from Cl, check for back bonding that could form pi dative bonds.
- **Option C**: Similar analysis as above. Look for the presence of both bond types.
- **Option D**: Evaluate the bonding in this species. If it has only sigma dative bonds, it does not meet the criteria.

4. **Conclusion**:
- After analyzing the options, it is found that **Option B** contains both sigma-dative and pi-dative bonds due to the presence of back bonding from chlorine to beryllium.

### Final Answer:
**Option B** is the correct answer, as it contains both sigma-dative and pi-dative bonds.

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