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 solve the question of which molecular species contains both sigma-dative and pi-dative bonds, we will analyze each option given in the question: BF4^-, Be2Cl4, NH4^+, and BeF4^2-.

### Step-by-Step Solution:

1. **Understanding Dative Bonds**:
- A dative bond (or coordinate bond) is formed when one atom donates both electrons to form a bond with another atom that has an empty orbital.
- **Sigma-dative bond**: This occurs when the orbitals overlap head-on, allowing for the sharing of a pair of electrons.
- **Pi-dative bond**: This occurs when the orbitals overlap sidewise, allowing for the sharing of a pair of electrons.

2. **Analyzing BF4^-**:
- BF4^- has a tetrahedral structure with boron at the center and four fluorine atoms around it.
- The bond between boron and fluorine is a sigma bond, and the additional electron from the fluoride ion forms a sigma-dative bond.
- There are no pi-dative bonds present in BF4^-.
- **Conclusion**: BF4^- contains only sigma-dative bonds.

3. **Analyzing Be2Cl4**:
- Be2Cl4 consists of beryllium and chlorine atoms. Beryllium is electron-deficient and can accept electron pairs from the chlorine atoms.
- The bonds formed between beryllium and chlorine are sigma-dative due to head-on overlap.
- Additionally, due to the electron deficiency of beryllium, there can be back-bonding from chlorine, leading to pi-dative bonds.
- **Conclusion**: Be2Cl4 contains both sigma-dative and pi-dative bonds.

4. **Analyzing NH4^+**:
- NH4^+ (ammonium ion) is formed when ammonia (NH3) donates a pair of electrons to H^+.
- The bond formed is a sigma-dative bond.
- There are no pi-dative bonds present in NH4^+.
- **Conclusion**: NH4^+ contains only sigma-dative bonds.

5. **Analyzing BeF4^2-**:
- BeF4^2- has a tetrahedral structure similar to BF4^-.
- The bonds formed between beryllium and fluorine are sigma bonds, and the additional electron pair donated by the fluoride ion forms a sigma-dative bond.
- There are no pi-dative bonds present in BeF4^2-.
- **Conclusion**: BeF4^2- contains only sigma-dative bonds.

### Final Answer:
The molecular species that contains both sigma-dative and pi-dative bonds is **Be2Cl4**.