In which of the following molecular species `sigma-` dative bond is present?

Options

A. `BF_(3)`
B. `Be_(2)Cl_(4)`
C. `NH_(3)`
D. `BH_(3)`

Answer

To determine which molecular species contains a sigma dative bond, we need to understand what a sigma dative bond is. A sigma dative bond, also known as a coordinate bond, occurs when one atom donates a pair of electrons to another atom, forming a bond.

Let's analyze the given molecular species one by one:

1. **BF3 (Boron Trifluoride)**:
- BF3 has a central boron atom bonded to three fluorine atoms.
- Each bond is a sigma bond formed by the overlap of orbitals.
- There are no lone pairs on the boron atom to donate; hence, there is no sigma dative bond present.

2. **Be2Cl4 (Beryllium Dichloride Tetrafluoride)**:
- In Be2Cl4, beryllium is bonded to chlorine atoms.
- The structure involves bridge bonding, where chlorine atoms can donate lone pairs to beryllium.
- This results in the formation of sigma dative bonds because chlorine donates its lone pair to beryllium, creating a coordinate bond.
- Therefore, Be2Cl4 has sigma dative bonds.

3. **NH3 (Ammonia)**:
- NH3 has a nitrogen atom bonded to three hydrogen atoms.
- The nitrogen atom has one lone pair, but it does not donate this pair to form a bond with another atom in this structure.
- All bonds in NH3 are simple sigma bonds, with no dative bond present.

4. **BH3 (Boron Hydride)**:
- BH3 has a boron atom bonded to three hydrogen atoms.
- Similar to BF3, all bonds are sigma bonds, and there are no lone pairs available for donation.
- Thus, there are no sigma dative bonds in BH3.

**Conclusion**:
The only molecular species that contains a sigma dative bond is **Be2Cl4**.

### Summary of Steps:
1. Identify the molecular species.
2. Analyze the bonding structure of each species.
3. Determine the presence of lone pairs and their ability to donate electrons.
4. Conclude which species has sigma dative bonds.