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 of the given molecular species contains a sigma dative bond, we need to analyze each option step by step.

### Step 1: Understand the Concept of Sigma Dative Bond
A sigma dative bond (also known as a coordinate bond) is formed when one atom donates both of the electrons in the bond to another atom. This typically occurs when a lone pair from one atom is shared with another atom that has an empty orbital.

### Step 2: Analyze Each Option

1. **Option 1: BF3**
- BF3 (Boron Trifluoride) has a central boron atom bonded to three fluorine atoms. Boron has only three valence electrons and forms three sigma bonds with fluorine atoms. There are no lone pairs on boron to donate, hence no sigma dative bond is present.

2. **Option 2: Be2Cl4**
- In Be2Cl4 (Beryllium Dichloride), beryllium can form dative bonds with chlorine atoms. Chlorine has lone pairs that can be donated to beryllium, which can accept these pairs due to its empty orbitals. Therefore, a sigma dative bond is formed here.

3. **Option 3: NH3**
- NH3 (Ammonia) consists of one nitrogen atom bonded to three hydrogen atoms. Nitrogen has one lone pair, but it forms three sigma bonds with hydrogen. There are no dative bonds since nitrogen does not donate its lone pair to another atom.

### Conclusion
After analyzing all options, we find that the only species that contains a sigma dative bond is **Be2Cl4**.

### Final Answer
The molecular species that contains a sigma dative bond is **Be2Cl4**.

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