The species which acts as a Lewis acid but not as Bronsted acid is

Options

A. `NH^(-2)`
B. `O^(-2)`
C. `BF_3`
D. `OH^(-1)`

Answer

To determine the species that acts as a Lewis acid but not as a Brønsted acid, we need to understand the definitions of both types of acids.

### Step-by-Step Solution:

1. **Understand the Definitions**:
- **Brønsted Acid**: A species that donates a proton (H⁺).
- **Lewis Acid**: A species that accepts an electron pair.

2. **Analyze the Options**:
We are given four options to evaluate:
- **Option 1: NH₂⁻ (Amide ion)**
- **Option 2: O²⁻ (Oxide ion)**
- **Option 3: BF₃ (Boron trifluoride)**
- **Option 4: OH⁻ (Hydroxide ion)**

3. **Evaluate Each Option**:
- **Option 1: NH₂⁻**
- Acts as a Lewis base (donates electron pair) and can also act as a Brønsted base (accepts H⁺). Thus, it does not fit our criteria.
- **Option 2: O²⁻**
- Also acts as a Lewis base (donates electron pair) and can accept protons (H⁺), making it a Brønsted base. Thus, it does not fit our criteria.
- **Option 3: BF₃**
- Boron trifluoride has an incomplete octet and can accept an electron pair, making it a Lewis acid. It does not have a hydrogen to donate, so it cannot act as a Brønsted acid. This fits our criteria.
- **Option 4: OH⁻**
- Hydroxide ion acts as a Lewis base (donates electron pair) and can also act as a Brønsted base (accepts H⁺). Thus, it does not fit our criteria.

4. **Conclusion**:
The only species that acts as a Lewis acid but not as a Brønsted acid is **BF₃** (Option 3).

### Final Answer:
**Option 3: BF₃**