Any species capable of accepting a lone pair of electrons acts as a lewis acid, where as a species capable of donating a proton acts as a Bronstead acid.
Which of the following can act as a lewis acid as well as a lewis base ?

Options

A. a) `NH_(4)^(+)`
B. b) `CO`
C. c) `CO_(2)`
D. d) `SO_(2)`

Answer

To determine which species can act as both a Lewis acid and a Lewis base, we need to analyze the given options based on their ability to accept and donate electron pairs.

### Step-by-Step Solution:

1. **Understanding Lewis Acids and Bases**:
- A Lewis acid is defined as a species that can accept a pair of electrons.
- A Lewis base is defined as a species that can donate a pair of electrons.

2. **Analyzing the Given Options**:
- The options provided are:
- A. NH4+
- B. CO
- C. CO2
- D. SO2

3. **Evaluating Each Option**:
- **A. NH4+**:
- NH4+ (ammonium ion) cannot act as a Lewis base because it has no lone pairs to donate. It can only act as a Lewis acid by accepting electrons.
- **B. CO**:
- CO (carbon monoxide) can act as a Lewis base due to the lone pair on carbon, but it is less likely to act as a Lewis acid.
- **C. CO2**:
- CO2 (carbon dioxide) has a linear structure and can accept electrons at the carbon atom (making it a Lewis acid) and can also donate lone pairs from the oxygen atoms (making it a Lewis base).
- **D. SO2**:
- SO2 (sulfur dioxide) has a bent structure with lone pairs on the oxygen atoms. It can accept electrons at the sulfur atom (acting as a Lewis acid) and also donate lone pairs from the oxygen atoms (acting as a Lewis base).

4. **Conclusion**:
- Both CO2 and SO2 can act as Lewis acids and Lewis bases. However, based on the context of the question and the structures discussed, the most straightforward answer is **D. SO2** as it clearly exhibits both properties.

### Final Answer:
The species that can act as both a Lewis acid and a Lewis base is **D. SO2**.