Which of the following cannot behave like a Lewis acid ?

Options

A. `CO_3^(2-)`
B. `Zn^(2+)`
C. `SO_3`
D. `SiCl_4`

Answer

To determine which of the given species cannot behave like a Lewis acid, we need to understand the definition of a Lewis acid. A Lewis acid is defined as a species that can accept an electron pair. This typically requires the species to have a vacant orbital available to accommodate the incoming electron pair.

### Step-by-Step Solution:

1. **Identify the Species**: The question provides several species to consider. We need to analyze each one to see if it can accept an electron pair.

2. **Analyze CO3^2- (Carbonate Ion)**:
- The carbonate ion has a central carbon atom bonded to three oxygen atoms.
- The carbon atom in CO3^2- is fully bonded with no vacant orbitals available for accepting an electron pair.
- The resonance structures of CO3^2- show that all bonds have a degree of double bond character, which stabilizes the structure and prevents it from acting as a Lewis acid.
- **Conclusion**: CO3^2- cannot behave like a Lewis acid.

3. **Analyze Zn^2+ (Zinc Ion)**:
- The Zn^2+ ion has a complete d-orbital (3d^10) and can accept electron pairs, making it a Lewis acid.
- **Conclusion**: Zn^2+ can behave like a Lewis acid.

4. **Analyze SO3 (Sulfur Trioxide)**:
- SO3 has a sulfur atom that can accept electron pairs due to the presence of empty d-orbitals.
- It can form coordinate covalent bonds by accepting electron pairs from Lewis bases.
- **Conclusion**: SO3 can behave like a Lewis acid.

5. **Analyze SiCl4 (Silicon Tetrachloride)**:
- SiCl4 has a silicon atom that can also accept electron pairs. Silicon has empty d-orbitals and can expand its octet.
- **Conclusion**: SiCl4 can behave like a Lewis acid.

6. **Final Conclusion**: The species that cannot behave like a Lewis acid is CO3^2-.

### Summary:
- **CO3^2-**: Cannot behave like a Lewis acid (no vacant orbitals).
- **Zn^2+**: Can behave like a Lewis acid.
- **SO3**: Can behave like a Lewis acid.
- **SiCl4**: Can behave like a Lewis acid.

### Answer:
**CO3^2- cannot behave like a Lewis acid.**