Which of the following species is not a pseudo halide ?

Options

A. `CNO^(Theta)`
B. `RCOO^(Theta)`
C. `OCN^(Theta)`
D. `N N N^(Theta)`

Explanation

To determine which of the given species is not a pseudohalide, we will analyze each option based on the definition of pseudohalides. Pseudohalides are ions that consist of two or more electronegative elements, with at least one of them being nitrogen.

### Step-by-Step Solution:

1. **Understanding Pseudohalides**:
- Pseudohalides are ions that resemble halides in their properties. They must contain at least one nitrogen atom and one or more other electronegative atoms.

2. **Analyzing the Options**:
- **Option 1: C-N-O⁻ (Cyanate ion)**:
- This ion consists of carbon (C), nitrogen (N), and oxygen (O). It has nitrogen and meets the criteria for being a pseudohalide.
- **Option 2: R-COO⁻ (Carboxylate ion)**:
- This ion consists of carbon (C) and oxygen (O) only. There is no nitrogen present, so it does not meet the criteria for being a pseudohalide.
- **Option 3: O-C-N⁻ (Isocyanate ion)**:
- This ion consists of oxygen (O), carbon (C), and nitrogen (N). It contains nitrogen and meets the criteria for being a pseudohalide.
- **Option 4: N-N-N⁻ (Azide ion)**:
- This ion consists of three nitrogen atoms. It meets the criteria for being a pseudohalide since it contains nitrogen.

3. **Conclusion**:
- Based on the analysis, the species that is not a pseudohalide is **Option 2: R-COO⁻ (Carboxylate ion)**.

### Final Answer:
**Option 2: R-COO⁻ is not a pseudohalide.**