`CN^(-)` ion is known but `CP^(-)` ion is not known.

Explanation

To understand why the CN⁻ ion is known while the CP⁻ ion is not, we can break down the reasoning into a few key steps:

### Step 1: Analyze the Elements Involved
- **Carbon (C)** is a non-metal that can form multiple bonds due to its ability to share electrons.
- **Nitrogen (N)** is also a non-metal with a small atomic size and high electronegativity.

### Step 2: Formation of CN⁻ Ion
- The CN⁻ ion is formed when carbon and nitrogen bond together.
...