Which of the following is not acid ligand?

Options

A. CO
B. `F^(-)`
C. `C_(2)H_(4)`
D. `NO^(+)`

Answer

To determine which of the following is not an acid ligand, we need to understand the definition of acid ligands and analyze the given options accordingly.

### Step-by-Step Solution:

1. **Definition of Acid Ligands**:
Acid ligands are those that can both donate a lone pair of electrons to form a coordinate bond with a central metal atom and accept electron density from the metal atom into their vacant low-lying d-orbitals. This type of interaction is known as synergic or back bonding.

2. **Identifying the Ligands**:
We need to evaluate the ligands provided in the options to see which one does not exhibit the characteristics of an acid ligand.

3. **Analyzing Each Option**:
- **Option A: CO (Carbon monoxide)**:
CO is a well-known acid ligand. It can donate a lone pair to the metal and also accept electron density back into its vacant π* orbitals, thus exhibiting synergic bonding.

- **Option B: C2H4 (Ethylene)**:
C2H4 can also act as an acid ligand. It has π-bonds that can overlap with the metal's d-orbitals, allowing for back bonding and hence qualifies as an acid ligand.

- **Option C: NO+ (Nitrosonium ion)**:
NO+ is another acid ligand. It can donate electrons and also accept electron density from the metal, thus showing back bonding.

- **Option D: F- (Fluoride ion)**:
Fluoride is a simple anion that does not have vacant d-orbitals to accept electron density from the metal. It can only donate its lone pair to form a coordinate bond but does not exhibit back bonding. Therefore, it does not qualify as an acid ligand.

4. **Conclusion**:
Based on the analysis, the ligand that is not an acid ligand is **F- (Fluoride ion)**.

### Final Answer:
**Fluoride ion (F-) is not an acid ligand.**

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