Metal carbonyls have the metal ions in zero or unusually lower oxidation states. This is because :

Options

A. carbonyl ligands is reducing in nature.
B. carbonyl is a highly electron rich ligands.
C. carbonyl is a strongly o-bonding ligand.
D. carbonyl is a strongly p-acidic ligand.

Answer

To answer the question, "Why do metal carbonyls have the metal ions in zero or unusually lower oxidation states?", we can break down the explanation into several key steps:

### Step-by-Step Solution:

1. **Understanding Metal Carbonyls**:
- Metal carbonyls are coordination compounds where a metal is bonded to carbon monoxide (CO) ligands.

2. **Nature of CO as a Ligand**:
- Carbon monoxide (CO) acts as a ligand that can form coordinate bonds with metal ions. It is classified as a **pi-acceptor ligand**.

3. **Oxidation States of Metals in Carbonyls**:
- In metal carbonyl complexes, the metal typically exhibits a zero or low oxidation state. This is crucial for the stability of the complex.

4. **Back Bonding Concept**:
- The reason for the low oxidation state is due to **back bonding**. In this process, the metal donates electron density from its d-orbitals to the empty pi* (pi star) antibonding orbitals of the CO ligand.

5. **Electron Rich Metals**:
- For effective back bonding to occur, the metal must be electron-rich. This condition is met when the metal is in a low or zero oxidation state, allowing it to donate electrons easily.

6. **Strengthening of Metal-CO Bond**:
- The back bonding enhances the strength of the metal-CO bond, making the overall complex more stable. It also leads to an increase in the bond length of the CO bond due to the nature of the bonding interaction.

7. **Conclusion**:
- Therefore, the presence of CO as a pi-acceptor ligand necessitates that the metal be in a low or zero oxidation state to facilitate back bonding, which stabilizes the metal carbonyl complex.

### Final Answer:
Metal carbonyls have the metal ions in zero or unusually lower oxidation states because carbon monoxide (CO) is a pi-acceptor ligand that requires the metal to be electron-rich. This condition is satisfied when the metal is in a low or zero oxidation state, allowing for effective back bonding, which strengthens the metal-CO bond.

---