Select the true statement from the following for metal carbonyls ?

Options

A. `pi` back bonding strengthens M - C bond order as well as CO bond order .
B. `pi` back bonding weakens M - C bond order as well as CO bond order
C. `pi` back bonding weakens M - C bond order but strengthens CO bond order
D. `pi` back bonding strengthens M - C bond order and weakens CO bond order .

Explanation

### Step-by-Step Solution:

1. **Understanding Metal Carbonyls**:
Metal carbonyls are coordination compounds where carbon monoxide (CO) acts as a ligand, bonding to a metal center.

2. **Synergic Bonding**:
In metal carbonyls, the bonding involves a synergic interaction. This means that there are two types of bonding occurring: sigma (σ) bonding and pi (π) backbonding.

3. **Formation of Sigma Bond**:
The carbon atom of the CO molecule donates a lone pair of electrons to an empty d-orbital of the metal. This forms a sigma bond between the metal and the carbon.

4. **Formation of Pi Bond**:
The metal can also donate electron density from its filled d-orbitals into the empty π* (antibonding) orbital of the CO ligand. This forms a pi bond, which is known as π backbonding.

5. **Effect on Bond Order**:
Due to π backbonding, the electron density in the bonding orbital of CO decreases, while the electron density in the antibonding orbital increases. This results in a decrease in the bond order of the CO bond, leading to a weaker CO bond.

6. **Effect on Metal-Carbon Bond**:
Conversely, the π backbonding strengthens the metal-carbon bond because the interaction enhances the overall stability of the metal-carbon bond.

7. **Evaluating Statements**:
Now, let's analyze the statements provided in the question:
- **Statement A**: Pi bonding strengthens the metal and CO bond order. (Incorrect)
- **Statement B**: Pi backbonding weakens the metal and CO bond. (Incorrect)
- **Statement C**: Pi backbonding weakens MC bond order but strengthens CO bond order. (Incorrect)
- **Statement D**: Pi backbonding strengthens metal-carbon bond order and weakens CO bond order. (Correct)

8. **Conclusion**:
The true statement regarding metal carbonyls is that pi backbonding strengthens the metal-carbon bond order and weakens the CO bond order.

### Final Answer:
**The correct statement is: Pi backbonding strengthens metal-carbon bond order and weakens CO bond order.**

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