Which of the following involves synergic bonding?

Options

A. `[Fe(C_(5)H_(5))_(2))]`
B. `(CH_(3))_(2)Zn`
C. `[Fe(CO)_(5)]`
D. `CH_(3)MgBr`

Answer

To determine which of the given options involves synergic bonding, we need to understand the concept of synergic bonding in coordination compounds.

### Step-by-Step Solution:

1. **Understanding Synergic Bonding**:
- Synergic bonding occurs when there is both sigma bonding (coordinate bond) and pi back-bonding between the ligand and the central metal atom.
- In this process, the ligand donates a pair of electrons to the metal (sigma bond), and in return, the metal can donate electron density back to the ligand's vacant anti-bonding orbitals (pi back-bonding).

2. **Identifying Pi-Acid Ligands**:
- For synergic bonding to occur, the ligands must be pi-acidic, meaning they have vacant anti-bonding orbitals that can accept electrons from the metal.
- Common examples of pi-acid ligands include CO (carbon monoxide), NO2+, and C2H4.

3. **Analyzing the Given Options**:
- We need to evaluate each option to see if it contains a pi-acid ligand:
- **Option A: C6H5 (Phenyl group)** - This is not a pi-acid ligand; it does not have vacant anti-bonding orbitals.
- **Option B: CH3 (Methyl group)** - This is also not a pi-acid ligand; it lacks the necessary orbitals for synergic bonding.
- **Option C: CO (Carbon monoxide)** - This is a well-known pi-acid ligand that can participate in synergic bonding due to its vacant anti-bonding orbitals.
- **Option D: Grignard Reagent** - This does not involve pi-acid ligands and therefore cannot exhibit synergic bonding.

4. **Conclusion**:
- Among the given options, only **Option C (CO)** involves synergic bonding due to its nature as a pi-acid ligand.

### Final Answer:
**The correct answer is Option C: CO.**