Which of the following carbonyls will have the strongest C–O bond ?

Options

A. `Fe (CO)_(5)`
B. `Mn (CO)_(6)^(+)`
C. `Cr (CO)_(6)`
D. `V(CO)_(6)^(-)`

Explanation

To determine which carbonyl complex has the strongest C–O bond, we need to analyze the given options based on the oxidation states of the metal ions and the resulting electron donation to the carbonyl ligands.

### Step-by-Step Solution:

1. **Identify the Carbonyl Complexes:**
- Option A: Fe(CO)₅
- Option B: Mn(CO)₆⁺
- Option C: Cr(CO)₆
- Option D: V(CO)₆⁻

2. **Determine the Oxidation States of the Metal:**
- For Fe(CO)₅: Oxidation state of Fe = 0
- For Mn(CO)₆⁺: Oxidation state of Mn = +1
- For Cr(CO)₆: Oxidation state of Cr = 0
- For V(CO)₆⁻: Oxidation state of V = -1

3. **Evaluate the Electron Count:**
- Fe(CO)₅: 24 electrons (Fe = 0)
- Mn(CO)₆⁺: 24 electrons (Mn = +1)
- Cr(CO)₆: 24 electrons (Cr = 0)
- V(CO)₆⁻: 26 electrons (V = -1)

Note: The first three complexes (Fe, Mn, Cr) are isoelectronic, meaning they have the same number of electrons.

4. **Analyze the Bonding:**
- In metal carbonyl complexes, there is a phenomenon known as synergic bonding. This involves the donation of electrons from the ligand (CO) to the metal and the back-donation of electrons from the metal to the CO's π* anti-bonding orbitals.
- The more positive the oxidation state of the metal, the smaller its size, which leads to a decreased tendency to donate electrons to the π* orbitals of CO.

5. **Determine the Effect of Oxidation State on Bond Strength:**
- As the oxidation state of the metal increases, the size of the metal ion decreases, leading to less back-donation to the anti-bonding orbitals of CO. Consequently, this results in a higher bond order and stronger C–O bond.
- Therefore, we can conclude that the complex with the highest oxidation state will have the strongest C–O bond.

6. **Conclusion:**
- Among the given options, Mn(CO)₆⁺ has the highest oxidation state (+1) compared to the others (Fe = 0, Cr = 0, V = -1).
- Thus, the carbonyl complex with the strongest C–O bond is **Option B: Mn(CO)₆⁺**.

### Final Answer:
**Option B: Mn(CO)₆⁺ has the strongest C–O bond.**