Which of the following carbonyls will have the strongest C-O bond

Options

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

Answer

To determine which of the given carbonyls will have the strongest C-O bond, we need to analyze the nature of the metal center in each carbonyl complex and its oxidation state, as well as the extent of back-bonding (synergic bonding) that occurs between the metal and the carbonyl ligand.

### Step-by-Step Solution:

1. **Identify the Carbonyl Complexes**:
We have several carbonyl complexes to consider. Let's denote them as:
- Complex A: [M(CO)6] (where M is a metal)
- Complex B: [M(CO)5] (where M is a metal)
- Complex C: [M(CO)4] (where M is a metal)
- Complex D: [M(CO)3] (where M is a metal)

2. **Determine the Oxidation States**:
For each complex, we need to calculate the oxidation state of the metal (M):
- For Complex A: If the overall charge is -1, then the oxidation state of M is -1.
- For Complex B: If the overall charge is 0, then the oxidation state of M is 0.
- For Complex C: If the overall charge is +1, then the oxidation state of M is +1.
- For Complex D: If the overall charge is 0, then the oxidation state of M is 0.

3. **Analyze Back-Bonding**:
Back-bonding occurs when the metal donates electron density to the π* orbitals of the CO ligand. The extent of back-bonding is influenced by the oxidation state of the metal:
- Higher oxidation states typically lead to greater back-bonding because the metal has more d electrons available for donation.
- Conversely, lower oxidation states lead to less back-bonding.

4. **Identify the Complex with Minimum Back-Bonding**:
- Complex A (M = -1): Likely has significant back-bonding due to the negative charge.
- Complex B (M = 0): Moderate back-bonding.
- Complex C (M = +1): Higher back-bonding due to positive charge and availability of d electrons.
- Complex D (M = 0): Similar to Complex B with moderate back-bonding.

5. **Conclusion**:
The complex with the least back-bonding will have the strongest C-O bond. In this case, Complex C (with M in the +1 oxidation state) will have the least back-bonding due to the positive charge, leading to a stronger C-O bond.

### Final Answer:
The carbonyl complex with the strongest C-O bond is **Complex C (M in +1 oxidation state)**.