To determine which of the following complexes can be easily reduced, we need to analyze the electron count of each complex and apply the 18-electron rule. The 18-electron rule states that transition metal complexes are most stable when they have 18 valence electrons. If a complex has fewer than 18 electrons, it can gain electrons (be reduced) to achieve stability.
Let's analyze each option step by step:
### Step 1: Analyze Option A - \( \text{V(CO)}_6 \)
1. **Identify the metal and its electron configuration**: Vanadium (V) has an atomic number of 23, and its electron configuration is \( [\text{Ar}] 4s^2 3d^3 \). Thus, it has 5 valence electrons.
2. **Count the electrons contributed by CO ligands**: Each CO ligand donates 2 electrons. Since there are 6 CO ligands, the total contribution is \( 6 \times 2 = 12 \) electrons.
3. **Total electron count**:
\[
\text{Total} = 5 + 12 = 17 \text{ electrons}
\]
4. **Determine if reduction is possible**: Since the complex has 17 electrons, it can gain 1 electron to reach 18 electrons, making it stable. Therefore, \( \text{V(CO)}_6 \) can be easily reduced to \( \text{V(CO)}_6^- \).
### Step 2: Analyze Option B - \( \text{Mo(CO)}_6 \)
1. **Identify the metal and its electron configuration**: Molybdenum (Mo) has an atomic number of 42, and its electron configuration is \( [\text{Kr}] 5s^2 4d^4 \). Thus, it has 6 valence electrons.
2. **Count the electrons contributed by CO ligands**: Again, with 6 CO ligands, the contribution is \( 6 \times 2 = 12 \) electrons.
3. **Total electron count**:
\[
\text{Total} = 6 + 12 = 18 \text{ electrons}
\]
4. **Determine if reduction is possible**: Since the complex already has 18 electrons, it is stable and cannot be reduced further.
### Step 3: Analyze Option C - \( \text{Co(CO)}_4^- \)
1. **Identify the metal and its electron configuration**: Cobalt (Co) has an atomic number of 27, and its electron configuration is \( [\text{Ar}] 4s^2 3d^7 \). Thus, it has 9 valence electrons.
2. **Count the electrons contributed by CO ligands**: With 4 CO ligands, the contribution is \( 4 \times 2 = 8 \) electrons.
3. **Account for the negative charge**: The negative charge adds 1 electron.
4. **Total electron count**:
\[
\text{Total} = 9 + 8 + 1 = 18 \text{ electrons}
\]
5. **Determine if reduction is possible**: Since it has 18 electrons, it cannot be reduced further.
### Step 4: Analyze Option D - \( \text{Fe(CO)}_5 \)
1. **Identify the metal and its electron configuration**: Iron (Fe) has an atomic number of 26, and its electron configuration is \( [\text{Ar}] 4s^2 3d^6 \). Thus, it has 8 valence electrons.
2. **Count the electrons contributed by CO ligands**: With 5 CO ligands, the contribution is \( 5 \times 2 = 10 \) electrons.
3. **Total electron count**:
\[
\text{Total} = 8 + 10 = 18 \text{ electrons}
\]
4. **Determine if reduction is possible**: Since it has 18 electrons, it cannot be reduced further.
### Conclusion
The only complex that can be easily reduced is **Option A: \( \text{V(CO)}_6 \)**.
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