On the basic of stability of complex ion in the solution, complexes may be of two types, perfect and imperfect complexes. The stability depends upon the extent of dissociation which in turn depends upon charge on central metal atom, basic nature of ligand, chelation, and nature of metal ion and ligand according to HSAB principle.
Q> Which one of the following does not follow EAN rule?

Options

A. `Fe(CO)_(5)`
B. `V(CO)_(6)`
C. `K_(4)[Fe(CN)_(6)]`
D. `Mn_(2)(CO)_(10)`

Answer

To solve the question of which complex does not follow the Effective Atomic Number (EAN) rule, we will analyze each complex given in the options step by step.

### Step 1: Understand the EAN Rule
The EAN rule states that a complex will be stable if the total number of electrons around the central metal atom equals the number of electrons in the nearest noble gas configuration. This means we need to calculate the total number of electrons contributed by the metal and the ligands.

### Step 2: Analyze Each Complex

#### Complex 1: [Fe(CO)5]
1. **Identify the metal and its atomic number**: Iron (Fe) has an atomic number of 26.
2. **Calculate the electrons from ligands**: Each carbonyl (CO) donates 2 electrons. With 5 CO ligands, the total from ligands is \(5 \times 2 = 10\) electrons.
3. **Total electrons**:
\[ 26 \text{ (from Fe)} + 10 \text{ (from CO)} = 36 \text{ electrons} \]
4. **Noble gas configuration**: 36 electrons correspond to the noble gas Krypton. Hence, this complex follows the EAN rule.

#### Complex 2: [V(CO)6]
1. **Identify the metal and its atomic number**: Vanadium (V) has an atomic number of 23.
2. **Calculate the electrons from ligands**: Each CO donates 2 electrons. With 6 CO ligands, the total from ligands is \(6 \times 2 = 12\) electrons.
3. **Total electrons**:
\[ 23 \text{ (from V)} + 12 \text{ (from CO)} = 35 \text{ electrons} \]
4. **Noble gas configuration**: 35 electrons do not correspond to any noble gas configuration (36 is needed for Krypton). Hence, this complex does not follow the EAN rule.

#### Complex 3: [K4[Fe(CN)6]]
1. **Identify the metal and its oxidation state**: Iron (Fe) is in the +2 oxidation state here.
2. **Calculate the electrons from ligands**: Each cyanide (CN) donates 2 electrons. With 6 CN ligands, the total from ligands is \(6 \times 2 = 12\) electrons.
3. **Total electrons**:
\[ 26 \text{ (from Fe)} - 2 + 12 = 36 \text{ electrons} \]
4. **Noble gas configuration**: 36 electrons correspond to Krypton. Hence, this complex follows the EAN rule.

#### Complex 4: [Mn2(CO)10]
1. **Identify the metal and its atomic number**: Manganese (Mn) has an atomic number of 25. There are 2 Mn atoms.
2. **Calculate the electrons from ligands**: Each CO donates 2 electrons. With 10 CO ligands, the total from ligands is \(10 \times 2 = 20\) electrons.
3. **Total electrons**:
\[ 2 \times 25 \text{ (from 2 Mn)} + 20 \text{ (from CO)} + 2 \text{ (metal-metal bond)} = 50 + 20 + 2 = 72 \text{ electrons} \]
4. **Electrons per Mn**:
\[ \frac{72}{2} = 36 \text{ electrons} \]
5. **Noble gas configuration**: 36 electrons correspond to Krypton. Hence, this complex follows the EAN rule.

### Conclusion
After analyzing all complexes, we find that the only complex that does not follow the EAN rule is **[V(CO)6]**.

### Final Answer
**The complex that does not follow the EAN rule is [V(CO)6].**