Consider the following complexes :
1. `Fe(CO)_(5)` 2. `Ni(CO)_(4)`
3. `K_(4)[Fe(CN)_(6)]` 4. `Na_(3)[Cu(SCN)_(4)]`
Which of the above complexes obey EAN rule ?

Options

A. 1, and 2 only
B. 2, 3 and 4 only
C. 1, 2, 3 and 4
D. none of the above

Explanation

To determine which of the given complexes obey the Effective Atomic Number (EAN) rule, we need to calculate the EAN for each complex. The EAN is calculated using the formula:

\[ \text{EAN} = Z + \text{Charge} + \text{Number of electrons donated by ligands} \]

Where \( Z \) is the atomic number of the central metal atom.

Let's analyze each complex step by step:

### Step 1: Calculate EAN for `Fe(CO)₅`
1. **Identify the atomic number of Iron (Fe):**
Atomic number \( Z = 26 \)

2. **Determine the charge of the complex:**
Charge = 0 (neutral complex)

3. **Count the number of CO ligands:**
Each CO donates 2 electrons. For 5 CO ligands:
\( 5 \times 2 = 10 \) electrons

4. **Calculate EAN:**
\[ \text{EAN} = 26 + 0 + 10 = 36 \]

### Step 2: Calculate EAN for `Ni(CO)₄`
1. **Identify the atomic number of Nickel (Ni):**
Atomic number \( Z = 28 \)

2. **Determine the charge of the complex:**
Charge = 0 (neutral complex)

3. **Count the number of CO ligands:**
For 4 CO ligands:
\( 4 \times 2 = 8 \) electrons

4. **Calculate EAN:**
\[ \text{EAN} = 28 + 0 + 8 = 36 \]

### Step 3: Calculate EAN for `K₄[Fe(CN)₆]`
1. **Identify the atomic number of Iron (Fe):**
Atomic number \( Z = 26 \)

2. **Determine the oxidation state of Iron in the complex:**
The charge of the complex is \( +2 \) (since K is \( +1 \) and there are 4 K ions, total \( +4 \), thus \( Fe \) must be \( +2 \) to balance with \( CN \) ligands).

3. **Count the number of CN ligands:**
Each CN donates 2 electrons. For 6 CN ligands:
\( 6 \times 2 = 12 \) electrons

4. **Calculate EAN:**
\[ \text{EAN} = 26 - 2 + 12 = 36 \]

### Step 4: Calculate EAN for `Na₃[Cu(SCN)₄]`
1. **Identify the atomic number of Copper (Cu):**
Atomic number \( Z = 29 \)

2. **Determine the oxidation state of Copper in the complex:**
The charge of the complex is \( +1 \) (since Na is \( +1 \) and there are 3 Na ions, total \( +3 \), thus \( Cu \) must be \( +1 \) to balance with \( SCN \) ligands).

3. **Count the number of SCN ligands:**
Each SCN donates 2 electrons. For 4 SCN ligands:
\( 4 \times 2 = 8 \) electrons

4. **Calculate EAN:**
\[ \text{EAN} = 29 - 1 + 8 = 36 \]

### Conclusion
All four complexes have an EAN of 36, which means they all obey the EAN rule.

### Final Answer:
All complexes: `Fe(CO)₅`, `Ni(CO)₄`, `K₄[Fe(CN)₆]`, and `Na₃[Cu(SCN)₄]` obey the EAN rule.