Which of the following doesn’t follow effective atomic number rule ?

Options

A. `[Cu(NH_(3))_(4)]^(+2)`
B. `[Zn(OH)_(4)]^(-2)`
C. `[HgI_(4)]^(2-)`
D. `Fe(CO)_(5)`

Answer

To solve the question of which compound does not follow the effective atomic number (EAN) rule, we will calculate the effective atomic number for each of the given compounds using the formula:

**Effective Atomic Number (EAN) = Atomic Number - Oxidation State + Electrons from Ligands**

Let's analyze each option step by step:

### Step 1: Analyze Cu(NH3)2+
1. **Atomic Number of Copper (Cu)**: 29
2. **Oxidation State**: In Cu(NH3)2+, the oxidation state of Cu is +2 (since NH3 is a neutral ligand).
3. **Electrons from Ligands**: NH3 donates 2 electrons (1 from each NH3), and there are 2 NH3 ligands.
- Total electrons from ligands = 2 × 2 = 4
4. **Calculate EAN**:
\[ \text{EAN} = 29 - 2 + 4 = 31 \]
- 31 is not a multiple of 18.

### Step 2: Analyze Zn(OH)4^2-
1. **Atomic Number of Zinc (Zn)**: 30
2. **Oxidation State**: In Zn(OH)4^2-, the oxidation state of Zn is +2.
3. **Electrons from Ligands**: OH- donates 2 electrons (1 from each OH-), and there are 4 OH- ligands.
- Total electrons from ligands = 4 × 2 = 8
4. **Calculate EAN**:
\[ \text{EAN} = 30 - 2 + 8 = 36 \]
- 36 is a multiple of 18.

### Step 3: Analyze Hg(I4)2-
1. **Atomic Number of Mercury (Hg)**: 80
2. **Oxidation State**: In Hg(I4)2-, the oxidation state of Hg is +2.
3. **Electrons from Ligands**: I- donates 2 electrons (1 from each I-), and there are 4 I- ligands.
- Total electrons from ligands = 4 × 2 = 8
4. **Calculate EAN**:
\[ \text{EAN} = 80 - 2 + 8 = 86 \]
- 86 is a multiple of 18.

### Step 4: Analyze Fe(CO)5
1. **Atomic Number of Iron (Fe)**: 26
2. **Oxidation State**: In Fe(CO)5, the oxidation state of Fe is 0 (since CO is a neutral ligand).
3. **Electrons from Ligands**: CO donates 2 electrons (1 from each CO), and there are 5 CO ligands.
- Total electrons from ligands = 5 × 2 = 10
4. **Calculate EAN**:
\[ \text{EAN} = 26 - 0 + 10 = 36 \]
- 36 is a multiple of 18.

### Conclusion
From the calculations:
- Cu(NH3)2+ has an EAN of 31 (not a multiple of 18).
- Zn(OH)4^2- has an EAN of 36 (multiple of 18).
- Hg(I4)2- has an EAN of 86 (multiple of 18).
- Fe(CO)5 has an EAN of 36 (multiple of 18).

Thus, the compound that does not follow the effective atomic number rule is **Cu(NH3)2+**.