Which of the following co-ordinate complexes is an exception to EAN rule ?
(Given atomic number `Pt = 078, Fe = 26, Zn = 30, Cu = 29`)

Options

A. `[Pt(NH_3)_6]^(4+)`
B. `[Fe(CN)_6]^(4-)`
C. `[Zn(NH_3)_4]^(2+)`
D. `[Cu(H_2O)_4]^(2+)`

Answer

To determine which of the following coordinate complexes is an exception to the Effective Atomic Number (EAN) rule, we will analyze the given complexes one by one. The EAN rule states that the central metal atom in a complex tends to achieve an electronic configuration equivalent to that of the nearest noble gas.

### Step-by-Step Solution:

1. **Understand the EAN Rule:**
- The EAN is calculated using the formula:
\[ \text{EAN} = Z - x + 2n \]
where:
- \( Z \) = atomic number of the central metal atom
- \( x \) = oxidation state of the central metal atom
- \( n \) = number of lone pair donating sites (ligands)

2. **Analyze Each Complex:**

**a. For \( \text{[Pt(NH}_3\text{)}_6]^{4+} \):**
- Atomic number of Pt = 78
- Oxidation state of Pt = +4 (since NH3 is neutral)
- Number of NH3 ligands = 6
- Calculate EAN:
\[ \text{EAN} = 78 - 4 + 2 \times 6 = 78 - 4 + 12 = 86 \]
- Nearest noble gas (Radon) atomic number = 86.
- This complex follows the EAN rule.

**b. For \( \text{[Fe(CN)}_6]^{4-} \):**
- Atomic number of Fe = 26
- Oxidation state of Fe = +2 (since CN is -1 and there are 6 CN ligands)
- Calculate EAN:
\[ \text{EAN} = 26 - 2 + 2 \times 6 = 26 - 2 + 12 = 36 \]
- Nearest noble gas (Krypton) atomic number = 36.
- This complex follows the EAN rule.

**c. For \( \text{[Zn(NH}_3\text{)}_4]^{2+} \):**
- Atomic number of Zn = 30
- Oxidation state of Zn = +2 (NH3 is neutral)
- Number of NH3 ligands = 4
- Calculate EAN:
\[ \text{EAN} = 30 - 2 + 2 \times 4 = 30 - 2 + 8 = 36 \]
- Nearest noble gas (Krypton) atomic number = 36.
- This complex follows the EAN rule.

**d. For \( \text{[Cu(H}_2\text{O)}_4]^{2+} \):**
- Atomic number of Cu = 29
- Oxidation state of Cu = +2 (H2O is neutral)
- Number of H2O ligands = 4
- Calculate EAN:
\[ \text{EAN} = 29 - 2 + 2 \times 4 = 29 - 2 + 8 = 35 \]
- Nearest noble gas (Krypton) atomic number = 36.
- This complex does not follow the EAN rule.

3. **Conclusion:**
- The complex that is an exception to the EAN rule is \( \text{[Cu(H}_2\text{O)}_4]^{2+} \) as its EAN (35) does not match the atomic number of the nearest noble gas (36).

### Final Answer:
The coordinate complex that is an exception to the EAN rule is \( \text{[Cu(H}_2\text{O)}_4]^{2+} \).