Which of the following compound has/have effective atomic number equal to the atomic number of a noble gas?

Options

A. `K[Co(CO)_(4)]`
B. `K_(2)[Fe(CO)_(4)]`
C. `[Co(NH_(3))_(6)]Cl_(2)`
D. `[CoCl_(3)(H_(2)O)_(3)]`

Answer

To determine which of the given compounds has an effective atomic number (EAN) equal to the atomic number of a noble gas, we will calculate the EAN for each compound using the following formula:

**EAN = Z - oxidation state + contribution from ligands**

Where:
- Z = atomic number of the central metal atom
- Oxidation state = the charge of the complex
- Contribution from ligands = total number of electrons donated by the ligands

### Step-by-Step Solution:

1. **Identify the Compounds**:
Let's denote the compounds as follows:
- Compound 1: K2[Co(CO)4]
- Compound 2: [Fe(CO)4]2-
- Compound 3: [Co(NH3)6]2+
- Compound 4: [CoCl3(H2O)3]

2. **Calculate EAN for Compound 1: K2[Co(CO)4]**
- Atomic number of Co (Cobalt) = 27
- Oxidation state of Co: Assume X, CO is neutral (0).
\[ X + 0 \times 4 = -2 \implies X = -2 \]
- Contribution from ligands: CO contributes 2 electrons each, and there are 4 CO ligands.
\[ 4 \times 2 = 8 \]
- EAN Calculation:
\[ EAN = 27 - (-2) + 8 = 27 + 2 + 8 = 37 \]
- **Result**: EAN = 37 (not equal to any noble gas).

3. **Calculate EAN for Compound 2: [Fe(CO)4]2-**
- Atomic number of Fe (Iron) = 26
- Oxidation state of Fe: Assume X, CO is neutral (0).
\[ X + 0 \times 4 = -2 \implies X = -2 \]
- Contribution from ligands:
\[ 4 \times 2 = 8 \]
- EAN Calculation:
\[ EAN = 26 - (-2) + 8 = 26 + 2 + 8 = 36 \]
- **Result**: EAN = 36 (equal to Krypton, a noble gas).

4. **Calculate EAN for Compound 3: [Co(NH3)6]2+**
- Atomic number of Co = 27
- Oxidation state of Co: Assume X, NH3 is neutral (0).
\[ X + 0 \times 6 = +2 \implies X = +2 \]
- Contribution from ligands:
\[ 6 \times 2 = 12 \]
- EAN Calculation:
\[ EAN = 27 - 2 + 12 = 27 - 2 + 12 = 37 \]
- **Result**: EAN = 37 (not equal to any noble gas).

5. **Calculate EAN for Compound 4: [CoCl3(H2O)3]**
- Atomic number of Co = 27
- Oxidation state of Co: Assume X, Cl is -1 and H2O is neutral (0).
\[ X + (-1) \times 3 + 0 \times 3 = 0 \implies X = +3 \]
- Contribution from ligands:
- Cl contributes 2 electrons each (3 Cl) = 6
- H2O contributes 2 electrons each (3 H2O) = 6
\[ Total = 6 + 6 = 12 \]
- EAN Calculation:
\[ EAN = 27 - 3 + 12 = 27 - 3 + 12 = 36 \]
- **Result**: EAN = 36 (equal to Krypton, a noble gas).

### Final Results:
- **Compounds with EAN equal to a noble gas**:
- Compound 2: [Fe(CO)4]2- (EAN = 36)
- Compound 4: [CoCl3(H2O)3] (EAN = 36)