The d-electron configurations of `Cr^(2+), Mn^(2+), Fe^(2+)` and `Co^(2+)` are `d^4, d^5, d^6` and `d^7` respectively. Which one of the following will exhibit minimum paramagnetic behaviour?

Options

A. (a) `[Fe(H_(2)O)_(6)]^(2+)`
B. (b) `[Co(H_(2)O)_(6)]^(2+)`
C. (c) `[Cr(H_(2)O)_(6)]^(2+)`
D. (d) `[Mn(H_(2)O)_(6)]^(2+)`

Explanation

To determine which of the given compounds exhibits the minimum paramagnetic behavior, we need to analyze the d-electron configurations of the central metal ions in each complex and count the number of unpaired electrons.

### Step-by-Step Solution:

1. **Identify the oxidation states**:
- All complexes have water (H2O) as a neutral ligand, so the oxidation state of the central metal ion in each complex is +2.
- Therefore, we have:
- Fe in [Fe(H2O)6]²⁺: +2
- Co in [Co(H2O)6]²⁺: +2
- Cr in [Cr(H2O)6]²⁺: +2
- Mn in [Mn(H2O)6]²⁺: +2

2. **Determine the d-electron configurations**:
- For each metal ion in the +2 oxidation state:
- **Fe²⁺**: d⁶
- **Co²⁺**: d⁷
- **Cr²⁺**: d⁴
- **Mn²⁺**: d⁵

3. **Fill the d-orbitals and count unpaired electrons**:
- **Fe²⁺ (d⁶)**:
- Configuration: ↑↓ ↑↓ ↑ ↑ ↑
- Unpaired electrons: 4
- **Co²⁺ (d⁷)**:
- Configuration: ↑↓ ↑↓ ↑ ↑ ↑
- Unpaired electrons: 3
- **Cr²⁺ (d⁴)**:
- Configuration: ↑ ↑ ↑ ↑
- Unpaired electrons: 4
- **Mn²⁺ (d⁵)**:
- Configuration: ↑ ↑ ↑ ↑ ↑
- Unpaired electrons: 5

4. **Compare the number of unpaired electrons**:
- Fe²⁺: 4 unpaired electrons
- Co²⁺: 3 unpaired electrons
- Cr²⁺: 4 unpaired electrons
- Mn²⁺: 5 unpaired electrons

5. **Determine which has the minimum paramagnetic behavior**:
- Paramagnetic behavior is directly proportional to the number of unpaired electrons.
- Co²⁺ has the least number of unpaired electrons (3) compared to the others.

### Conclusion:
The compound that exhibits the minimum paramagnetic behavior is **[Co(H2O)6]²⁺**.