Crystal field theory provides correct electronic distribution of central metal under surrounding ligannd field, hence it clearly explains magnetic moment, colour of a complex.
Q. Which of the following hydrated complex ion has high intensity colour in aqueous solution.

Options

A. `[Mn(H_(2)O)_(6)]^(3+)`
B. `[Co(H_(2)O)_(6)]^(2+)`
C. `[Ni(H_(2)O)_(6)]^(2+)`
D. `[Mn(H_(20O)_(6)]^(2+)`

Explanation

To determine which hydrated complex ion has high intensity color in aqueous solution, we will analyze the electronic configurations of the metal ions involved and their respective colors. Here’s a step-by-step solution:

### Step 1: Identify the Metal Ions and Their Oxidation States
We need to identify the metal ions and their oxidation states in the hydrated complexes. Common metal ions in coordination compounds include Mn, Co, and Ni.

### Step 2: Determine the Electronic Configuration
For each metal ion, we will determine the electronic configuration based on its oxidation state and the number of d-electrons:

- **Mn³⁺**: The electronic configuration is [Ar] 3d⁴. This means there are 4 unpaired electrons.
- **Co²⁺**: The electronic configuration is [Ar] 3d⁷. This means there are 3 unpaired electrons.
- **Ni²⁺**: The electronic configuration is [Ar] 3d⁸. This means there are 2 unpaired electrons.
- **Mn²⁺**: The electronic configuration is [Ar] 3d⁵. This means there are 5 unpaired electrons.

### Step 3: Analyze the Ligand Field Strength
The ligand in question is water (H₂O), which is a neutral and weak field ligand. Weak field ligands do not cause significant splitting of the d-orbitals, leading to a specific arrangement of electrons.

### Step 4: Determine the Color of Each Complex Ion
Based on the electronic configurations and the presence of unpaired electrons, we can determine the color of each complex ion:

- **Mn³⁺**: Exhibits a blue color due to the presence of unpaired electrons.
- **Co²⁺**: Exhibits a pink color.
- **Ni²⁺**: Exhibits a green color.
- **Mn²⁺**: Exhibits a pale pink color.

### Step 5: Compare the Intensity of Colors
The intensity of color is related to the number of unpaired electrons and the energy transitions possible within the d-orbitals. The more unpaired electrons present, the higher the potential for color intensity.

- **Mn³⁺**: 4 unpaired electrons (blue color, high intensity)
- **Co²⁺**: 3 unpaired electrons (pink color)
- **Ni²⁺**: 2 unpaired electrons (green color)
- **Mn²⁺**: 5 unpaired electrons (pale pink color, lower intensity)

### Conclusion
Among the given options, **Mn³⁺** with its blue color and 4 unpaired electrons has the highest intensity color in aqueous solution.