In which of the following ions, the colour is not due to d-d transition?

Options

A. `[Ti(H_2O)_6]^(3+)`
B. `[Cu(NH_3)_4]^(2+)`
C. `[CoF_6]^(3-)`
D. `CrO_4^(2-)`

Explanation

To determine which of the following ions has a color that is not due to d-d transitions, we will analyze the electronic configurations and the mechanisms responsible for the color in each case.

### Step-by-Step Solution:

1. **Identify the Ions**: We need to analyze the color mechanisms for the following ions:
- Titanium ion (Ti³⁺)
- Copper ion (Cu²⁺)
- Cobalt ion (Co³⁺)
- Chromate ion (CrO₄²⁻)

2. **Titanium Ion (Ti³⁺)**:
- **Oxidation State**: +3
- **Electronic Configuration**: The initial configuration of titanium (Ti) is [Ar] 3d² 4s². After losing three electrons (2 from 4s and 1 from 3d), the configuration becomes 3d¹.
- **Color Mechanism**: The presence of one d-electron allows for d-d transitions, which are responsible for the color of Ti³⁺. Thus, Ti³⁺ has color due to d-d transitions.

3. **Copper Ion (Cu²⁺)**:
- **Oxidation State**: +2
- **Electronic Configuration**: The initial configuration of copper (Cu) is [Ar] 3d¹⁰ 4s¹. After losing two electrons (1 from 4s and 1 from 3d), the configuration becomes 3d⁹.
- **Color Mechanism**: The presence of nine d-electrons allows for d-d transitions, which are responsible for the color of Cu²⁺. Thus, Cu²⁺ has color due to d-d transitions.

4. **Cobalt Ion (Co³⁺)**:
- **Oxidation State**: +3
- **Electronic Configuration**: The initial configuration of cobalt (Co) is [Ar] 3d⁷ 4s². After losing three electrons (2 from 4s and 1 from 3d), the configuration becomes 3d⁶.
- **Color Mechanism**: The presence of six d-electrons allows for d-d transitions, which are responsible for the color of Co³⁺. Thus, Co³⁺ has color due to d-d transitions.

5. **Chromate Ion (CrO₄²⁻)**:
- **Oxidation State**: +6 (for Cr in CrO₄²⁻)
- **Electronic Configuration**: The initial configuration of chromium (Cr) is [Ar] 3d⁵ 4s¹. In the +6 oxidation state, the configuration becomes 3d⁰.
- **Color Mechanism**: In the case of CrO₄²⁻, the color is not due to d-d transitions. Instead, it is due to charge transfer from the ligand (O²⁻) to the metal (Cr). This is known as ligand-to-metal charge transfer (LMCT). Therefore, CrO₄²⁻ does not exhibit color due to d-d transitions.

### Conclusion:
The ion whose color is not due to d-d transitions is **CrO₄²⁻**.

### Final Answer:
**D. CrO₄²⁻**