For `[FeF_(6)]^(3-) and [CoF_(6)]^(3-)`, the correct statement is

Options

A. both are coloured
B. both are colourless
C. `[FeF_(6)]^(3-)` is coloured and `[CoF_(6)]^(3-)` is colourless
D. `[FeF_(6)]6(3-)` is colourless and `[CoF_(6)]^(3-)` is coloured

Answer

To solve the question regarding the color properties of the complexes \([FeF_6]^{3-}\) and \([CoF_6]^{3-}\), we will analyze the electronic configurations and the effects of the ligands on these complexes.

### Step-by-Step Solution:

1. **Identify the Oxidation States:**
- For \([FeF_6]^{3-}\):
- Let the oxidation state of Fe be \(x\).
- The charge from 6 fluoride ions (F) is \(-6\).
- The overall charge of the complex is \(-3\).
- Therefore, the equation is:
\[ x + (-6) = -3 \implies x = +3 \]
- For \([CoF_6]^{3-}\):
- Let the oxidation state of Co be \(y\).
- The charge from 6 fluoride ions (F) is \(-6\).
- The overall charge of the complex is \(-3\).
- Therefore, the equation is:
\[ y + (-6) = -3 \implies y = +3 \]

2. **Determine the Electronic Configurations:**
- For \(Fe^{3+}\) (with atomic number 26):
- The electron configuration is \([Ar] 3d^5\).
- For \(Co^{3+}\) (with atomic number 27):
- The electron configuration is \([Ar] 3d^6\).

3. **Analyze the Ligand Field Strength:**
- Fluoride (F) is a weak field ligand. This means it does not cause significant splitting of the d-orbitals.
- In the case of \([FeF_6]^{3-}\):
- The 5 d-electrons will occupy the available d-orbitals without pairing due to weak field strength, resulting in a symmetrical arrangement.
- Hence, it does not have any d-d transitions and is colorless.
- In the case of \([CoF_6]^{3-}\):
- The 6 d-electrons can undergo d-d transitions due to the presence of available orbitals.
- This results in a color being observed due to the absorption of light corresponding to the energy gap between the split d-orbitals.

4. **Conclusion:**
- \([FeF_6]^{3-}\) is colorless because it does not have any d-d transitions due to the symmetrical arrangement of its d-electrons.
- \([CoF_6]^{3-}\) is colored because it allows for d-d transitions due to the presence of unpaired electrons in the d-orbitals.

### Final Statement:
- The correct statement is that \([FeF_6]^{3-}\) is colorless, while \([CoF_6]^{3-}\) is colored.