Colour of compounds occurs due to phenomenon of polarisation, charge transfer, d-d transition and H.O.M.O.L.U.M.O transition. Most of the transition metal transition metal complex compound are coloured either due to d-d electrons transition or charge transfer and ionic compounds are coloured due to polarisation of anion. Q. Which of the following is correct about `KFe^(II)[underset(III)(Fe)(CN_(6))] and KFe^(III)[underset(II)(Fe)(CN_(6))]` complex compounds?

Options

A. Both are blue coloured compounds because colour arises due to d-d electrons transition in Fe cation present outside the complex ion.
B. Both are blue coloured compounds because colour arises due to transfer of electron between `Fe^(II) and Fe^(III)` cation
C. Both are blue coloured compounds because in complexes `Fe^(II)` cation shows same d-d transition bond
D. Complex (I) has blue colour becouse (II) has brown colour

Answer

To solve the question regarding the color of the complex compounds \( KFe^{II}[Fe^{III}(CN)_6] \) and \( KFe^{III}[Fe^{II}(CN)_6] \), we will analyze the oxidation states of iron in each complex and the resulting color due to electronic transitions.

### Step-by-Step Solution:

1. **Identify the Complexes**:
- The first complex is \( KFe^{II}[Fe^{III}(CN)_6] \).
- The second complex is \( KFe^{III}[Fe^{II}(CN)_6] \).

2. **Determine the Oxidation States**:
- In \( KFe^{II}[Fe^{III}(CN)_6] \), the oxidation state of iron in the outer complex is +2 (Fe²⁺), and in the inner complex, it is +3 (Fe³⁺).
- In \( KFe^{III}[Fe^{II}(CN)_6] \), the oxidation state of iron in the outer complex is +3 (Fe³⁺), and in the inner complex, it is +2 (Fe²⁺).

3. **Color of the Complexes**:
- The first complex \( KFe^{II}[Fe^{III}(CN)_6] \) is known to be Turnbull's blue, which is a blue-colored compound.
- The second complex \( KFe^{III}[Fe^{II}(CN)_6] \) is known to be Prussian blue, which is also a blue-colored compound.

4. **Reason for Color**:
- The color in these complexes arises primarily due to charge transfer between the Fe²⁺ and Fe³⁺ ions rather than d-d transitions. Charge transfer occurs as the oxidation states of iron can shift between +2 and +3, leading to a color change.

5. **Conclusion**:
- Both complexes are blue in color due to charge transfer between the Fe²⁺ and Fe³⁺ ions, confirming that the correct statement regarding these complexes is that both are blue-colored because the color arises due to the electron charge transfer between Fe²⁺ and Fe³⁺ cations.

### Final Answer:
The correct statement about \( KFe^{II}[Fe^{III}(CN)_6] \) and \( KFe^{III}[Fe^{II}(CN)_6] \) is that both are blue-colored because the color arises due to the electron charge transfer between Fe²⁺ and Fe³⁺ cations.