JEE Chemistry 2025 Question with Solution

Given: We need the increasing order of stability of the complexes based on $\Delta$.

Find: The correct option representing increasing stability.

From the solution, the complexes are:

• $\text{I} = [\text{Mn(CN)}_6]^{3-}$
• $\text{II} = [\text{Co(CN)}_6]^{4-}$
• $\text{III} = [\text{Fe(CN)}_6]^{4-}$
• $\text{IV} = [\text{Fe(CN)}_6]^{3-}$

For strong field ligand $\text{CN}^-$, larger crystal field splitting $\Delta_0$ implies greater stability. The solution states that $\Delta_0$ increases with increasing oxidation state and charge density.

Using the extracted analysis, the increasing order of $\Delta_0$ is:

$$[\text{Mn(CN)}_6]^{3-} < [\text{Co(CN)}_6]^{4-} < [\text{Fe(CN)}_6]^{3-} < [\text{Fe(CN)}_6]^{4-}$$

Therefore,

$$\text{I} < \text{II} < \text{IV} < \text{III}$$

So, the correct option is B.

There is a discrepancy in the provided answer key and one part of the solution, but the worked order extracted from the detailed solution corresponds to $\text{I} < \text{II} < \text{IV} < \text{III}$.

Conclusion: The correct increasing order of stability is $\text{I} < \text{II} < \text{IV} < \text{III}$, so the correct option is B.

Given: Four cyanide complexes are to be arranged in increasing order of stability based on crystal field splitting energy.

Find: The order of increasing stability.

The detailed the solution lists:

1. $[\text{Mn(CN)}_6]^{3-}$
2. $[\text{Co(CN)}_6]^{4-}$
3. $[\text{Fe(CN)}_6]^{4-}$
4. $[\text{Fe(CN)}_6]^{3-}$

Since $\text{CN}^-$ is a strong field ligand, stability follows increasing $\Delta_0$.

The extracted detailed solution concludes:

$$[\text{Mn(CN)}_6]^{3-} < [\text{Co(CN)}_6]^{4-} < [\text{Fe(CN)}_6]^{3-} < [\text{Fe(CN)}_6]^{4-}$$

Mapping these back to Roman numerals gives:

$$\text{I} < \text{II} < \text{IV} < \text{III}$$

This matches option B among the provided choices.

Conclusion: The correct option is B.