JEE Chemistry 2025 Question with Solution

Given: Four cobalt complexes are to be arranged in increasing order of crystal field stabilization energy.

Find: The correct increasing order of CFSE and hence the correct option.

The solution states that CFSE depends mainly on:

1. Nature of the ligand
2. Oxidation state of the metal
3. Geometry of the complex

From the extracted working:

• en is a stronger field ligand than NH$3$.
• Higher oxidation state of cobalt gives greater splitting and hence larger CFSE.
• The complexes are treated as octahedral unless mentioned otherwise.

Now compare the complexes:

1. $[\text{Co(NH}_3)_4]^{2+}$ has cobalt in the $+2$ oxidation state with NH$3$ ligands, so it has the lowest CFSE among the listed species.
2. $[\text{Co(NH}_3)_6]^{2+}$ also has cobalt in the $+2$ oxidation state, but the coordination environment is considered to give higher stabilization than $[\text{Co(NH}_3)_4]^{2+}$.
3. $[\text{Co(NH}_3)_6]^{3+}$ has cobalt in the $+3$ oxidation state, so the crystal field splitting is greater and CFSE increases further.
4. $[\text{Co(en)}_3]^{3+}$ has cobalt in the $+3$ oxidation state and en is taken as the stronger field ligand, so this complex has the maximum CFSE.

Therefore, the increasing order is: $[\text{Co(NH}_3)_4]^{2+} < [\text{Co(NH}_3)_6]^{2+} < [\text{Co(NH}_3)_6]^{3+} < [\text{Co(en)}_3]^{3+}$

Therefore, the correct option is C.

Given: The complexes are $[\text{Co(NH}_3)_4]^{2+}$, $[\text{Co(NH}_3)_6]^{2+}$, $[\text{Co(NH}_3)_6]^{3+}$ and $[\text{Co(en)}_3]^{3+}$.

Find: Their increasing order of crystal field stabilization energy.

Use the qualitative rules stated in the solution:

• Higher metal oxidation state gives larger crystal field splitting.
• Stronger field ligands give larger splitting.

So first compare oxidation states: $\text{Co}^{3+} > \text{Co}^{2+} \quad \text{in crystal field splitting}$ Hence both $\text{Co}^{3+}$ complexes must lie above the $\text{Co}^{2+}$ complexes in CFSE.

Now compare ligands for the $\text{Co}^{3+}$ complexes:

• $[\text{Co(NH}_3)_6]^{3+}$ contains NH$3$
• $[\text{Co(en)}_3]^{3+}$ contains en

According to the provided solution, en gives the stronger field here, so: $[\text{Co(NH}_3)_6]^{3+} < [\text{Co(en)}_3]^{3+}$

Among the $\text{Co}^{2+}$ complexes, the provided working places: $[\text{Co(NH}_3)_4]^{2+} < [\text{Co(NH}_3)_6]^{2+}$

Combining both comparisons: $[\text{Co(NH}_3)_4]^{2+} < [\text{Co(NH}_3)_6]^{2+} < [\text{Co(NH}_3)_6]^{3+} < [\text{Co(en)}_3]^{3+}$

Thus, the correct option is C.