Question

The correct order of the following complexes in terms of their crystal field stabilization energies is:

• A. \( [\text{Co(NH}_3)_6]^{2+} < [\text{Co(NH}_3)_6]^{3+} < [\text{Co(NH}_3)_4]^{2+} < [\text{Co(en)}_3]^{3+} \)
• B. \( [\text{Co(en)}_3]^{3+} < [\text{Co(NH}_3)_6]^{3+} < [\text{Co(NH}_3)_6]^{2+} < [\text{Co(NH}_3)_4]^{2+} \)
• C. \( [\text{Co(NH}_3)_4]^{2+} < [\text{Co(NH}_3)_6]^{2+} < [\text{Co(NH}_3)_6]^{3+} < [\text{Co(en)}_3]^{3+} \)
• D. \( [\text{Co(NH}_3)_4]^{2+} < [\text{Co(NH}_3)_6]^{2+} < [\text{Co(en)}_3]^{3+} < [\text{Co(NH}_3)_6]^{3+} \)

Hint:

For coordination compounds, the crystal field stabilization energy is influenced by the oxidation state of the metal and the ligand field strength. Stronger field ligands and higher oxidation states generally lead to higher CFSE.

Answer 1

The Correct Option is C

The order of Crystal Field Stabilization Energies (CFSE) for the complexes is determined by the ligand's field strength, the metal's oxidation state, and the complex's geometry. Octahedral geometry is assumed unless otherwise specified.

1. Ligand Field Strength: Stronger ligands induce greater d-orbital splitting. Ethylenediamine (en) is a stronger ligand than ammonia (NH₃) per the spectrochemical series.
2. Metal Oxidation State: Higher oxidation states enhance d-orbital splitting.
3. Complex Geometry: Assumed octahedral.

Individual complex analyses:

• \([\text{Co(NH}_3)_4]^{2+}\): Co is +2; NH₃ is a moderate ligand.
• \([\text{Co(NH}_3)_6]^{2+}\): Co is +2; increased coordination with six NH₃ ligands slightly raises field strength compared to four.
• \([\text{Co(NH}_3)_6]^{3+}\): Co is +3; the higher positive charge significantly increases field splitting.
• \([\text{Co(en)}_3]^{3+}\): Co is +3; en ligands are stronger than NH₃, leading to maximum complex stability.

Based on these factors and the spectrochemical series, the increasing order of CFSE is:

1. [\text{Co(NH}_3)_4]^{2+}
2. [\text{Co(NH}_3)_6]^{2+}
3. [\text{Co(NH}_3)_6]^{3+}
4. [\text{Co(en)}_3]^{3+}

This order reflects the combined effects of stronger ligands and higher oxidation states, which yield the greatest CFSE.

The definitive order is:

\( [\text{Co(NH}_3)_4]^{2+} < [\text{Co(NH}_3)_6]^{2+} < [\text{Co(NH}_3)_6]^{3+} < [\text{Co(en)}_3]^{3+} \)