Understanding Oxalate as a Ligand
The classification of a ligand as a strong field or weak field is based on its ability to split the d-orbitals of a transition metal ion in a coordination complex. Oxalate ion (C₂O₄²⁻) is an interesting case in this context.
Nature of Oxalate Ligand
- Oxalate is a bidentate ligand, meaning it can coordinate to a metal ion at two sites.
- Its chelating ability enhances its overall strength as a ligand, allowing for more stable complexes.
Crystal Field Theory
- According to Crystal Field Theory, ligands can be classified based on the strength of the electric field they create around the metal ion.
- Strong field ligands cause a large splitting of the d-orbitals, favoring low-spin configurations.
- Weak field ligands create a smaller splitting, allowing for high-spin configurations.
Oxalate's Position
- Oxalate is generally considered a moderate to strong field ligand.
- It tends to cause significant splitting of the d-orbitals, especially in octahedral complexes, and can lead to low-spin states in certain metal complexes.
Examples of Metal Complexes
- Complexes like [Fe(C₂O₄)₃]³⁻ can exhibit low-spin configurations due to the strong field nature of oxalate.
- In contrast, with some metals, oxalate may not exhibit strong field characteristics, illustrating its moderate nature.
Conclusion
- In summary, oxalate is primarily viewed as a moderate to strong field ligand, mainly due to its bidentate nature and the ability to create stable complexes with transition metals. Its classification can vary depending on the specific metal ion involved and the overall geometry of the complex formed.

Weaker than H2O