Here's why:

* Structure: Oxalic acid (H₂C₂O₄) has two carboxyl groups (-COOH) which can each donate a lone pair of electrons from the oxygen atom.

* Coordination: These two carboxyl groups can coordinate to a metal ion, forming two separate bonds.

* Bidentate: Because it forms two bonds with the metal ion, it is classified as a bidentate ligand.

Let me know if you'd like to know more about ligands or coordination chemistry!