Oxalate (\( C_2O_4^{2-} \)), a bidentate ligand, possesses two donor atoms for simultaneous metal ion coordination: the oxygen atoms within its carboxylate groups (-COO).
Structure: \[ \text{[O-C=O]} - \text{[O-C=O]}. \]
In contrast, \( Cl^- \) and \( I^- \) are monodentate ligands, each with a single donor atom. Similarly, \( H_2O \) functions as a monodentate ligand, utilizing its single oxygen atom as the donor.
Applications: Oxalate is a component of complexes such as potassium ferrioxalate (\( K_3[Fe(C_2O_4)_3] \)).