Concept:A ligand is classified by the number of donor atoms it uses to coordinate to a metal ion.Explanation:The oxalate ion, C2​O42−​, contains two negatively charged oxygen atoms.Each of these oxygen atoms can donate a lone pair of electrons to the metal ion.Therefore, it forms two coordinate bonds with the central metal ion.A ligand that binds through two donor atoms is called a bidentate ligand.
Answer:The oxalate ion is a bidentate ligand. Hence, the correct option is B. Bidentate.
