Bidentate ligands are molecules or ions that have the ability to form two separate coordinate covalent bonds with a central metal atom or ion in a coordination complex. These ligands contain two atoms or groups that can each donate a lone pair of electrons to the metal, forming two bonds. Bidentate ligands play a significant role in coordination chemistry and often lead to the formation of more stable and geometrically interesting complexes.

Here are some examples of bidentate ligands:

1. Ethylene diamine (en): Ethylene diamine is a common bidentate ligand. It has two amino (-NH2) groups, each with a lone pair of electrons, allowing it to form two bonds with a metal center.
2. Oxalate ion (C2O4^2-): The oxalate ion contains two carboxylate groups (-COO^-), each with a lone pair of electrons. It can form two coordinate bonds with a metal ion.
3. 1,2-Diaminocyclohexane: This organic molecule contains two amino groups attached to a cyclohexane ring, making it a bidentate ligand.
4. Glycinato ion (Gly^- or NH2CH2COO^-): The glycinate ion has both an amino group and a carboxylate group, which can each bond to a metal ion.
5. Acetylacetonate ion (acac^- or C5H7O2^-): This ion has two oxygen atoms with available lone pairs, allowing it to bond to a metal center through both oxygen atoms.
6. Bipyridine (bipy): Bipyridine is an aromatic organic compound with two nitrogen atoms that can each form a coordinate bond with a metal ion.
7. Phenanthroline (phen): Phenanthroline is another aromatic compound with two nitrogen atoms that can serve as bidentate ligands.

The formation of bidentate complexes results in increased stability due to the chelation effect, where the ligands form a ring-like structure around the metal center. This chelate effect enhances the overall structural integrity of the coordination complex. Bidentate ligands also contribute to the determination of complex geometries and properties, including colors and reactivity.