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How are polydentate ligands different from monodentate ligands?

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Structural diagram of the vitamin B12 coenzyme molecule, highlighting its organometallic core with five nitrogen-cobalt bonds and one cobalt-carbon bond at the center, surrounded by complex organic groups. — coordination compound Coordination compounds contain a central metal atom surrounded by nonmetal atoms or groups of atoms, called ligands. For example, vitamin B₁₂ is made up of a central metallic cobalt ion bound to multiple nitrogen-containing ligands.

In chemistry, the primary difference between monodentate and polydentate ligands lies in the number of atoms through which they attach to a central metal atom or ion in a coordination compound.1

Monodentate ligands: These ligands attach to the central metal atom through a single donor atom. Common examples include water ( $H_{2} O$ ), ammonia ( $N H_{3}$ ), and various anions like chloride ( $C l^{-}$ ) or cyanide ( $C N^{-}$ ).1
Polydentate ligands: These ligands attach to the central metal atom through two or more donor atoms. When a polydentate ligand binds to a metal, it often forms a ring structure, resulting in a complex known as a chelate. Because these ligands form multiple bonds with the metal, the resulting chelate complexes tend to be very stable.12

Examples of polydentate ligands include didentate ligands (such as ethylenediamine) and hexadentate ligands (such as ethylenediaminetetraacetate, or EDTA). Notably, some ligands can function as either monodentate or polydentate depending on the specific chemical environment.2