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In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom or ion to form a coordination complex. Ligands are classified based on the number of donor atoms that attach to the metal center.

Ethylenediamine (en) is an organic molecule with the formula H₂N–CH₂–CH₂–NH₂. It contains two nitrogen atoms, each with a lone pair of electrons capable of coordinating to a metal ion. Since it uses two donor sites to bind to the central metal atom, ethylenediamine is called a bidentate ligand.

Bidentate ligands form chelate complexes, which are more stable than complexes formed with monodentate ligands. This stability arises from the chelate effect, where the ligand forms a ring including the metal ion, reducing the entropy loss during complex formation. Ethylenediamine is commonly used in coordination compounds with transition metals like Ni²⁺, Co³⁺, and Cu²⁺, producing colorful and stable complexes.

Other ligand types include monodentate ligands, which bind through a single donor atom (e.g., NH₃, H₂O), tridentate ligands that coordinate through three donor atoms, and polydentate ligands, which contain multiple donor atoms capable of forming several bonds to the metal center.

Recognizing the denticity of ligands, such as ethylenediamine being bidentate, is essential for understanding complex geometry, coordination number, and stability in inorganic chemistry. Bidentate ligands also play important roles in analytical chemistry, catalysis, and bioinorganic systems due to their ability to stabilize metal ions efficiently.