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Coordination compounds contain a central metal atom or ion surrounded by ligands bonded through coordinate bonds. Werner's theory (1893) introduced two types of valencies: primary valence (oxidation state, ionisable) and secondary valence (coordination number, non-ionisable, directional). The coordination entity is enclosed in square brackets and retains its identity in solution.
Ligands are classified by denticity: monodentate (1 donor atom — NH3, Cl-, H2O), bidentate (2 — en, ox), polydentate (3+ — EDTA with 6). Ambidentate ligands like CN-, NO2-, and SCN- can bond through different atoms. The coordination number equals the total number of donor atoms bonded to the metal, which differs from the number of ligands when polydentate ligands are present.
The chelate effect explains why polydentate ligands form more stable complexes than equivalent monodentate ligands — primarily driven by favourable entropy change. Five-membered chelate rings (formed by en and ox) are most stable. The EAN rule (Sidgwick) predicts that stable complexes often achieve the electron configuration of the nearest noble gas, though many exceptions exist.