Complex compounds in chemistry are compounds with a complex structure. They consist of central atoms or ions surrounded by molecules or ions called ligands. Ligands in complex compounds form covalent bonds to the central atom via donor atoms. They are characterized by coordination number, indicating how many ligands are attached to the central atom. Coordination numbers can range from 2 to 12, but the most common ones are 4 and 6. Complex compounds play a vital role in industrial catalytic processes, in medicine as drugs, and in biological systems