Complex ions are charged species formed when a central metal atom or ion bonds with surrounding molecules or ions, known as ligands. These ligands can be neutral molecules, like water or ammonia, or negatively charged ions, such as chloride or cyanide. The central metal typically belongs to the transition metals group of the periodic table and can coordinate with multiple ligands, creating a stable structure. The formation of complex ions is essential in various chemical processes, including catalysis and biological functions. For example, the hemoglobin in blood contains a complex ion of iron that binds to oxygen, allowing for efficient transport throughout the body. Understanding complex ions is crucial in fields like coordination chemistry and biochemistry.