A complex ion is a 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 or negatively charged ions that donate electron pairs to the metal, creating a coordination complex. The central metal often belongs to the transition metals group, such as those found in the Periodic Table. Complex ions play a crucial role in various chemical processes, including catalysis and biological functions. They are essential in areas like coordination chemistry and are involved in the formation of colored solutions, which is significant in fields such as analytical chemistry and biochemistry.