The Free Dictionary describes a complexation reaction as a chemical reaction between a ligand and a metal ion. A ligand is an ionic entity with at least one atom that contains an unshared pair of electrons.
The Chemistry Learning Center explains that complexation reactions form chemical complexes due to the increased concentration of reactive elements that result in flooding. The resulting ion is called a complex ion, and it features a noticeably brighter shade of color.
Chem Wiki describes complexation reactions as being divided into four categories, including: catalysis, enzymes, chain reactions and ionic mobility, and electrophoresis. Catalysis reactions are some of the most important complexation reactions due to the fact that they cause reactions between other substances to occur. Chem Wiki states that catalysts create both forward and reverse reactions without distinguishing between them, which means that they do not affect the equilibrium of a substance in the process.
The Chemistry Learning Center also notes that complexation ions involve more than one substance species and carry a positive charge like all other ions do when they are present in a solution. The positivity found within complexation ions created from complexation reactions makes them an essential component of many ionic solutions.