A metal aquo complex is a coordination compound formed by the reaction of a metal ion with one or more water molecules. Such complexes are commonly encountered in aqueous chemistry and are essential in many biological processes. Some common synonyms for metal aquo complexes include metal-water complexes, metal-aqua species, hydrated metal ions, and aqua ligands. These terms essentially refer to the same phenomenon and describe the same type of chemical species, but are used interchangeably depending on the context of use. Other related terms that are used in this context include coordination compounds, chelates, and metal complexes, which are also important in the realm of inorganic chemistry.