text: Given that a virtually uncountable variety of octahedral complexes exist, it is not surprising that a wide variety of reactions have been described. These reactions can be classified as follows:
  * Ligand substitution reactions (via a variety of mechanisms)
  * Ligand addition reactions, including among many, protonation
  * Redox reactions (where electrons are gained or lost)
  * Rearrangements where the relative stereochemistry of the ligand changes within the coordination sphere.
  Many reactions of octahedral transition metal complexes occur in water. When an anionic ligand replaces a coordinated water molecule the reaction is called an anation. The reverse reaction, water replacing an anionic ligand, is called aquation. For example, the  slowly yields  in water, especially in the presence of acid or base. Addition of concentrated HCl converts the aquo complex back to the chloride, via an anation process.