The formation of a complex between a metal ion, M, and a ligand, L, is in fact usually a substitution reaction. For example, In aqueous solutions, metal ions will be present as aquo ions, so the reaction for the formation of the first complex could be written as
:[M(HO)] + L  [M(HO)L] + HO
However, since water is in vast excess, the concentration of water is usually assumed to be constant and is omitted from equilibrium constant expressions. Often, the metal and the ligand are in competition for protons. For the equilibrium
:p M + q L + r H  MLH
a stability constant can be defined as follows:
The definition can easily be extended to include any number of reagents. It includes hydroxide complexes because the concentration of the hydroxide ions is related to the concentration of hydrogen ions by the self-ionization of water
Stability constants defined in this way, are association constants. This can lead to some confusion as pK values are dissociation constants. In general purpose computer programs it is customary to define all constants as association constants. The relationship between the two types of constant is given in association and dissociation constants.
In biochemistry, an oxygen molecule can bind to an iron(II) atom in a heme prosthetic group in hemoglobin. The equilibrium is usually written, denoting hemoglobin by Hb, as
: Hb + O  HbO
but this representation is incomplete as the Bohr effect shows that the equilibrium concentrations are pH-dependent. A better representation would be
: [HbH] + O  HbO + H
as this shows that when hydrogen ion concentration increases the equilibrium is shifted to the left in accordance with Le Châtelier's principle. Hydrogen ion concentration can be increased by the presence of carbon dioxide, which behaves as a weak acid.
:HO + CO   + H
The iron atom can also bind to other molecules such as carbon monoxide. Cigarette smoke contains some carbon monoxide so the equilibrium 
:HbO + CO   + O
is established in the blood of cigarette smokers.
Chelation therapy is based on the principle of using chelating ligands with a high binding selectivity for a particular metal to remove that metal from the human body.
Complexes with polyamino carboxylic acids find a wide range of applications. EDTA in particular is used extensively.