Olation
At low pH, many metal ions exist in aqueous solution as aqua coordination complexes, often with the formula [M(HO)]. As the pH increases, one O-H bond ionizes to give the hydroxide complex, the conjugate base of the parent hexaaqua complex:
The hydroxo complex is poised to undergo olation, which is initiated by displacement of one water by a neighboring complex:
In this product, the hydroxide ligand bridges between the two metals, this bridge is denoted with the symbol μ.
In the resulting 5+ ion, the remaining water and hydroxo ligands are highly acidic and the ionization and condensation processes can continue at still higher pHs. The formation of the oxo-dimer is a process called "oxolation," although sometimes olation and oxolation are not distinguished:
Ultimately one observes the formation of the metal oxide:
Bisulfide
At physiological pH, hydrogen sulfide is usually fully ionized to bisulfide (HS) so in biochemical settings, "hydrogen sulfide" is often used to mean, bisulfide or hydrosulfide. Hydrosulfide has been identified as the third gasotransmitter along with nitric oxide and carbon monoxide. Its specific role and direct interaction with signaling molecules is the subject of ongoing research.