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:
Osmium tetroxide
Osmium(VIII) oxide forms monoclinic crystals. It has a characteristic acrid chlorine-like odor. The element name osmium is derived from "osme", Greek for "odor". OsO is volatile: it sublimes at room temperature. It is soluble in a wide range of organic solvents. It is also moderately soluble in water, with which it reacts reversibly to form osmic acid (see below). "Pure" osmium(VIII) oxide is probably colourless and it has been suggested that its yellow hue is due to osmium dioxide (OsO) impurities. The osmium tetroxide molecule is tetrahedral and therefore non-polar. This nonpolarity helps OsO penetrate charged cell membranes. OsO is 518 times more soluble in carbon tetrachloride than in water.