Hydroxide
The hydroxides of the transition metals and post-transition metals usually have the metal in the +2 (M = Mn, Fe, Co, Ni, Cu, Zn) or +3 (M = Fe, Ru, Rh, Ir) oxidation state. None are soluble in water, and many are poorly defined. One complicating feature of the hydroxides is their tendency to undergo further condensation to the oxides, a process called olation. Hydroxides of metals in the +1 oxidation state are also poorly defined or unstable. For example, silver hydroxide Ag(OH) decomposes spontaneously to the oxide (AgO). Copper(I) and gold(I) hydroxides are also unstable, although stable adducts of CuOH and AuOH are known. The polymeric compounds M(OH) and M(OH) are in general prepared by increasing the pH of an aqueous solutions of the corresponding metal cations until the hydroxide precipitates out of solution. On the converse, the hydroxides dissolve in acidic solution. Zinc hydroxide Zn(OH) is amphoteric, forming the tetrahydroxidozincate ion in strongly alkaline solution.
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.