Osmium Tetroxide and Its Applications By W. P. Griffith Department of Inorganic Chemistry, Imperial College, London Osmium tetroxide, OsO,, is the most im- (maximum permitted atmospheric level portant and most easily prepared compound 2 x 10-6 glm3). It attacks the eyes, causing of osmium. It has a number of specific blurring of vision and, in very severe cases, applications in organic chemistry and in temporary blindness, and it also attacks and biochemistry, and it is with these that this irritates the nose and throat linings and may article is chiefly concerned; it is also a useful exacerbate bronchial conditions. Fortunately source of osmium compounds. It is re- it has a characteristic, penetrating, rather markable in that it is one of the few volatile ozone-like smell and this, together with its oxides of a heavy metal and that although the irritant effects, makes accidents with it rare. osmium is octavalent (of all elements only In the event of OsO, vapour attacking the eyes osmium and ruthenium reach as high an or the skin the remedy is washing with copious oxidation state) it is a reasonably controllable quantities of water. Spilled OsO, may be oxidising agent. It is from this latter property reduced by dissolving it in alkali in 50 per cent that most of its applications derive. water-ethanol solution, and the non-volatile osmate salt may then be washed away. Discovery and Preparation The compound was discovered in 1S03 by Physical and Chemical Properties Smithson Tennant (1761-1815), and in the The properties of OsO, have been reviewed same year he isolated metallic osmium from by Griffith (3). The solid forms pale yellow it (I). Fusion with alkali of the black residue monoclinic crystals (M.P. 40.6”C, B.P. remaining after treatment of native platinum 121.2T,density 4.906). It is fairly soluble in with aqua regia followed by extraction and water (7.2 g/Ioo ml at 25°C) and extremely acidification of the melt gave soluble in inert organic solvents (e.g. “a pungent and peculiar smell . from the 350 g/Ioo ml. of carbon tetrachloride). It extrication of a very volatile metal oxide; and, exists in the gaseous, solid or solution state as as this smell is one of its most distinguishing discrete molecular tetrahedra (Os-0 distance characters, I should on that account incline to call this metal Osmium” (I) (oopq-smell, 1.717 A). The thermodynamic properties of odour). OsO, have been reviewed (4). Industrially, OsO, is made from crude In almost all of its chemical reactions, a platinum concentrates by oxidative acid number of which are summarised in the distillation and is then separated from ru- diagram, OsO, is reduced to compounds con- thenium tetroxide. In the laboratory it is best taining lower oxidation states. With ammonia, made by direct oxidation of osmium metal (2) however, the tetrahedral osmiamate ion or by the acid distillation with chlorate of [OsO,N]- is formed, which is isoelectronic almost any osmium compound. with OsO,. Toxicity Applications in Organic Chemistry The solid has an appreciable vapour pres- The valuable function of OsO, in the sure at room temperature and should be oxidation of olefins has been known and handled with care. The vapour is poisonous applied since 1913, and now constitutes one of Platinum Metals Rev., 1974, 18, (3), 94-96 94 the major industrial and laboratory uses of the The reagent may be used either alone in an compound, particularly for reactions of inert solvent, the resulting osmate (VI) ester steroids and sugars. The subject has been being decomposed by bisulphite or hydrogen well reviewed by Rylander (5) and also by sulphide (6,7), or, more commonly and Fieser and Fieser (6). It is used mainly for the economically, in the presence of an additional cis-hydroxylation of olefinic double bonds to oxidant such as chlorate, hydrogen peroxide give glycols, for which purpose it is the or periodate. These regenerate the OsO, smoothest and most efficient general reagent which is therefore functioning as a catalyst. known. It tends to react faster with strained There is little doubt that cyclic osmate (VI) olefins, particularly in the presence of esters are involved in these reactions, and a pyridine (7). Aromatic hydrocarbons are recent X-ray study showed that mono-esters hydroxylated only at the most reactive are dimeric (I) with a dioxo bridge, the osmium aromatic site (e.g. phenanthrene at the having square-based pyramidal coordination 9,10 position (8)). (8). The overall reaction is shown below: Platinum Metals Rev., 1974, 18, (3), 95 Examples of simple hydroxylations with attack of double bonds in unsaturated lipids OSO, are the production of glycerol from to give osmate (VI) esters. There is evidence ally1 alcohol and of ethylene glycol from that mono-esters of type (I) above may be ethylene. The compound has been used in involved, in which case the fixation property the synthesis of such species as cortisone, may possibly arise from the formation of progesterone, and of reserpine-type alkaloids, dioxo bridges (9). and also in the degradative investigation of There are still many questions to be natural products such as columbin (6). The answered, however-whether mono- or di- glycol cleavage properties of periodate may be esters are normally formed (12), and whether used together with the oxidising properties of in the dehydrated tissue the osmium is further OsO, to convert olefins to aldehydes (e.g. reduced to osmium (IV) and perhaps shifted trans-stilbene to benzaldehyde, cyclohexene away from the original double-bond sites. to adipaldehyde), to ketones or to epoxides The resolution of these problems is important (10). since OsO, is so extensively used, and it is necessary to know to what extent the fixed and In Biochemistry stained tissue is representative of the once The compound is extensively used (nor- living organism. mally in 2 per cent aqueous solution, often called “osmic acid”) for cell and tissue studies, Corrosion Prevention its unique fixation and staining properties Like some other heavy metal tetra-oxo having been recognised and used since 1861. species, osmium tetroxide in electrolytes It is used for both visible and electron has the property of passifying iron electrodes microscopy of biological materials, but now (13). the latter application is probably the more important. References I D. McDonald, Platinum Metals Rev., 1961, 5, The purpose of fixation is to “freeze” cells 146; Smithson Tennant, Phil. Trans., 1804, without destruction or disruption of their 94, 411 organisation or structure; staining is necessary 2 G. Brauer, Handbook of Preparative Chemis- try, Academic Press, New York, 1965, p. 1603 for the resolution of cellular structure by in- 3 W. P. Griffith, Chemistry of the Rarer creasing the apparent density of some parts of Platinum Metals, Interscience, New York, 1967; Quart. Rev., 1965, 19,254 the tissue only. OsO, is unique in that it both 4 B. N. Goldberg and L. G. Hepler, Chem. Rev., fixes and stains biological material. For the x968,68,229 electron microscopist its most important 5 P. N. Rylander, Engelhard Ind. Tech. Bull., 1968, 9, 90; F. R. Gunstone, Adv. Org. functions are the preservation of sub-cellular Chem., 1960, I, 103 ultrastructure and its ability to fix and stain 6 L. F. Fieser and M. Fieser, Reagents for membranes. For staining purposes it is often Organic Synthesis, Wiley, New York, 1967, PP 475,759 used with polar species such as uranyl or lead 7 R. Criegee, B. Marchand and H. Wannowius, ions. The normal method used is to pre-treat Ann., 1942, 550, 99 the tissue with aldehydes, then to treat it by 8 G. M. Badger, Qzturt. Rev., 1951,5,160 9 R. J. Collin, W. P. Griffith, F. Phillips and immersion in a dilute aqueous solution of A. C. Skapski, Biochim. Biophys. Acra, 1973, OsO, (or the tissue is exposed to OsO, vapour) 320, 745; J. Chem. Soc., Dalton Trans., 1974, I094 followed by washing, additional staining if I0 R. Pappo, D. S. Allen, R. U. Lemieux and required, dehydration with alcohol, em- W. S. Johnson, J. Org. Chem., 1956,z1,478 bedding in resin and cutting into thin sections I1 M. A. Hayat, Principles and Techniques of Electron Microscopy, Vol. I, van Nostrand, suitable for microscopy. I970 The mechanism of tissue fixation and I2 J. Riemersma, Biochim. Biophyx. Acta, 1968, 718; E. Korn,J. Cell Biol., 1967, staining by OsO, is far from clear, although it 152, D. 34, 627 is generally accepted that the first step is the 13 G. H. Cartledge, Corrosion, 1967,18,316t Platinum Metals Rev., 1974, 18, (3), 94 .