Osmium tetroxide properties

Chemical ingenuity in using the properties of the elements and their compounds has allowed analyses to be carried out by processes analogous to the generation of hydrides. Osmium tetroxide is very volatile and can be formed easily by oxidation of osmium compounds. Some metals form volatile acetylacetonates (acac), such as iron, zinc, cobalt, chromium, and manganese (Figure 15.4). Iodides can be oxidized easily to iodine (another volatile element in itself), and carbonates or bicarbonates can be examined as COj after reaction with acid. 

Among oxo-metals, osmium tetroxide is a particularly intriguing oxidant since it is known to oxidize various types of alkenes rapidly, but it nonetheless eschews the electron-rich aromatic hydrocarbons like benzene and naphthalene (Criegee et al., 1942 Schroder, 1980). Such selectivities do not obviously derive from differences in the donor properties of the hydrocarbons since the oxidation (ionization) potentials of arenes are actually less than those of alkenes. The similarity in the electronic interactions of arenes and alkenes towards osmium tetroxide relates to the series of electron donor-acceptor (EDA) complexes formed with both types of hydrocarbons (26). Common to both arenes and alkenes is the immediate appearance of similar colours that are diagnostic of charge-transfer absorp-... 

For this purpose it is necessaiy to give sufficient contrast to a thin film of the frozen sample, for example, by use of osmium tetroxide. Then the sample can be viewed directly in the TEM (at — 196°C). The adjustment of the temperature to — 196°C produces a very low vapor pressure, especially of water, so that the examination of the probe is possible by preservation of the microstructure despite the high vacuum. A disadvantage of cryo-TEM is the classification of vesicles according to their size. Due to the fluid property of the vesicle dispersion prior to freezing, the thickness of the sample film varies from the center to the outside. Hence the smaller vesicles stay in the center, where the film is thin, while the larger ones remain at the outside margin in the thicker part of the film. In this outer part, the vesicles evade... 

Other functionalized supports that are able to serve in the asymmetric dihydroxylation of alkenes were reported by the groups of Sharpless (catalyst 25) [88], Sal-vadori (catalyst 26) [89-91] and Cmdden (catalyst 27) (Scheme 4.13) [92]. Commonly, the oxidations were carried out using K3Fe(CN)g as secondary oxidant in acetone/water or tert-butyl alcohol/water as solvents. For reasons of comparison, the dihydroxylation of trons-stilbene is depicted in Scheme 4.13. The polymeric catalysts could be reused but had to be regenerated after each experiment by treatment with small amounts of osmium tetroxide. A systematic study on the role of the polymeric support and the influence of the alkoxy or aryloxy group in the C-9 position of the immobilized cinchona alkaloids was conducted by Salvadori and coworkers [89-91]. Co-polymerization of a dihydroquinidine phthalazine derivative with hydroxyethylmethacrylate and ethylene glycol dimethacrylate afforded a functionalized polymer (26) with better swelling properties in polar solvents and hence improved performance in the dihydroxylation process [90]. 

One of the important properties of osmium is the formation of gases when the metal is exposed to air. These fumes are extremely toxic, which limits osmiums usefulness. Osmium is a hard, tough, brittle, bluish-white metal that is difficult to use except in a powder form that oxidizes into osmium tetroxide (OsO ), which not only has objectionable odor but also is toxic. 

There are other stereospecific olefin addition processes which occur with cis or syn stereochemistry. Common examples include catalytic hydrogenation, hydroboration/oxidation, and dihydroxylation using osmium tetroxide. The stereospecificity of these syn additions requires that die facial properties of the olefinic bond be maintained throughout die addition process and that both new bonds are formed to the same face of the olefin. This is normally accomplished by a concerted syn addition to the n system. 

CXXXI. Its properties are those to be expected of such a structure for example, the UV-spectrum shows absorption at 224 m/u. (e = 14000) corresponding to one diene chromophore in the C4o-molecule, and oxidation by osmium tetroxide-sodium chlorate followed by periodate fission gives both acetaldehyde and formaldehyde. The Hofmann degradation can be completed by treatment of descurarine with alkali to yield the ditertiary ether base CXXVIII (129). 

Properties. Osmium tetroxide forms pale yellow crystals with a very characteristic odour (a possible description is that of a mixture of ozone and damp hay). It has a considerable vapour pressure at room temperature and so must be kept in stoppered container or (preferably) in sealed ampoules. The vapour is toxic (TLV 2.5 p.p.m.).558a The long-term occupational exposure limit (OEL) is 0.002mgm 3. 558b... 

The physical properties, preparation and reactions of ruthenium tetroxide have been reviewed by Lee and van den Engh, Rylander," Haines and Hetuy and Lange. A more vigorous oxidant than osmium tetroxide, its reaction with double bonds produces only cleavage products. " Under neutral conditions aldehydes are formed from unsaturated secondary carbons while carboxylic acids are obtained under alkaline or acidic conditions. For example, Shalon and Elliott" found that ruthenium tetroxide reacted with compound (11) to give the corresponding aldehyde under neutral conditions, but that a carboxylic acid was formed in acidic or alkaline solvents (equation 23). 

Fig. 4. Frequently cited ascidian blood cell types. The middle column refers to staining properties of each type of cell with the two reagents osmium tetroxide and the pH indicator neutral red. Free tunichrome would reduce osmium tetroxide, as would lower oxidation states of vanadium.

Ideas about the octet were really intrinsic to the periodic table and the law of octaves, the periodic repetition of chemical properties every eighth element (noble gases were unknown in the 1860s). Mendeleev observed that the highest known valence of any element was 8 [for example, osmium tetroxide (OSO4), the valence of oxygen is 2]. He found that if one summed the (periodic table) column number and multiplied by the number of equivalents, the total was commonly 8 HCl (1 + 7) H2O ([1 times 2] + 6) ASH3 (5 + [1 times 3]). 

The synthesis of porphyrinoids containing nonpyrrolic heterocycles is of interest for studying the intrinsic properties of porphoryins. These porphyrin-like molecules possess varying absorption and emission properties and abilities to interact with metals or other analytes. Akhigbe and Bruckner converted meso-tetraphenylporphyrine 140 to diketone 141 using osmium tetroxide and DDQ oxidation (Scheme 5.33T Oxime formation followed by a... 

Cundari et aZ. have used an effective core potential model to study the nonlinear optical properties of a transition metal complex, osmium tetroxide. 

Incident 7.2.3.1 involved potential exposure to osmium tetroxide (OSO4). OsOa is hazardous due to its high toxicity and the fact that it easily sublimes at room temperature and pressure. It has little or no warning properties since you can t smeU it at concentrations that can seriously harm you. (See also Chemical Connection 7.1.4.1 If I Can Smell It, Am I in Danger )... 

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