Osmium oxidation states

The most common oxidation states and the corresponding electronic configurations of osmium ate +2 and + (t5 ), which ate usually octahedral. Stable oxidation states that have various coordination geometries include —2 and 0 to +8 (P] The single most important appHcation is OsO oxidation of olefins to diols. Enantioselective oxidations have also been demonstrated. 

Coordination Compounds. Osmium in oxidation states from +2 to +8 forms a wide range of complexes with nitrogen ligands. Amine... 

Like mthenium, amines coordinated to osmium in higher oxidation states such as Os(IV) ate readily deprotonated, as in [Os(en) (NHCH2CH2NH2)] [111614-75-6], This complex is subject to oxidative dehydrogenation to form an imine complex (105). An unusual Os(IV) hydride, [OsH2(en)2] [57345-94-5] has been isolated and characterized. The complexes of aromatic heterocycHc amines such as pyridine, bipytidine, phenanthroline, and terpyridine ate similar to those of mthenium. Examples include [Os(bipy )3 [23648-06-8], [Os(bipy)2acac] [47691-08-7],... 

Organometallic Compounds. Osmium forms numerous mononuclear and polynuclear organometaUic complexes, primarily iu lower oxidation states. There are many complexes of carbon monoxide, such as [Os(CO)3] [16406-49-8], [Os(CO) H2] [22372-70-9], [Os3(CO)2 H2] [56398-24-4],... 

Table 25.3 Oxidation states and stereochemistries of some compounds of iron, mthenium and osmium...

Ruthenium and osmium have no oxides comparable to those of iron and, indeed, the lowest oxidation state in which they form oxides is -t-4. RUO2 is a blue to black solid, obtained by direct action of the elements at 1000°C, and has the rutile (p. 961) structure. The intense colour has been suggested as arising from the presence of small amounts of Ru in another oxidation state, possibly - -3. 0s02 is a yellowish-brown solid, usually prepared by heating the metal at 650°C in NO. It, too, has the rutile structure. 

Iron forms barely any complexes in oxidation states above +3, and in the +8, +7 and +6 states those of ruthenium are less numerous than those of osmium. complexes are confined... 

This is the second of the common oxidation states for iron and is familiar for ruthenium, particularly with Group 15-donor ligands (Ru probably forms more nitrosyl complexes than any other metal). Osmium(II) also produces a considerable number of complexes but is usually more strongly reducing than Ru". 

Unlike ruthenium (and other platinum metals) osmium forms chlorides and bromides in a range of oxidation states [11,12]. 

The complexes of ruthenium and osmium in the same oxidation state are generally similar and are, therefore, treated together the structural (Table 1.3) and vibrational data (Table 1.4) have been set out in some detail to demonstrate halogen-dependent trends. 

Within the osmium complexes in oxidation states (II-IV) [11,12] the stability of the +4 oxidation state becomes more important. Ammine and tertiary phosphine complexes have been selected for detailed examination. 

Comparison with data (mainly obtained from EXAFS measurements) on osmium diarsine complexes (Table 1.14) shows that as the oxidation state increases, osmium—halogen bonds shorten whereas Os-P and Os—As bonds lengthen. Bond shortening is predicted for bonds with ionic character,... 

There is a vigorously expanding chemistry of compounds of ruthenium and osmium in high oxidation states [3, 4, 11, 12], particularly of dioxo and nitrido compounds, though recently some striking developments have taken place in imide chemistry. 

The olive-green osmium(VI) octaethylporphyrin complex 0s02(0EP) (IR v(0s—0) 825 cm-1) is representative of a number of osmyP porphyrins [185] they can readily be transformed into a number of osmium porphyrins in lower oxidation states (Figure 1.73). 

The change in formal oxidation state from osmium(VI) to osmium(II) is noteworthy [192],... 

Organic imide ligands have also been used to stabilize high oxidation states. The best example of this is the osmium(VIII) compound Os(NBul)4, which has a distorted tetrahedral OsN4 core (N—Os—N 104.6-111.9° Os—N 1.750 A) [194],... 

Ruthenium and osmium form some remarkably stable alkyls and aryls compounds, often in unusually high oxidation states. 

The chemistry of ruthenium, osmium, rhodium, iridium, palladium and platinum in the higher oxidation states. D. J. Gulliver and W. Levason, Coord. Chem. Rev., 1982,46,1-127 (1131). 

Notice that ruthenium and osmium have all possible oxidation states and that even gold and mercury, which lie near the end of the block, can be found in three oxidation states. 

Polynuclear transition metal cyanides such as the well-known Prussian blue and its analogues with osmium and ruthenium have been intensely studied Prussian blue films on electrodes are formed as microcrystalline materials by the electrochemical reduction of FeFe(CN)g in aqueous solutionThey show two reversible redox reactions, and due to the intense color of the single oxidation states, they appear to be candidates for electrochromic displays Ion exchange properties in the reduced state are limited to certain ions having similar ionic radii. Thus, the reversible... 

Many carbonyl and carbonyl metallate complexes of the second and third row, in low oxidation states, are basic in nature and, for this reason, adequate intermediates for the formation of metal— metal bonds of a donor-acceptor nature. Furthermore, the structural similarity and isolobal relationship between the proton and group 11 cations has lead to the synthesis of a high number of cluster complexes with silver—metal bonds.1534"1535 Thus, silver(I) binds to ruthenium,15 1556 osmium,1557-1560 rhodium,1561,1562 iron,1563-1572 cobalt,1573 chromium, molybdenum, or tungsten,1574-1576 rhe-nium, niobium or tantalum, or nickel. Some examples are shown in Figure 17. 

Silvery, shiny, and hard. Unique metal, gives off an odor as it forms volatile 0s04 on the surface (oxidation states 81). Osmium is the densest element (22.6 g cm3 record ). Was replaced in filaments (Osram) by the cheaper tungsten. Used in platinum alloys and as a catalyst. Haber s first catalyst in ammonia synthesis was osmium, which fortunately could be replaced by doped iron. The addition of as little as 1 to 2 % of this expensive metal increases the strength of steel (e.g. fountain-pen tips, early gramophone needles, syringe needles). 

Thus the reactivity of transition metal-carbene complexes, that is, whether they behave as electrophiles or nucleophiles, is well explained on the basis of the frontier orbital theory. Studies of carbene complexes of ruthenium and osmium, by providing examples with the metal in either of two oxidation states [Ru(II), Os(II) Ru(0), Os(O)], help clarify this picture, and further illustrations of this will be found in the following sections. 

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