Osmium 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. 

The platinum-group metals (PGMs), which consist of six elements in Groups 8— 10 (VIII) of the Periodic Table, are often found collectively in nature. They are mthenium, Ru rhodium, Rh and palladium, Pd, atomic numbers 44 to 46, and osmium. Os indium, Ir and platinum, Pt, atomic numbers 76 to 78. Corresponding members of each triad have similar properties, eg, palladium and platinum are both ductile metals and form active catalysts. Rhodium and iridium are both characterized by resistance to oxidation and chemical attack (see Platinum-GROUP metals, compounds). 

Ruthenium and osmium have hep crystal stmetures. These metals have properties similar to the refractory metals, ie, they are hard, britde, and have relatively poor oxidation resistance (see Refractories). Platinum and palladium have fee stmetures and properties akin to gold, ie, they are soft, ductile, and have excellent resistance to oxidation and high temperature corrosion. 

High Temperature Properties. There are marked differences in the abihty of PGMs to resist high temperature oxidation. Many technological appHcations, particularly in the form of platinum-based alloys, arise from the resistance of platinum, rhodium, and iridium to oxidation at high temperatures. Osmium and mthenium are not used in oxidation-resistant appHcations owing to the formation of volatile oxides. High temperature oxidation behavior is summarized in Table 4. 

Selected physical properties of rhenium are summarized ia Table 1. The metal is silvery-white and has a metallic luster. It has a high density (21.02 g/cm ). Only platinum, iridium, and osmium have higher densities. The melting poiat of rhenium is higher than that of all other elements except tungsten (mp 3410°C) and carbon (mp 3550°C). 

Table 25.1 Some properties of the elements iron, ruthenium and osmium...

Osmium(II) forms no hexaaquo complex and [Os(NH3)g] +, which may possibly be present in potassium/liquid NH3 solutions, is also unstable. [Os(NH3)5N2] and other dinitrogen complexes are known but only ligands with good 7r-acceptor properties, such as CN, bipy, phen, phosphines and arsines, really stabilize Os , and these form complexes similar to their Ru analogues. 

In the iron, ruthenium, and osmium derivatives, there are eases of r] re-switeh on thermolysis followed by the elimination of small ligands. Organo-ruthenium speeies eontaining pyrazol-l-ylborate or -methane ligands with bulky substituents often have uneoordinated pyrazol-l-yl moieties and agostie R—B(C) - - - M interaetion. The latter sometimes influenees the properties of the jj -eoordinated speeies as well. 

Osmium makes a sluggish carbonyl hydrogenation catalyst but has the unusual property of reducing a, -unsaturated aldehydes to the unsaturated alcohol in good yield (85). The system has proved erratic high selectivity can only be obtained through prereduction of the catalyst just before use. 

Ruthenium, iridium and osmium The use of a fused cyanide electrolyte is the most effective means for the production of sound relatively thick coatings of ruthenium and iridium, but this type of process is unattractive and inconvenient for general purposes and does not therefore appear to have developed yet to a significant extent for industrial application. This is unfortunate, since these metals are the most refractory of the platinum group and in principle their properties might best be utilised in the form of coatings. However, several interesting improvements have been made in the development of aqueous electrolytes. 

Catalysts. The methanation of CO and C02 is catalyzed by metals of Group VIII, by molybdenum (Group VI), and by silver (Group I). These catalysts were identified by Fischer, Tropsch, and Dilthey (18) who studied the methanation properties of various metals at temperatures up to 800°C. They found that methanation activity varied with the metal as follows ruthenium > iridium > rhodium > nickel > cobalt > osmium > platinum > iron > molybdenum > palladium > silver. 

Plutonium-noble metal compounds have both technological and theoretical importance. Modeling of nuclear fuel interactions with refractory containers and extension of alloy bonding theories to include actinides require accurate thermodynamic properties of these materials. Plutonium was shown to react with noble metals such as platinum, rhodium, iridium, ruthenium, and osmium to form highly stable intermetallics. 

The platinum-group metals comprise ruthenium (Ru), rhodium (Rh) and palladium (Pd) from the second transition series and osmium (Os), iridium(Ir), and platinum (Pt) from thethird transition series. Little or no C VD investigation of palladium and osmium have been reported and these metalsarenotincludedhere. The properties of the other platinum-group metals are summarized in Table 6.9. 

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... 

Ruthenium-copper and osmium-copper clusters (21) are of particular interest because the components are immiscible in the bulk (32). Studies of the chemisorption and catalytic properties of the clusters suggested a structure in which the copper was present on the surface of the ruthenium or osmium (23,24). The clusters were dispersed on a silica carrier (21). They were prepared by wetting the silica with an aqueous solution of ruthenium and copper, or osmium and copper, salts. After a drying step, the metal salts on the silica were reduced to form the bimetallic clusters. The reduction was accomplished by heating the material in a stream of hydrogen. 

The results of the EXAFS studies on osmium-copper clusters lead to conclusions similar to those derived for ruthenium-copper clusters. That is, an osmium-copper cluster Is viewed as a central core of osmium atoms with the copper present at the surface. The results of the EXAFS investigations have provided excellent support for the conclusions deduced earlier (21,23,24) from studies of the chemisorption and catalytic properties of the clusters. Although copper is immiscible with both ruthenium and osmium in the bulk, it exhibits significant interaction with either metal at an interface. 

Because of- the similarity in the backscattering properties of platinum and iridium, we were not able to distinguish between neighboring platinum and iridium atoms in the analysis of the EXAFS associated with either component of platinum-iridium alloys or clusters. In this respect, the situation is very different from that for systems like ruthenium-copper, osmium-copper, or rhodium-copper. Therefore, we concentrated on the determination of interatomic distances. To obtain accurate values of interatomic distances, it is necessary to have precise information on phase shifts. For the platinum-iridium system, there is no problem in this regard, since the phase shifts of platinum and iridium are not very different. Hence the uncertainty in the phase shift of a platinum-iridium atom pair is very small. 

S.M. Chen and C.J. Liao, Preparation and characterization of osmium hexacyanoferrate films and their electrocatalytic properties. Electrochim. Acta 50, 115-125 (2004). 

Some second- and third-row transition metals are, for good reason, known as precious metals. These include silver, palladium, rhodium, iridium, osmium, gold, and platinum. As this is written, gold is over 900 per ounce and silver is over 15 per ounce. Some of the other metals such as rhodium, osmium, and rhenium are also extremely expensive. Most of the second- and third-row transition metals are found as minor constituents in ores of other metals. Consequendy, we will not enumerate the sources, minerals, or the processes by which these metals are obtained. Some of their most important properties are shown in Table 11.3. 

We have recently extended our interest to the analogous halfsandwich osmium-arene complexes and are exploring the chemical and biological properties of [Os(r 6-arene)(XY)Z]ra 1 complexes (Fig. 25) (105). Both the aqueous chemistry and the biological activity of osmium complexes have been little studied. Third-row transition metals are usually considered to be more inert than those of the first and second rows. Similar to the five orders of magnitude decrease in substitution rates of Pt(II) complexes compared to Pd(II), the [Os(ri6-arene)(L)X]"+ complexes were expected to display rather different kinetics than their Ru(II)-arene analogs. A few other reports on the anticancer activity of osmium-arene complexes have also appeared recently (106-108). 

Fig. 24. Comparison between the osmium- and ruthenium-arenes, exemplified by the respective [M(ri6-bip)Cl(en)]+ complexes. Although the crystal structures show the complexes to be isostructural with similar M-Cl bond lengths (a), the properties of the complexes are quite different, illustrated by the differences in hydrolysis rate h1/2), pAa, and 5 -GMP binding (the black box denotes the amount of OP03-bound 5 -GMP) (b).

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