Os systems

The binary compounds of palladium and platinum show a higher diversity of structures than found for the Rh-Ir and Ru-Os systems. Aside from several other reported binary phases, four general families can be distinguished ... 

Figure 4.42. Mo alloys multi-diagram. Notice that along the vertical axis, from Ta to Pt, the sequence of the isothermal sections Ta-W, W-Re, Re-Os, etc. is shown. In the same axis an approximate representation is also suggested for the other binary combinations (Ta-Re Ta-Os, Ta-Ir, etc., W-Os, W-Ir, etc.). A confirmation of the (even partial) validity of this representation is given by the phase sequences observed in the W-Re and Re-Os systems in comparison with that of the W-Os system. (For the symbols see Fig.4.41).

A suitable comparison of the Os(VI)/ Os(III) couple is the well-studied three-electron oxidant HCr04, where the intermediate oxidation states Cr(V)/Cr(IV) appear to be unstable with respect to disproportionation. However, the Os system shows a dramatically enhanced electrochemical reversibihty compared to Cr(IV). This feature may play an important role in the ability of related systems that act as redox catalysts. 

Ravizza G. and Tnrekian K. K. (1989). Application of the Re- Os system to black shale geochronometry. Geochim. Cosmochim. Acta, 53 3257-3262. 

The next part of this section focuses on di- and polynuclear Ru"/Os" systems (organized according to the bridging ligands), and pertinent reviews covering multicomponent molecular arrays should be consulted. The cyano-bridged complexes [(bpy)(tpy)Ru(//-CN)Ru" (NH3)5]"+ ( = 3 or 4) have been studied. Picosecond excitation of [(bpy)(tpy)Ru-(//-CN)Ru(NH3)5] results in the observation of a transient intermediate which decays by an... 

This chapter will describe primarily the crystal chemistry and crystallography of the superconducting compounds in this family. I begin with descriptions of the basic compounds BaBiOs and BaPbOs. Next, the Ba(Pb,Bi)Os solid solutions, which have been studied in some detail, and the relatively new Ba(Pb,Sb)Os system are described. Finally, I describe what is known about (Ba,K)BiOs, which, although it is a material of considerable importance, is not completely characterized as of yet because of the difficulties in materials preparation and crystal growth. 

The Re- Os system in cosmochemistry has been primarily used to investigate the crystallization paths and ages of asteroidal cores through measurements of magmatic iron... 

The Re- Os system does not lend itself to the determination of internal isochrons for most meteorites. Chen et al. (1998) produced an internal isochron for the St. Severin (LL6) chondrite using metal separates and whole-rock samples (Fig. 8.21). The slope of the isochron gives a date of 4.60-0.15 Ga (X = 1.666 x 10 11 yr 1), consistent with expectations based on other chronometers, but not precise enough to improve upon other techniques. 

As with the Sm- Nd and Re- Os systems, careful chemistry is required to cleanly separate the parent and daughter elements because mass spectrometry cannot resolve 176Lu from 176Hf. The ion-exchange chemistry is similar to that for samarium-neodymium. In fact, fractions of samarium, neodymium, lutetium and hafnium are often produced in a single procedure. Mass spectrometry is done by ICPMS because this is the only method that effectively ionizes hafnium. 

Figure 6.28 Cyclic voltammogram (voltage vs. SCE) of the TiC>2-Ru-Os system, with a surface coverage, r, of 1.0 x 1014 molecules cm-2, measured in 0.3 M LiC104/acetonitrile, at a scan rate of v = 50 mV s 1. Reprinted with permission from A. C. Lees, C. J. Kleverlaan, C. A. Bignozzi and J. G. Vos, Inorg. Chem., 40, 5343 (2001). Copyright (2001) American Chemical Society...

Phenol complexes of [Os] display pronounced reactivity toward Michael acceptors under very mild conditions. The reactivity is due, in part, to the acidity of the hydroxyl proton, which can be easily removed to generate an extended enolate. Reactions of [Os]-phenol complexes are therefore typically catalyzed using amine bases rather than Lewis acids. The regio-chemistry of addition to C4-substituted phenol complexes is dependent upon the reaction conditions. Reactions that proceed under kinetic control typically lead to addition of the electrophile at C4. In reactions that are under thermodynamic control, the electrophile is added at C2. These C2-selective reactions have, in some cases, allowed the isolation of o-quinone methide complexes. As with other [Os] systems, electrophilic additions to phenol complexes occur anti to the face involved in metal coordination. 

Until recently there has been surprisingly little interest in high oxidation state complexes of terpy. Meyer and co-workers have demonstrated that the ruthenium(IV) complex [Ru(terpyXbipy)0] is an effective active catalyst for the electrocatalytic oxidation of alcohols, aromatic hydrocarbons, or olefins (335,443,445,446). The redox chemistry of the [M(terpy)(bipy)0] (M = Ru or Os) systems has been studied in some detail, and related to the electrocatalytic activity (437,445,446). The complexes are prepared by oxidation of [M(terpy)(bipyXOH2)] . The related osmium(VI) complex [Os(terpyXO)2(OH)] exhibits a three-electron reduction to [Os(terpyXOH2)3] (365,366). The complex [Ru(terpy)(bipyXH2NCHMe2)] undergoes two sequential two-electron... 

Thin-layer Chromatography. System TP—Rf 42 system TQ—Rf OS system TR—Rf 0 system TS—Rf 01. DPST solution.)... 

Because of the numerous secondary effects that modify the whole-rock elemental and isotopic compositions, our discussion below will focus solely on mineral-separate analyses for the Rb-Sr, Sm-Nd, Lu-Hf, and U-Pb isotope systems. In comparison, the Re-Os system is much more robust to the effects of... 

Improved analytical capabilities have led to the analysis of several hundred xenoliths for osmium isotopic composition. The compatible nature of osmium during mantle melting means that, unlike incompatible-element-based isotope systems, peridotite residues have much higher osmium contents than mantle melts and thus the system is less readily disturbed by later metasomatism (see Section 2.05.2.5.3). This is clearly shown by rhenium and osmium abundances (Figure 21). The vast majority of rhenium contents of both cratonic and noncratonic peridotite xenoliths are below the PUM value proposed by Morgan et al (1981) and many are P-PGE depleted. This contrasts with almost universal TREE enrichment of whole-rock peridotites. That the Re-Os system is not immune from the effects of metasomatism is illustrated by the consideration of extended PGE patterns (Figure 20 Section 2.05.2.5.3 Pearson et al., 2002, 2004). Dismption of both rhenium and osmium in some mantle environments may have occurred (Chesley et al, 1999), especially where sulhde metasomatism is involved (Alard et al, 2000). However, Pearson et al. (2002, 2004) and Irvine et al (2003) have shown that coupled PGE and Re-Os isotope analyses can effectively assess the level of osmium isotope disturbance in peridotite suites. 

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