Os couple

The electrochemistry of dioxoosmium(VI) complexes has also been extensively studied. The tra 5-dioxoosmium(VI) complexes of polypyridyl and macrocyclic tertiary amine ligands display very similar proton-coupled electron transfer couples. In aqueous solutions at pH < 5-7 the cyclic voltammograms of n-a i-[0s (0)2(bpy)2] show a remarkable reversible three-electron couple and a one-electron Os coimle. In the Pourbaix diagram two break points are observed in the pH dependence of the Os couple, which correspond to the pAa values of Os —OH2 and Os —(OHXOH2) (Figure 10). The redox reactions are shown in Equations (41)-(43). At pH >8 the 3e Os wave splits into a pH-independent le Os wave and a 2e/2H" Os wave (Equations (44) and (45)). 

It appears that the observation of a reversible Os couple is a general feature of trans-dioxoosmium(VI) complexes in acidic media, indicating that the intermediates Os and Os species undergo rapid acid-catalyzed disproportionation. 

This species is more stable than [Os (en-H)2(en)f+, which undergoes oxidative dehydrogenation." The Os—NH distances are 1.896 A, which are 0.3 A shorter than the average Os— NH2 distance, indicative of substantial rr-bonding. Cyclic voltammetry of (115b) shows a pH-dependent irreversible wave. However, a reversible pH-dependent Os couple is observed for tram-[Os (tmen)2(Cl)2]+." ... 

The skutterudites do not melt congruently and involve pnicogens (P, As, Sb) that generally have high vapor pressures at the formation temperatures of the compounds. The high melting temperatures of Fe, Ru and Os coupled with the reactivity of the lanthanide metals with convenient crucible materials (e.g., Si02) makes the synthesis of many of these compounds difficult. As a result, variations in the reported properties of a particular lanthanide skutterudite compound can often be traced to differences in sample composition and quality. 

The vast majority of the coordination compounds of Os that have been prepared are in the oxidation states 11 and III. Moreover, many of these compounds show reversible or well defined Os / couples in which the electronic and redox properties at the metal are controlled by the a-donor, 7r-acceptor, and r-donor properties of the ligands. Indeed, the study of the redox behavior in Os / and Ru / species, metal ions in which octahedral coordination is almost universally retained in both redox partners, has been central in recent developments to parameterize metal centered redox processes as a function of ligand donor and acceptor capacity. The chemistries of Os and Os are, therefore, intimately linked, and have been extended to studies of important mixed valence Os / binuclear and polynuclear species (see Mixed Valence Compounds). For the purposes of brevity and convenience, this section will deal with Os and Os complexes together. The extensive literature on Os / complexes has been developed with a very wide range of donor ligands a comprehensive assessment of this work is beyond the scope of this article, and the reader is directed to published comprehensive reviews. " ... 

The complexes [Os(LL)3] + can be oxidized chemically or electrochemically to [Os(LL)3] +, the Os couples being related to the photophysical behavior of the 2+ complexes. A substantial amount of work has been carried out on the outer-sphere electron transfer to [Os(LL)3] + (a good one-electron oxidant that is co-ordinatively stable) with a range of reductants such [Fe(CN)6]" (see Electron Transfer in Coordination Compounds). 

However, photochemical systems are not suitable for characterizing the ground-state Os /Os couple, because the back reaction is extremely exergonic. 

It is interesting to note that in the Os -n(NH3)n family (X = Cl, Br, 1- n = 2,3,4), where the measured change in the Os couple is of the same order (0.33 V/X-), the halide to metal charge-transfer bands do not shift appreciably as the stoichiometry alters. Clearly, this is well accomodated by our model. Thus, in addition to the familiar effects of the ligands on the metal center, a "ligand field theory for ligands" is required. The present analysis is an early step in the quantification of these terms. 

Unlike Os(bpy)32+, substution-labile M(bpy)3 + complexes can be exchanged into zeolite Y. For example, the Co I complex, which is nearly identical in size to the Os complex, can dissociate and then re-assemble inside the 13 A diameter supercages. Consequently the maximum loading of Co(bpy)32+ is about 2 X 10 4 moles/gram in zeolite Y, whereas the maximum for the Os complex (on the external surface of the particles) is 7 x lO moles/gram. Figure 2 dso shows the electrochemistry of Os(bpy)32+/Co(bpy)32+/zeolite Y electrodes. In this case the surface Os couple appears to be unable to mediate electron transfer from the Co complex entrained in the zeolite. Since the size of the Co2+/3+ and Os2+/3+ waves are comparable in these experiments, it is likely that only the Co(bpy)33+/2+ molecules on the zeolite external surface can be oxidized and reduced, and that the contribution of bulk charge transport diffusion to the current, on this timescale, is minimal. 

---