Ruthenium valence states

Ruthenium and osmium are decidedly less noble than the other four metals of the platinum group. Both exist in numerous valency states and very readily form complexes. Ruthenium is not attacked by water or non-complexing acids, but is easily corroded by oxidising alkaline solutions, such as peroxides and alkaline hypochlorites. 

Physical properties of binary or ternary Ru/Ir based mixed oxides with valve metal additions is still a field which deserves further research. The complexity of this matter has been demonstrated by Triggs [49] on (Ru,Ti)Ox who has shown, using XPS and other techniques (UPS, Mossbauer, Absorption, Conductivity), that Ru in TiOz (Ti rich phase) adopts different valence states depending on the environment. Possible donors or acceptors are compensated by Ru in the respective valence state. Trivalent donors are compensated by Ru5+, pentavalent acceptors will be compensated by Ru3+ or even Ru2+. In pure TiOz ruthenium adopts the tetravalent state. The surface composition of the titanium rich phase (2% Ru) was found to be identical to the nominal composition. 

Ruthenium like iridium is known for its ability of adopting various valence states which make these elements rather attractive in catalysis. Kim and Winograd [52] were the first who studied the chemical in XPS of different Ru compounds. The results of their extensive work still serve as reference for today s investigators. Kim and Winograd have identified binding energies of Ru-oxygen species (Table 1). 

Increasing attention is being given to the reactivity of ruthenium species which show unusual behavior compared with their Co analogs. Aspects of current interest are mixed valence states, ruthenated proteins to probe electron transfer in them (Chap. 5) and the photochemistry and photophysics of Ru(II) polypyridine eomplexes. 

ESCA spectra have been recorded for a series of pyrazine-bridged ruthenium dimers including [(NH3)5Ru(pyz)Ru(NH3)5]s H and [(bipy)2ClRu(pyz)RuCl(bipy)2]3+.50 The data show that both Ru and Ruin are present in those complexes although the metal sites are equivalent. The speed with which ESCA monitors electron distribution (ca. 10"17 s) would seem to make it an excellent spectrosocpic tool for such valence-state assignments. 

From the second cycle the plutonium goes through anion exchange for final purification (Fig. 21.21). The principal problem here is due to ruthenium, which is difficult to remove because of its many valence states. The uranium stream goes through silica sorption primarily to remove zirconium, which seems to be carried along as a colloid. 

These two intriguing classes of compounds have a common point the possibility to write an alternate mesomeric structure, either quinoidal or cumulenic, for the fully oxidized form containing ruthenium(III) or iron(III) [23, 68, 73, 96]. The relationship with the high value of the coupling in the mixed-valence state is at present unknown, but deserves further investigation. 

Ruthenium has a complex chemistry with up to seven valence states, although the 2-I-, 3-I-, and 4- - valences are encountered most commonly. Essington et al. 

While high surface area and metallic conductivity are beneficial to electrocatalysis, they do not alone explain the high catalytic activity. We speculate that the variable oxygen stoichiometry of the pyrochlore lattice, and the multiple valence states of the cations, particularly the ruthenium, are essential to the catalytic activities of these pyrochlores. 

In the solution, americium, curium, and most of the fission products are in a single, relatively inextractable valence state. Iodine and ruthenium are important exceptions. Iodine may appear as inextractable iodide or iodate or as elemental iodine, which would be extracted by the solvent and react with it. Ruthenium may appear iii any valence state between 0... 

Mixed Oxidation State Ruthenium. In view of the growing number of mixed valence state ruthenium complexes reported, especially Ru compounds, all the pertinent references are collected in this section. The new pyrazine-bridged... 

It was found that binuclear complexes with delocalized mixed-valence states form an asymmetric 1 3 adduct to 18C6, in which there is a 1 1 stoichiometry at one ruthenium moiety and 1 2 stoichiometry at the other. From the results of this study, it was clear that the valence of the metal center significantly affects the composition of crown ether adducts of metal complexes. 

In mixed valence species, especially symmetric species such as the dinuclear ruthenium Creutz-Taube (CT) ion, questions arise as to whether the valence states are localized, in the CT ion Ru Ru , or delocalized, formally, Ru Ru. The former can become the latter if an electron exchanges rapidly between the two ruthenium centers. Thus, a snapshot taken faster than... 

In the solution, americium and curium, most of the FPs are in a single relatively nonextractable valence state. Iodine and ruthenium are important exceptions. Iodine may appear as nonextractable iodide or iodate or as elemental iodine, which would be extracted by the solvent and react with it. Ruthenium may appear in any valence state between 0 (insoluble metal) and 8 (volatile ruthenium tetroxide), and, at valence 4, it may form a number of nitrosyl ruthenium (Ru(IV)NO) complexes of varying extractabil-ity. An important objective of dissolution and the preconditioning of the feed solution prior to extraction is to convert these FP elements into states that will not contaminate uranium, plutonium, or solvent in subsequent solvent extraction (Benedict, Pigford, and Levi, 1981). 

Using simple electrochemical series, the most probable valency states of fission products in UO2 fuel presented in Table 3.10. can be defined the setting of an element in brackets means that it is able to exist in different valency states. With increasing bumup, the electrochemical situation in the fuel is assumed to change due to changing concentration ratios of the fission products. These variations are compensated for by changes in the valency states of different fission products (e. g. iodine, ruthenium, molybdenum) thus, redox reactions at the U(IV) matrix atoms are not to be expected. 

On the other hand, the ruthenium complex of ferrocenyl-alkynyl dimer shows a separation of redox potentials of two ferrocene units through the alkyne-Ru-alkyne ligand at 0.22 This result indicates that the mixed-valence state of ferrocenyl groups in Ru(-C=C Fc)2(dppm)2 complex are much more stable because of the strong donor-acceptor interaction through the ruthenimn core metal with two a-bonding alkynyl chains. The four oxidation states could be experimentally foimd for Ru(H =C Fc)2(dppm)2 complex (Fe(II)-Ru(II) Fe(II) Fe(III) Ru(II) Fe(II) Fe(III) Ru(II)-Fe(in) Fe(III)-Ru(III)-Fe(III)), where the redox potential of the ruthenium center is positively shifted as noted above. 

Ru3 (CO) 12 is widely studied as precursor of chlorine-free ruthenium compoimd. A lot of studies have been conducted on Rus (CO) 12 due to its high activity caused by low valence states of Ru in Rus (CO) 12, high diserpsion on support and absence of chlorine which can poison the ruthenium catalyst. 

For other elements of variable valence, su( h as technetium, the amount of the element in solution is determined by the stable valence state under reactor conditions. In general, the higher valence states lietter resist hydrolysis and remain in solution. Thus at 275°C in 0.02 m UO2SO4 Tc(VII) is reduced to Tc(IV) if hydrogen is present, and only 12 mg/kg H2O remains in solution. However, a slurry of TCO2 in the same solution but with oxygen present dissolves to give a solution at 275 C with a technetium concentration of more than 9 g/kg H2O. The same qualitative behavior is observed with ruthenium. Selenium and tellurium in the hexapositive state are much more soluble than when in the tetrapositive state [4]. 

A good example is the excited state of the tris(bipyridine)ruthenium(2+) ion, Ru(bpy)5+. This species results from the transfer of an electron from the metal to a ligand. In the language of localized valences, it is a ruthenium(3+) ion, coordinated to two bipyridines and to one bipyridyl radical anion in other words, [Ru3+(bpy)2(bpy )]2+. This excited state is a powerful electron donor and acceptor.17 The following equations show an example of each quenching mode ... 

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