Silver species

McMillan has reviewed the chemistry of Ag(II) and Ag(IlI). Paramagnetism and electron spin resonance studies confirm the presence of Ag(ll) (as opposed to equimolar Ag(I)+Ag(III)). The colours of Ag(II) solutions in various mineral acids indicate the existence of complexes, the oxidising power of which is apparent from their decomposition even at 0 °C, although high acidity promotes stability. Rapid isotope exchange between Ag(I) and Ag(n) is considered to result from the equilibrium 

Both Ag(ll) and Ag(III) have been considered to be the active species in the Ag(I)-catalysed oxidation of many compounds by persulphate ion. Salts of Ag(III) have been prepared but only a single kinetic study (of the decomposition of water by the ethylene dibiguanide nitrate) has been reported (p. 366). 

Oxidations by persulphate ion have been reviewed by House . Silver-ion catalysed reactions normally obey the rate expression 

Several mechanisms have been put forward which are consistent with the 

The species AgS208 may represent only a transition state and not a reactive intermediate. 

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Cyanide Complexes. Insoluble silver cyanide, AgCN, is readily dissolved in an excess of alkah cyanide. The predominant silver species present in such solutions is Ag(CN) 2) with some Ag(CN) 3 and Ag(CN) 4. Virtually all silver salts, including the insoluble silver sulfide, dissolve in the presence of excess cyanide because the dissociation constant for the Ag(CN) 2 complex is only 4 x 10 (see Cyanides). 

Sulfur Complexes. Silver compounds other than sulfide dissolve in excess thiosulfate. Stable silver complexes are also formed with thiourea. Except for the cyanide complexes, these sulfur complexes of silver are the most stable. In photography, solutions of sodium or ammonium thiosulfate fixers are used to solubilize silver hahdes present in processed photographic emulsions. When insoluble silver thiosulfate is dissolved in excess thiosulfate, various silver complexes form. At low thiosulfate concentrations, the principal silver species is Ag2(S203) 2j high thiosulfate concentrations, species such as Ag2(S203) 3 are present. Silver sulfide dissolves in alkaline sulfide solutions to form complex ions such as Ag(S 2 Ag(HS) 4. These ions are... 

In secondary wastewater treatment plants receiving silver thiosulfate complexes, microorganisms convert this complex predominately to silver sulfide and some metallic silver (see Wastes, INDUSTRIAL). These silver species are substantially removed from the treatment plant effluent at the settling step (47,48). Any silver entering municipal secondary treatment plants tends to bind quickly to sulfide ions present in the system and precipitate into the treatment plant sludge (49). Thus, silver discharged to secondary wastewater treatment plants or into natural waters is not present as the free silver ion but rather as a complexed or insoluble species. 

Cellophane or its derivatives have been used as the basic separator for the silver—ziac cell siace the 1940s (65,66). Cellophane is hydrated by the caustic electrolyte and expands to approximately three times its dry thickness iaside the cell exerting a small internal pressure ia the cell. This pressure restrains the ziac anode active material within the plate itself and renders the ziac less available for dissolution duriag discharge. The cellophane, however, is also the principal limitation to cell life. Oxidation of the cellophane ia the cell environment degrades the separator and within a relatively short time short circuits may occur ia the cell. In addition, chemical combination of dissolved silver species ia the electrolyte may form a conductive path through the cellophane. 

Figure 6. Proposed mode of formation for Pt multipods in the presence of silver species.

Investigations by Vermeer and co-workers have shown that 3-substituted allenyl methyl thioethers 309 can be prepared by regioselective addition of an alkyl silver species to the terminal C=C bond of enyne sulfides 308 (Scheme 8.83) [172], Remarkably, this method can also be applied to the preparation of several allenyl-phosphines starting from the corresponding phosphorus-substituted alkynes. 

An oxidation catalyst containing cobalt, copper, manganese and silver species. It adsorbs nitroalkanes strongly, which may then ignite. Respirator cartridges containing it should not be used in high concentrations of nitroalkanes. 

Iglesias-Juez, A Hungria, AB Martinez-Arias, A Fuerte, A Femandez-Garcia, M Anderson, JA Conesa, JC Soria, J. Nature and catalytic role of active silver species in the lean NOx reduction with CsIL in the presence of water, J. Catal, 2003, Volume 217, Issue 2, 310-323. 

Rueping has recently reported an interesting alknylation reaction of a-imino esters employing both phosphoric acid Ip and AgOAc as orthogonal cocatalysts [35]. As seen in the catalytic cycle in Scheme 5.21, generation of chiral iminium ion pair I nucleophilic and alkynyl-silver species II proceeds simultaneously. Subsequent nucelophilic addition completes both parallel cycles [36]. 

Although there is an analogy in many aspects between the structural chemistry of heterometallic Au-Ag and Au-Cu compounds, the interest shown in recent years has resulted in a considerable increase in the structural diversity for the gold-silver species resulting from the different strategies used to obtain compounds with a pre-established design, but also due to many unexpected compounds. 

In addition, the separator must have a low electrical resistance, good thermal and chemical stability and must be light in order to retain the high energy density characteristics of the cell. Practical separators have a composite multilayer configuration. A silver-stopping layer of cellophane or non-woven synthetic polyamide is located next to the positive electrode which reduces soluble silver species back to the metal. A potassium titanate paper layer may be placed next to the zinc electrode, and a number of cellophane layers which swell in aqueous KOH make up the middle section. In most cells the separators are fabricated as envelopes or sacks which completely enclose the zinc electrodes. 

Chini and co-workers (42) found that [CofCOJJ- reacts with Cul and Agl to give high yields of [(CO)4CoCuCo(CO)4] and [(CO)4CoAgCo(CO)4] of structure 43. The copper complex is less stable than the silver species and readily dissociates in CH3CN. Reaction (108), however, is reversible, and the starting material can be recovered after solvent removal. 

The Nakamura group reported the [2+2] cycloaddition of alkoxymethylenecy-clopropanes to imines (Scheme 2.5).10 They identified Ag(fod) from an assortment of metal salts including gold (AuBr3), palladium [Pd(OAc)2], and other silver species [Ag(acac)]. 

The proposed mechanism involves silver-catalyzed attack by the imine function on the cyclopropyl system. This is followed by conjugate addition of the intermediate silver species on the tropone ring system and subsequent isomerization to afford 171. 

In the ion/neutral mass spectrometer the silver species identified were the atomic neutral and cationic species. 

The HT SIMS data indicate that the only silver species on the surface of the emitter is metallic. Hence, if metallic silver on the surface is subliming to give a mix of atomic neutral and cationic species, this looks like a pseudo-S-L process. Again, the data are not quantitative enough to compare to the S-L equation. 

Another concept that warrants mention is desolvation When silver metal in bulk is heated, it tends to sublime as neutral species. When both neutral and cationic silver species volatilize from these silica gel matrices they are exclusively monatomic. This indicates that silver atoms in the zero oxidation state are not solvated by each other or by a component in the matrix. This concept is admittedly speculative but does offer a concept as to how this ion emitter matrix may operate. 

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