Silver oxidation states

Monolayers can be transferred onto many different substrates. Most LB depositions have been perfonned onto hydrophilic substrates, where monolayers are transferred when pulling tire substrate out from tire subphase. Transparent hydrophilic substrates such as glass [18,19] or quartz [20] allow spectra to be recorded in transmission mode. Examples of otlier hydrophilic substrates are aluminium [21, 22, 23 and 24], cliromium [9, 25] or tin [26], all in their oxidized state. The substrate most often used today is silicon wafer. Gold does not establish an oxide layer and is tlierefore used chiefly for reflection studies. Also used are silver [27], gallium arsenide [27, 28] or cadmium telluride wafer [28] following special treatment. 

The more noble metals (for example copper, mercury and silver) can form oxides, and exhibit variable oxidation state in such compounds (for example CU2O, CuO), but it is not easy to prepare such oxides by direct action of oxygen on the metal, and elevated temperatures are necessary. Moreover, in the case of silver and mercury, loss of oxygen from the oxide by heating is easy. The oxidesare, however, basic (for example Ag20 - Ag, CuO - Cu in acids). 

Bromine has a lower electron affinity and electrode potential than chlorine but is still a very reactive element. It combines violently with alkali metals and reacts spontaneously with phosphorus, arsenic and antimony. When heated it reacts with many other elements, including gold, but it does not attack platinum, and silver forms a protective film of silver bromide. Because of the strong oxidising properties, bromine, like fluorine and chlorine, tends to form compounds with the electropositive element in a high oxidation state. 

Also, in anhydrous conditions, silver reacts with fluorine and forms silver difluoride AgFj and cobalt gives cobalt(III) fluoride, C0F3, these metals showing higher oxidation states than is usual in their simple salts. 

Halogens can act as ligands and are commonly found in complex ions the ability of fluorine to form stable complex ions with elements in high oxidation states has already been discussed (p. 316). However, the chlorides of silver, lead(Il) and mercury(l) are worthy of note. These chlorides are insoluble in water and used as a test for the metal, but all dissolve in concentrated hydrochloric acid when the complex chlorides are produced, i.e. [AgCl2] , [PbC ] and [Hg Clj]", in the latter case the mercury(I) chloride having also disproportionated. 

Silver has little tendency to formally lose more than one electron its chemistry is therefore almost entirely restricted to the + 1 oxidation state. Silver itself is resistant to chemical attack, though aqueous cyanide ion slowly attacks it, as does sulphur or a sulphide (to give black Ag S). hence the tarnishing of silver by the atmosphere or other sulphur-containing materials. It dissolves in concentrated nitric acid to give a solution of silver(I) nitrate. AgNOj. 

Some metals used as metallic coatings are considered nontoxic, such as aluminum, magnesium, iron, tin, indium, molybdenum, tungsten, titanium, tantalum, niobium, bismuth, and the precious metals such as gold, platinum, rhodium, and palladium. However, some of the most important poUutants are metallic contaminants of these metals. Metals that can be bioconcentrated to harmful levels, especially in predators at the top of the food chain, such as mercury, cadmium, and lead are especially problematic. Other metals such as silver, copper, nickel, zinc, and chromium in the hexavalent oxidation state are highly toxic to aquatic Hfe (37,57—60). 

Fig. 12. A possible mechanism for the dye-induced photooxidation of a silver center, x represents the distance across a silver haUde surface to which aggregated dye molecules are adsorbed. Steps 1, 4, and 5 represent the photohole (Q) formation, photohole migration, and silver oxidation processes which can ultimately lead to the total regression of the silver aggregate ( ) represents an energy state occupied by an electron.

Oxidation States. The common oxidation state of silver is +1, ie,, as found in AgCl, which is used with Mg in sea- or freshwater-activated batteries (qv) AgNO, the initial material for photographic materials, medical compounds, catalysts, etc and silver oxide, Ag20, an electrode in batteries (see Silver compounds). Few compounds are known. The aqua ion [Ag(H2 O), which has one unpaired electron, is obtained... 

Silver in the +3 oxidation state, including silver peroxide, ie, black oxide, marketed as AgO, is obtained by the action of the vigorous oxidising agent S20 g on Ag20 or other Ag compounds. X-ray and neutron diffraction analyses show the nominal AgO unit cell to be Ag20 Ag202- Both Ag" and Ag " are present. Another compound of potentially important commercial value is Ag O, which has a unit cell of two Ag and two Ag ions. Its preparation is as follows ... 

Silver(II) Compounds. Sdver(II) is stabilized by coordination with nitrogen heterocychc bases, such as pyridine and dipyridyl. These cationic complexes are prepared by the peroxysulfate oxidation of silver(I) solutions in the presence of an excess of the ligand. An extensive review of the higher oxidation states of silver has beenpubhshed (21). 

Some metal thiosulfates are inherently unstable because of the reducing properties of the thiosulfate ion. Ions such as Fe " and Cu " tend to be reduced to lower oxidation states, whereas mercury or silver, which form sulfides of low solubiUty, tend to decompose to the sulfides. The stabiUty of other metal thiosulfates improves in the presence of excess thiosulfate by virtue of complex thiosulfate formation. 

Fig. 1. Schematic representation of a battery system also known as an electrochemical transducer where the anode, also known as electron state 1, may be comprised of lithium, magnesium, zinc, cadmium, lead, or hydrogen, and the cathode, or electron state 11, depending on the composition of the anode, may be lead dioxide, manganese dioxide, nickel oxide, iron disulfide, oxygen, silver oxide, or iodine.

Oxidation catalysts are either metals that chemisorb oxygen readily, such as platinum or silver, or transition metal oxides that are able to give and take oxygen by reason of their having several possible oxidation states. Ethylene oxide is formed with silver, ammonia is oxidized with platinum, and silver or copper in the form of metal screens catalyze the oxidation of methanol to formaldehyde. Cobalt catalysis is used in the following oxidations butane to acetic acid and to butyl-hydroperoxide, cyclohexane to cyclohexylperoxide, acetaldehyde to acetic acid and toluene to benzoic acid. PdCh-CuCb is used for many liquid-phase oxidations and V9O5 combinations for many vapor-phase oxidations. 

Table 28.2 Oxidation states and stereochemistries of copper, silver and gold...

The -t-1 state is by far the best-known oxidation state of silver and salts with most anions are formed. These reveal the reluctance of Ag to coordinate to oxygen for, with the exceptions of the nitrate, perchlorate and fluoride, most are insoluble in water. The last two of these salts are also among the very few Ag salts which form... 

Ethylene oxide, the simplest epoxide, is an intermediate in the manufacture of both ethylene glycol, used for automobile antifreeze, and polyester polymers. More than 4 million tons of ethylene oxide is produced each year in the United States by air oxidation of ethylene over a silver oxide catalyst at 300 °C. This process is not useful for other epoxides, however, and is of little value in the laboratory. Note that the name ethylene oxide is not a systematic one because the -ene ending implies the presence of a double bond in the molecule. The name is frequently used, however, because ethylene oxide is derived pom ethylene by addition of an oxygen atom. Other simple epoxides are named similarly. The systematic name for ethylene oxide is 1,2-epoxyethane. 

Precipitation reactions. These depend upon the combination of ions to form a simple precipitate as in the titration of silver ion with a solution of a chloride (Section 10.74). No change in oxidation state occurs. 

The original blue (K.A. Hofmann, 1908) was obtained from the reaction of Pt(MeCN)2Cl2 with silver salts over some hours. Under these conditions, the nitrite is hydrolysed to acetamide. Very recently, the structure of the complex [(H3N)2Pt(MeCONH)2Pt(NH3)2]4(NO3)10 has been determined (Figure 3.37). The average oxidation state of the platinums in the octamer is 2.25. 

For many years, the chemistry of silver and gold was believed to be more similar than is now known to be the case [1-10]. In the Cu-Ag-Au triad, the stability of oxidation states does not follow the usual trend of increasingly stable high oxidation state on descending the group for copper, the +2 state is the most important, for silver it is the +1 state and, though oxidation states between -1 and +7 are claimed, for gold it is the +1 and +3 states that dominate its chemistry. The types of compound are summarized in Table 4.1. 

The halides of silver and gold are listed in Table 4.4 as expected gold has more in higher oxidation states [18c]. 

Stable compounds of silver(II) are found with N, O and F as donor atoms macrocycles are, as elsewhere, able to support the higher oxidation state. As a d9 system, Ag2+ imitates Cu2+ in displaying Jahn-Teller distortion. 

Gold, unlike silver, forms a wide range of cluster complexes [184] where the average oxidation state of the metal is below +1 they may be synthesized by reduction of gold(I) phosphine complexes ... 

Ag(I) and Cu(II) are readily used in neutral solution, although oxidations by Ag(I) are often affected by the deposition of silver metal. Theoretically Cu(II) can behave either as a two- or a one-equivalent reagent, but it usually functions as the latter. The tendency of Cu(I) to disproportionate is normally suppressed by working with solvents such as pyridine which form stable complexes with this oxidation state. 

Often, the oxides of certain metals are used as the oxidizer. In the names of systems and batteries, though, often only the metal is stated, so that the example reported above is called a silver-zinc, rather than silver oxide-zinc battery (or system). 

In the silver dithiocarbamato chemistry only univalent metal complexes could be isolated until now. EPR studies, however, revealed that the oxidation state + 2 can be achieved in solutions. The existence of the oxidation state -H 3 in solution is also reported. 

Attempts to oxidise silver dithiocarbamato complexes with halogens to compounds with the metal in higher oxidation states obviously failed. By addition of iodine to a solution of [Ag(Bu2 fc)]g in CHQ3, an insoluble product is formed with the composition rjA% S> X2dtc) 141). With other alkyl groups similar complexes are obtained. Investigations about the nature of this type of compounds are in progress. 

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