Gold oxidation states

The reduction potentials are dependent upon ligation of the gold species. Stabilization of either gold oxidation state can occur with suitable ligand choice [8, 9] (CyS is cysteinato) ... 

An example of how gold oxidation state can control the structure of the product formed has been provided recently3,10 when haloallenyl ketones were used in gold-catalysed cyclo-isomerisations to give furans (Scheme 12.2) ... 

C. Rearrangements Involving a Change of Gold Oxidation State. 440... 

X-ray absorption spectroscopy shows considerable promise as a powerful tool to aid in the elucidation of the role of gold-based pharmaceuticals in the management of rheumatoid arthritis. This technique has already given considerable new insight into gold oxidation state and binding in aurosomal samples prepared from rat kidneys. 

The gold oxidation states in this latter study were determined by XPS,... 

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. 

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. 

In the presence of air, it is attacked by potassium cyanide solution, to give the complex dicyanoaurate(I) ion, in which gold has an oxidation state + 1 ... 

It is dissolved by aqua regia (a mixture of concentrated hydrochloric and nitric acids). The product here is chlorauricil 11) acid, HAUCI4 in the complex chloraurate ion [AuClJ gold is in oxidation state + 3, auric gold. ... 

These elements formed Group IIB of Mendeleef s original periodic table. As we have seen in Chapter 13, zinc does not show very marked transition-metaf characteristics. The other two elements in this group, cadmium and mercury, lie at the ends of the second and third transition series (Y-Cd, La-Hg) and, although they resemble zinc in some respects in showing a predominantly - - 2 oxidation state, they also show rather more transition-metal characteristics. Additionally, mercury has characteristics, some of which relate it quite closely to its immediate predecessors in the third transition series, platinum and gold, and some of which are decidedly peculiar to mercury. 

Gold Compounds. The chemistry of nonmetallic gold is predominandy that of Au(I) and Au(III) compounds and complexes. In the former, coordination number two and linear stereochemistry are most common. The majority of known Au(III) compounds are four coordinate and have square planar configurations. In both of these common oxidation states, gold preferably bonds to large polarizable ligands and, therefore, is termed a class b metal or soft acid. 

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). 

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

Again, in the -(-3 oxidation state, only gold is known to form binary halides, though AUI3 has not been isolated. The chloride and the... 

For gold in oxidation states other than III, see H. ScHMiDBAUR and K. C. Dash, Adv. Inorg, Chem. 25, 239-66 (1982). 

For gold, by contrast, - -3 is the element s best-known oxidation state and Au is often compared with the isoelectronic Pt (p. 1161). The usual route to gold(III) chemistry is by dissolving the metal in aqua regia, or the compound AU2CI6 in cone HCl, after which evaporation yields yellow chloroauric acid, HAUCI4.4H2O, from which numerous salts of the square-planar ion [AuCLj]" can be obtained. 

Duncan and Frankenthal report on the effect of pH on the corrosion rate of gold in sulphate solutions in terms of the polarization curves. It was found that the rate of anodic dissolution is independent of pH in such solutions and that the rate controlling mechanism for anodic film formation and oxygen evolution are the same. For the open circuit behaviour of ferric oxide films on a gold substrate in sodium chloride solutions containing low iron concentration it is found that the film oxide is readily transformed to a lower oxidation state with a Fe /Fe ratio corresponding to that of magnetite . 

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]. 

A mononuclear compound containing gold in a formal oxidation state of +4 is shown in Figure 4.29 it was produced by electrochemical oxidation of the related gold(III) species [142],... 

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