Gold cations: disproportionation

These points are well illustrated by comparing Cu, Ag and Au with respect to the relative stabilities of their oxidation states. Although few compounds formed by these elements can properly be described as ionic, the model can quite successfully rationalise the basic facts. The copper Group 1 Id is perhaps the untidiest in the Periodic Table. For Cu, II is the most common oxidation state Cu(I) compounds are quite numerous but have some tendency towards oxidation or disproportionation, and Cu(III) compounds are rare, being easily reduced. With silver, I is the dominant oxidation state the II oxidation state tends to disproportionate to I and III. For gold, III is the dominant state I tends to disproportionate and II is very rare. No clear trend can be discerned. The relevant quantities are the ionization energies Iu l2 and A the atomisation enthalpies of the metallic substances and the relative sizes of the atoms and their cations. These are collected below / and the atomisation enthalpies AH%tom are in kJ mol-1 and r, the metallic radii, are in pm. 

A more extensive series of stability constants has been determined in acetonitrile solution, where disproportionation of gold(I) does not cause major problems. The stability constants follow the following series anionic complexes [AuX CNO cationic complexes PhsPO < MezS < py < AsPhs < NH3 PPhs and ArNC<... 

Gold salts such as AuCl and HAUCI4 and gold complexes have been widely investigated in many organic syntheses, especially for unsaturated C—C bonds [99, 100]. However, the ubiquity of gold salts and complexes has puzzled many chemists because they usually display very similar catalytic capability, irrespective of their oxidation state. Some chemists have spectdated that Au " " cations were reduced in situ to Au cations, and the formed Au" " cations were the real active sites. However, others have attributed this similar catalytic performance to the disproportionation of Au cations to Au + cations and metallic Au atoms [101-103). 

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