Silver and Gold

Abstract

These two metals are superficially the best known of those considered in this book, on account of their use in jewellery and as reserve currency metals, and their use, now passing, in coins. Considered from the chemical point of view, their behaviour is fairly straightforward compared with many of the other 4d and 5d metals. Silver has one particularly stable oxidation state, the d¹⁰ ion Ag(I), though several compounds of Ag(II) are known, as are a few Ag(III) compounds. In the case of gold, the d¹⁰ unipositive state is again stable but the heavier metal shows a marked increase in the stability of the +3 oxidation state compared with silver, so much so that Au(III) compounds are numerous and well-defined. Au(II), however, is extremely uncommon since it is sandwiched between the two very stable states Au(I) and Au(III) and is thus liable to immediate disproportionation. This situation is the converse of that obtaining for the oxidation states of copper, where Cu(II) is the most stable state, especially in aqueous solution or with oxygen or nitrogen ligands, but Cu(I) is stabilised by ligands having some π-acceptor character, such as phosphines, while Cu(III), although examples are known, is very rare. The d⁸ ions in the +3 oxidation state usually adopt a square planar four-co-ordinate array of ligands, the resulting crystal-field stabilisation energy being very considerable and probably a major factor in the stability of these compounds. The effect of chemical environment on the relative stability of the various oxidation states of copper, silver and gold forms an interesting study.