38 SECTIONAL ADDRESSES 



in which, as usual, the aureus gold atom is 2-covalent and has an Effective 

 Atomic Number of 82. In the light of available information we must 

 conclude that in all its compounds the aurous gold atom is co-ordinated 

 and, with the possible exception of the two compounds mentioned above, 

 it is always 2-covalent. At the present time there is no example of an 

 aurous compound in which the gold atom is known to be 4-covalent, 

 and attempts to produce such a compound have failed. Since the ter- 

 valent gold atom is also always co-ordinated and in its stable compounds 

 always 4-covalent — there may be a slight tendency for it to become 

 5-covalent — its chemistry can have little in common with that of other 

 tervalent metals which form normal salts. 



It will be shown later that the four valencies of the tervalent gold 

 atom have a planar configuration and, since 5-covalent and tervalent 

 gold compounds may always be too unstable, it would appear that the 

 only type of gold compound capable of exhibiting optical activity must 

 be a suitable 4-covalent aurous compound, if that can be prepared. 

 These four valencies would be expected, according to Pauling's theory, 

 to have a tetrahedral configuration. It has already been shown that 

 various 4-covalent cuprous and argentous compounds having a tetrahedral 

 configuration have been prepared and a 4-covalent argentous compound, 

 the silver derivative of 8-hydroxyquinoline, appears to have been obtained 

 by Hein and Regler (1936) in optically active forms. 



Much of the confusion of knowledge regarding the chemistry of gold 

 as described in almost all text-books and more comprehensive works 

 arises from the fact that the simple halide and cyanogen derivatives 

 are regarded as normal metallic salts and given the formulce AuCl, 

 AuBr, Aul, AuCN, AuClg, AuBrg according to the fundamental uni- or 

 tervalency of the metal. This is all the more surprising in view of the 

 long-established and well-known fact that whenever gold is in solution 

 or in the form of a soluble salt it is always present as a complex. There 

 is only need to mention as examples potassium auroc}^anide, probably 

 — on account of its application in the metallurgy of gold — the most 

 completely investigated derivative of the metal and the very interesting 

 sodium aurothiosulphate prepared as long ago as 1845 by Fordos and 

 Gelis. Even at the time of its discovery, this latter compound was 

 known to give neither the usual reactions for gold nor the usual reactions 

 of a thiosulphate. It has long been used for fixing and toning silver 

 photographic prints. Since its introduction in 1924 by the Danish 

 physician, Mollgaard, for the treatment of tuberculosis and, later, by 

 others for the treatment of rheumatoid arthritis it has been considerably 

 investigated and has formed the basis of the modern ' gold therapy.' 

 Curiously enough, in a standard text-book published as recently as 1937, 

 the formula, Au2Sa03.3Na2S20,.xH20 seems to be preferred to the 

 correct Na3[Au(S203)2].2H20 which may be fully written 



Na3 



■ O o - 



- t t - 



O— S— S— Au— S— S— O 



o o 



2H,0 



