40 SECTIONAL ADDRESSES 



method between 150° and 260° is Au2Clg and although it was not suggested 

 at the time, and, indeed, not until 1931, the constitution of this and 

 the analogous bromo compound is only adequately represented by the 

 general formula : 



XXX 



\ / \ X 



Au Au (X=C1, Br) 



XXX 



in keeping with the already recognised 4-covalency of auric gold. This 

 constitution followed from the constitution of the dialkylmonohalogeno 

 derivatives and from the direct determination of the molecular weight 

 of auric bromide (tribromogold) ^ in boiling bromine (Burawoy and 

 Gibson 1935). In a systematic discussion of the chemistry of gold, 

 and allowing for the differences in fundamental valencies and covalencies 

 of the metals, it would appear that gold is much more allied to palladium 

 and to platinum — the pair of metals having the lowest melting points 

 and the lowest densities of the six ' precious ' transitional metals — than 

 it is to any of the other metals. The comparison of gold with platinum 

 is also of historical interest. Mendeleeff (1871) placed gold and platinum 

 in the same horizontal series of his classification but for reasons which 

 do not concern the present discussion. 



1 My suggestion for a modified nomenclature of certain gold compounds may 

 be criticised as being, if not pedantic, unnecessary. It arises from obvious 

 analogies of the organic compounds of gold with similarly constituted inorganic 

 compounds of the metal ; its only object is to avoid further confusion in the 

 chemistry of gold. Such confusion is constantly occurring. At the present 

 time, in books of reference and even in original literature ' auric chloride ' may 

 imply hydrochloroauric acid in the presence or absence of hydrochloric acid, or 

 it may imply a neutral salt — generally the sodium salt — of hydrochloroauric 

 acid and, much less frequently gold trichloride or — to alter its name more pro- 

 foundly in order to indicate that the compound is not a salt — trichlorogold. 

 As a result of this confusion the statement is repeatedly found in the literature 

 that ' auric chloride is soluble in ether.' If this statement refers to the pure 

 compound having the molecular formula (AuCl3)2, it is not true. Hydro- 

 chloroauric acid and hydrobromoauric acid containing water of crystallisation, 

 the compounds HAUX4.3H2O, are soluble in ether but they are insoluble when 

 anhydrous. Although the fact was known long before, the definite statement 

 that gold chloride is soluble in ether appears to be due to Willstatter (1905) ; 

 but it is clear that the material he was investigating was not (AuCl3)2, but an 

 aqueous solution of hydrochloroauric acid which he termed gold chloride ; and, 

 as a result, the above erroneous statement is still in text-books published as 

 recently as 1937. The hygroscopic nature and solubility of 'auric chloride,' 

 i.e. gold trichloride, in water is not due to the solubility of the compound per 

 se, but to the formation in the first place of a compound diaquodichloroauric 

 chloride, 



rci OH2-] 



\ ^ 



Au CI 



LCI OH2_ 



a type of co-ordinated auric gold salt, frequently met with in the present series 

 of investigations, which is soluble in water and undergoes further changes in that 

 medium resulting in the formation of hydrochloroauric acid and aurous choride 

 (monochlorogold) . 



