B.— CHEMISTRY 53 



Smith (1922) as ' dibromogolddibenzylsulphide,' an addition compound 

 of ' gold bromide ' to which was assigned the formula, BrjAu-SEzj. 

 On the other hand, Ray and Sen (1930) described the chlorine analogue 

 of the same substance as auroaurichloride dibenzyl sulphide having 

 the constitution, AuClsBzaS.AuClBzaS. Ray and Sen, however, stated 

 that the molecular weight of their substance agreed with the molecular 

 formula they assigned to it. This does not agree with the results 

 obtained by previous workers nor by Dr. Tyabji and myself for the 

 bromine analogue (1937). 



There is nothing unusual concerning the structure and properties of 

 the colourless 2-covalent aurous compound (I), (dibenzylsulphidomono- 

 bromogold), or of the structure and properties of the deep red 4-covalent 

 auric compound (II), (dibenzylsulphidotribromogold) ; but the structure 

 and properties of the substances (III) and (IV), which, as compounds, 

 would be termed Z>w(dibenzylsulphidobromoiodogold) and ^wfdibenzyl- 

 sulphidodibromogold) respectively, present interesting features. Of these, 

 only (IV) in which the halogen atoms are the same needs to be con- 

 sidered in detail. All these substances are non-electrolytes and (IV), in 

 addition to the method already indicated, can be prepared by mixing 

 equimolecular quantities of compounds (I) and (II) in a suitable solvent, 

 for example chloroform. The molecular formula of (IV) cannot be the 

 same as its empirical formula otherwise it would be a 3-covalent derivative 

 of bivalent gold. The apparent molecular weight of each of the substances 

 (III) and (IV) in freezing bromoform is a little less than that indicated 

 by the empirical formula affording no information as to their constitutions. 



In this particular case, our knowledge of the chemical and physical 

 properties is inadequate for determining the constitution of the sub- 

 stance in the solid state. All that is possible is for the chemist to suggest 

 reasonable alternatives based on recognised principles of the constitution of 

 co-ordination compounds. None of these alternatives is capable of being 

 confirmed by chemical or physico-chemical methods and the only method 

 of determining the constitution is by careful crystallographic and X-ray 

 analysis. In the solid state the substance is obviously a mixed aurous- 

 auric complex of a new type, since the mixed aurous-auric compounds 

 already encountered are stable both in solution and in the solid condition. 

 For the constitution of the substance in the solid state, two not un- 

 reasonable possibilities immediately suggest themselves. In the linking 

 up of the aurous and auric compounds, (a) the aurous gold atom may 

 become 4-covalent, its four valencies probably assuming a tetrahedral 

 configuration, having now eight electrons in its outer shell and assuming 

 an Effective Atomic Number of 86, or (b) the auric gold atom may 

 become 5-covalent, its five valencies probably assuming a pyramidal con- 

 figuration, having now ten electrons in its outer shell and assuming an 

 Effective Atomic Number of 86. 



I have put the two alternatives in this order, because there seemed 

 to be a possibility, if crystallographic and X-ray analysis proved it, of 

 obtaining the first incontrovertible evidence of a 4-covalent aurous com- 

 pound which, up to the present, remains a theoretical conception un- 

 supported by experimental evidence to explain the constitution of certain 

 complex compounds (compare p. 37). Arising from suggestion (a) there 



