B.— CHEMISTRY 49 



aurocyanides (VI) are electrolytes and mixed 4-covalent auric and 2-co- 

 valent aurous compounds, a type of compound of which the first example 

 appears to have been described by Sir William Pope in 1929. It will be 

 appreciated that the conversion of compounds of type VI into those of 

 type IV by means of acid, which goes quite smoothly, is somewhat 

 complicated involving changes of electrovalencies into covalent and 

 coordinate linkages. 



The free radicals were not identified as such although there is evidence 

 for believing that they are initially evolved. The decompositions are of 

 either solid or liquid compounds and so far free radicals have only been 

 identified as such when they have produced by decompositions in the 

 vapour phase. Actually the free radicals were identified as the paraffin 

 hydrocarbons to which they should give rise, w-butane from the ethyl 

 derivatives and «-hexane from the w-propyl derivatives. This was the 

 first time that w-hexane had been obtained from the decomposition of a 

 n-propyl compound. Previously, when n-propyl radicals had been 

 anticipated as likely to be produced a mixture of ethane, butane and 

 ethylene (Frankland 1877) or a mixture of only ethane and ethylene 

 (Paneth and Lautsch 1931) had been obtained. 



(d) The structure of gold compounds. 



During the past two years, our knowledge of the structure of various 

 types of gold compounds has developed considerably as a result of X-ray 

 crystallographic investigations. Those on the organic compounds are 

 being carried out at Oxford by Powell (1937) and his collaborators, those 

 on Mann's co-ordination compounds of aurous gold at Cambridge by 

 Wells (1936) while Cox and Webster (1936) have carried out their investi- 

 gations on potassium bromoaurate at Birmingham. These investigations 

 have established the planar configuration of the four valencies of tervalent 

 gold and the linear configuration of the two valencies of aurous gold. 



The X-ray investigation of the simplest organic gold compound, 

 diethylmonobromogold, is attended with difficulties arising from the 

 instability of the crystals to X-rays and even to light. In spite of these, 

 Powell has been able to carry out his analysis satisfactorily. The results, 

 are summarised in figures (i) and (2), reproduced by permission of the 

 Chemical Society. The orientation of the molecule in the unit cell is 

 indicated in the perspective diagram (Fig. i) where, for convenience, the 

 origin has been moved to | |- ^. The molecule, projected on the plane 

 of the gold and bromine atoms, is shown in Fig. 2. The carbon atoms 

 marked @ and are, respectively, above and below the plane of the 

 other atoms. The distances marked on Fig. 2 are subject to a probable 

 error of ± o • i A. The results show that two gold atoms and two 

 bromine atoms lie close together near the origin and that the mole- 

 cule is Au2(C2H5)4Br2. These four atoms form a rough square in a plane 

 somewhat inclined to (001). In order that the molecule may fit into the 

 unit cell, all the atoms must lie approximately in one plane as is shown by 

 the very small c dimension, and the four gold valencies must accordingly 

 lie in one plane and will be approximately at right angles to one another. 

 The suggested structure is in agreement with the needle habit of the crystals 



