36 SECTIONAL ADDRESSES 



ordinary conditions the cuprous halides, the silver halides and the aureus 

 halides would appear to have the general formula : 



X 



M M 



\ / 

 X 



where X = halogen, indicating the 2-covalency of cuprous copper, 

 argentous silver and aureus gold in these compounds. 



Unlike bivalent copper, bivalent silver in its salts has been shown by 

 Morgan and Burstall (1928) to exist only as a complex ion, e.g. bis-cLo.'- 

 dipyridylargentic persulphate, [Ag adipyJSjOg, im-aa'-dipyridylargentic 

 nitrate, chlorate and perchlorate, [Ag 3dipy]X2, from which it is obvious 

 that bivalent silver may have co-oi-dination numbers of 4 and 6. On the 

 other hand, Cox, Wardlaw and Webster (1936) have shown that bivalent 

 silver is completely analogous to bivalent copper in giving like the latter 

 metal a derivative with picolinic acid and which like the cupric compound 

 has a planar structure. The constitution of these compounds is con- 

 veniently represented thus : 



C=0 



indicating the 4-covalency of bivalent copper and bivalent silver in this 

 type of compound. 



That cuprous copper and argentous silver can exhibit 2- and 4-covalency 

 is well established. It was proved by Bassett and Corbet (1924) in the course 

 of their phase-rule study of complex cyanides. They isolated the salts, 

 K[Cu(CN)2], K[Cu2(CN)3].H,0, K3[Cu(CN)J, K3[Cu(CN)J.H20, 

 K[Ag(CN)2], K[Ag2(CN)3].H20 and K3[Ag(CN)J.H20 ; but the only 

 complex cyanide of aurous gold of which they were able to prove the 

 existence and to isolate was the well-known potassium aurocyanide, 

 K[Au(CN)2]. The inability of aurous gold to exhibit a higher co-ordi- 

 nation number than 2 has also been recently emphasised by Mann, Wells 

 and Purdie (1936 and 1937) in their studies of the trialkylphosphine 

 and trialkylarsine derivatives of cuprous, silver and aurous halides. The 

 cuprous and silver compounds derived from the iodides have the general 

 formula [R3P(As)->Cu(Ag)I]4 as shown by their molecular weights and. 

 are systematically named by the authors as fe^/'«^w[iodotrialkylphosphine 

 (or arsine) copper (or silver)]. Crystallographic investigations strikingly 

 revealed the existence of these four-fold macro molecules in the solid 

 state and the tetrahedral configurations of the 4-covalent cuprous and 

 argentous silver complexes and, in addition, and for the first time, the 



