252 SCIENCE PROGRESS 



which it is associated. The maximum valency of such radicles, 

 whether basic or acidic, is with a trivalent metal such as cobalt 

 or chromium three, and this maximum valency is only exerted 

 when all the associated groups are of the same character and 

 it diminishes to zero as one to three groups of opposite sign 

 enter the molecule. The compounds cited are therefore fully 

 formulated thus : 



[Co(NH 3 ) 6 ]Cl 3 , [Co(NH 3 ) 2 (H 2 0) 8 Cl 2 ]Cl, [Co(NH 3 ) 3 (N0 3 ) 3 ], 



[Co(NH 3 ) a (NO a ) 4 ]K 



A very large number of substances of this nature are known 

 in which ammonia is replaced by other groups or radicles ; for 

 example, pyridine can replace ammonia molecule for molecule, 

 ethylene diamine and propylene diamine can each replace two 

 molecules of ammonia. 



The residues of dibasic acids, of oxalic (C 2 4 ), of sulphuric 

 (S0 4 ), of carbonic (C0 3 ) can in such radicles replace two mono- 

 valent negative groups or atoms such as NO a or CI. There 

 are besides, possible replacements of ammonia which leave the 

 valency of the complex unaffected; one of these is of great 

 importance — the replacement of ammonia by water molecule 

 for molecule. 



Such complexes may be represented as follows : 



[MeA:r ++ • . • [MeA^X;]™^"' . . . (MeX;]"" 



where Me is a metallic atom, A a molecule of ammonia, or its 

 equivalent in water, pyridine or a diamine, and X a negative 

 group or atom, such as NO a or CI or its equivalent, the valency 

 of the complex being represented by negative or positive signs, 

 as shown. These complexes are held together by the metallic 

 atom Me, the affinity of which is mainly exerted in holding 

 together in firm attachment the six groups associated with it. 

 The valency and chemical character of the complex is deter- 

 mined partly by the nature of the metallic atom, but very 

 largely by the nature of the six groups, the replacement of a 

 positive group such as ammonia by a negative group such as 

 CI diminishing the positive valency by one unit or increasing 

 the negative valency similarly. 



Werner assumes that in these complexes the six groups are 

 symmetrically distributed round the metallic atom and that 

 they effectively occupy all the available space within which 



