SECTION B.— CHEMISTRY. 



CO-ORDINATION COMPOUNDS. 



ADDRESS BY 



N. V. SIDGWICK, O.B.E., Sc.D., F.R.S., 



PRESIDENT OF THE SECTION. 



When the British Association last met in Leeds, thirty-seven years ago, 

 the attention of Section B was largely devoted to the discussion of 

 ionisation, and at a joint meeting with Section A the new theory of 

 Arrhenius was defended by van 't Hoff and Ostwald against the attacks 

 of such conservative die-hards as S. U. Pickering and Prof. H. E. 

 Armstrong. That meeting may be taken as marking the recognition in 

 this country of the distinction between ionised and non-ionised linkages. 

 It seems appropriate therefore that I should devote this address to the 

 discussion of a third species or sub-species of atomic linkage, that of 

 co-ordination. 



The theory of co-ordination is indeed by no means new : it is only a 

 few years younger than that of electrolytic dissociation ; but its inter- 

 pretation, and especially the establishment of its relation to the older 

 theory of structural chemistry, have only become possible through the 

 advance made in our knowledge of atomic structure in the last few years ; 

 and there are still many points in which its bearing on questions of general 

 chemistry is not yet fully realised. 



Werner's Theory of Co-ordination, which was first put forward in 

 1891, the year after our last meeting at Leeds, originated in an attempt 

 to explain the structure of certain compounds formed by apparently 

 saturated molecules with one another. A large number of such com- 

 pounds, often very stable, had been observed, but they were commonly 

 disregarded by chemists, or were shelved under the convenient name of 

 molecular compounds ; and such attempts as had been made to formulate 

 them on the lines of structural chemistry had been conspicuously 

 unsuccessful. The most marked peculiarities of these compounds were 

 three. In the first place their structure appeared to be quite independent 

 of the ordinary rules of valency, according to which the numerical value 

 of the valency of an atom element was primarily determined by the 

 group in the periodic table to which it belonged, first rising and then 

 falling by single units as we go from one group to the next. In these 

 compounds the structure was rather determined by the tendency of four 

 or six atoms or groups to arrange themselves round a central atom. 

 Secondly, in these complexes, a univalent atom or group of atoms such 

 as chlorine or N0 2 could be replaced by a whole apparently saturated 

 molecule such as water or ammonia without affecting the stability of the 

 complex. Thirdly, such replacement was always accompanied by a 



