38 SECTIONAL ADDRESSES. 



themselves are much less volatile than corresponds to their molecular 

 weights, as we should expect from the presence of the co-ordinate link. 

 The high values of the dielectric constant (water, f ormamide, and hydrogen 

 peroxide about 80, methyl alcohol 35, ethyl alcohol 27) are further evidence 

 of co-ordination. This reference to the dielectric constant raises a point 

 which is worth mentioning, although I cannot discuss it in detail here. 

 In some modern developments of the theory of organic reactions great 

 stress is laid on the dipole moment of such groups as hydroxy!. The 

 values of these moments are calculated from the dielectric constants of 

 the hydroxylic compounds, and are assumed to apply to the single 

 unassociated molecules. Now since we have seen that the association 

 itself must increase the dielectric constant owing to the co-ordination of 

 the molecules with one another, it is by no means certain that values so 

 obtained hold good for the unpolymerised hydroxyl group. It is, if course, 

 quite possible that the same conditions which make the hydroxyl group 

 so ready to polymerise also give it a high dipole moment even in the 

 simple molecule ; but the rise in the dielectric constant which the associa- 

 tion itself must produce is a factor which must be taken into account, 

 especially as it is one which will vary with the temperature. 



This view that association is due to co-ordination throws light on the 

 behaviour of a group of substances whose position was hitherto rather 

 puzzling. There are many substances such as sulphur dioxide, ethers, and 

 amines which behave in many ways like associated liquids, and yet when 

 they are directly tested are found not to be associated ; they are volatile, 

 and give simple values of the molecular weight in the' pure state and in 

 non-associated solvents. It has long been a problem how such substances 

 should be classified. It is now clear that they contain only one of the 

 two elements necessary to co-ordination : they have donor atoms (oxygen 

 or nitrogen) but no acceptor atoms (the acceptor properties of hydrogen 

 attached to nitrogen are for some reason very weak). They are thus 

 incapable of polymerisation, and in the presence of non-associated liquids 

 they behave as normal non-associated substances. But in the presence 

 of a substance capable of association, and so containing acceptor as well 

 as donor atoms, they behave as associated substances. 



These considerations emphasise a very important and far-reaching 

 characteristic of the co-ordinate link, its one-sided nature. The two 

 atoms taking part in it perform quite different functions ; and in deter- 

 mining the structure of a co-ordination compound it is essential to show 

 which of the two co-ordinated atoms is the donor and which the acceptor. 

 This distinguishes the electronic view of co-ordination from the subsidiary 

 valency theory of Werner and his school ; there was no apparent reason 

 why two atoms which could form subsidiary valencies with a third atom 

 should not also form them with one another. We can see now that such 

 a linkage is impossible : there must be the necessary opposition in 

 character between the two atoms before co-ordination can take place. 

 It is true that Werner himself was saved by his almost uncanny insight 

 into molecular structure from falling into this error, but there was nothing 

 in his theory to save him from it, and not all his followers had as true an 

 intuition as he had himself. The recognition of this distinction, to which 

 the electronic interpretation directly leads, is a definite advance. 



