42 SECTIONAL ADDRESSES. 



ease. Froni this we may conclude that when a chain of atoms is formed 

 by co-ordination which includes one or two double links, the sixth atom 

 of this chain will be able to approach the first very closely, and so may 

 be expected to react with it. In these and other ways the consideration 

 of possible intermediate co-ordination products may provide the clue to 

 many organic reactions. 



If this line of thought is to be pursued, there is a preliminary question 

 which requires investigation. We have seen that two conditions are 

 essential to the formation of a co-ordinate link, the presence of an atom 

 with an unshared pair of valency electrons (the donor), and of another 

 (the acceptor), which can add two electrons to its valency group. But 

 these conditions, though necessary, are not sufficient. They are both 

 fulfilled in most organic molecules other than those of hydrocarbons. The 

 normal hydrogen atom has only two electrons, and it can hold four : 

 every halogen atom, every oxygen atom, every trivalent nitrogen atom 

 has an unshared pair of valency electrons ; and yet halides, ethers, and 

 amines are not as a rule associated. For co-ordination to take place it 

 is necessary not only that such atoms should be present, but also that 

 they should be so linked that they are able to exercise their donor or 

 acceptor properties. Hydrogen, for example, is a powerful acceptor 

 when it is joined to oxygen or fluorine ; it is a weak acceptor when it is 

 joined to nitrogen ; it is practically not an acceptor at all when it is 

 combined with carbon or one of the heavier halogens. We cannot at 

 present explain these differences in behaviour, but it is quite easy to show 

 that they exist In the same way the donor properties of oxygen are 

 very largely influenced by its state of combination. If the influence of 

 co-ordination on reactivity in organic compounds is to be studied in 

 detail, the first necessity is a knowledge of the factors which promote 

 co-ordination itself, and this can only be attained by a careful examination 

 of the facts from this point of view ; a thorough investigation of the 

 influence of substitution on the tendency of molecules of a particular 

 type to associate with themselves, or to form addition compounds with 

 other substances, would no doubt throw much light on the question. 

 It would be particularly interesting to know what is the effect on activity, 

 both in donors and in acceptors, of the peculiar tendencies to reaction 

 which the modern organic chemist represents by positive and negative 

 signs. 



I have tried in these remarks to emphasise the fact that the modern 

 electronic interpretation of the theory of co-ordination has a value far 

 outside the range of those compounds which the theory was originally 

 devised to explain. There is too great a tendency even now to regard 

 the question of co-ordination as one which is of interest only in connection 

 with a highly special group of substances which the ordinary chemist 

 rarely meets, whereas in truth the study of this question has given us a 

 wider and a truer conception of the nature of the processes by which 

 molecules are built up. The determination of the factors which influence 

 chemical reaction is perhaps the most important of the fundamental 

 problems of chemistry, and it is essential that the factor of co-ordination, 

 with the new possibilities of reaction-mechanism which it opens up, should 

 be recognised and investigated. 



