210 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



a remarkable relationship between the number of ammonia molecules 

 associated with the complex salt and the properties of the salt, both in 

 solution and in a crystalline state. While the compounds containing 

 ammonia, or ammonia derivatives, are, in general, the most stable repre- 

 sentatives of this class, many other compounds are known where other 

 molecules function in a manner similar to those of ammonia. These 

 complex salts have been studied extensively by a number of investigators, 

 notably by Werner, 16 who has proposed a theory of the constitution of 

 these compounds which has met with remarkable success in accounting 

 for their properties. It is not proposed to give here an extended exposi- 

 tion of Werner's theory, since that is beyond the scope of this mono- 

 graph. However, a brief outline may be given here, in order to make 

 intelligible the relation between the constitution of the complex metal- 

 ammonia salts and their ionic properties. 



According to Werner's theory, the strongly electronegative elements 

 or groups of elements are attached to the nuclear atom by what are termed 

 principal valences, while neutral groups of molecules such as ammonia are 

 associated with the nuclear atom by auxiliary or secondary valences. A 

 definite number of atoms or groups is always attached to the nuclear atom, 

 either by principal or secondary valences, and this number, which is 

 usually 6 and sometimes 4, is fixed. The number of atoms or groups so 

 attached is called the co-ordination number. The charge on the nuclear 

 complex depends upon the number of principal valences comprised within 

 the co-ordination number. If N is the normal valence of the nuclear atom, 

 C the co-ordination number of the nuclear group, and n the number of 

 secondary valences satisfied in the nuclear group by neutral complexes, 

 such as ammonia, etc., then the number of charges on the nuclear group 

 or complex is: q = N C + n. Usually the co-ordination number is 

 6 or 4. If, for example, the co-ordination number is 6 and the normal 

 valence of the nuclear atom is 4, then, if n = 2, that is if two molecules 

 are associated in the nuclear complex, q = 0, and the charge on the com- 

 plex will be zero. If, on the other hand, n were 0, the charge on the 

 nuclear complex would be 2 ; that is, the nuclear complex would carry 

 two negative charges. On the other hand, if n were 6, q = + 4; that is, 

 the nuclear complex would carry 4 positive charges. For a co-ordination 

 number 6, the maximum variation in the charge on the nuclear complex 

 is from 4 positive to 2 negative charges. An example will serve to make 

 the relationships clear. Platinum chloride, PtCl 4 , in which platinum 

 appears with the principal valence of 4, forms with ammonia the follow- 

 ing series of complexes, all of which are known except the second. 



" Werner, New Ideas on Inorganic Chemistry. Trans, by E. P. Hedley, 1011. 



