Yu. I. Solov'ev
Institute for me History of Science and Technology 103012 MoscowK-12. U.S.S.R.
D. 1. Mendeleev's Conceptions Concerning the Structure of Complex Compounds
Mendeleev was one of the first to formulate the problem of discovering similar features of various classes of compounds (ammoniates, crystalline hydrates, double salts) and to use these features as the basis for developinggeneral conceptions about their structure. In Mendeleev's view, the development of the chemistry of complex compounds was of great importance. He envisaged that it would he exactly this kind of chemistry that would become of special interest for scientific research. For a long time chemistry nearly completely ignored.. . the compounds with water of crystallization, double salts, etc., hut. . .some of the well-known relations here are so closely associated with the solution of the most important problems of our science (e.g., with those of dissociation, of the transition from solutions to certain compounds, of the atomic nature of elements, ete.) that it is high time to clarify these aspects of chemical knowledge a s well (I,p. 1098). To pay no attention to that class of compounds which is called molecular means to give up [understanding-Yu.S.1 the fundamental properties of atoms forming such compounds (2, p. 842).
D. I. Mendeleev was one of the first who formulated the problem of discovering similar features of various classes of complex compounds (ammoniates, crystalline hydrates, double salts) and used i t as the basis for developing general eonceptioos about their structure. He wrote In the first case, it seems most important to us toelucidate the similarity in the formationof various complex compounds, and it is exactly this similarity between ammonium compounds, on the one hand, and the water compoundsand double salts, on the other, which constitutes the main objective of the primary generalization (1, p. 1110). As early as 1871 Mendeleev arrived a t an extremely interesting conclusion that the nature of ammoniates and crystalline hydrates was similar. Comparing these two classes of complex compounds, he pointed out with sagacity Ammonia adds to salts both qualitatively and quantitativelyin exactly the same manner as water of crystallization. There are very many similar features in the general nature of NHs and H20 compounds-both of them form not only anumher of stable compounds which in the ease of HzOare called hydrates and in the ease of NHs, ammonia salts, and also a number of very unstable substances that dissociate easily releasing HzO and NHs (3, p. 103fi). Referring to his own observations, Mendeleev pointed to "a remarkahle fact, that water and ammonia are capable of successive, molecule after molecule, displacement of each other" (3, p. 1037). In the chemistrv of c o m ~ l e xcom~oundsin the 1870-1880's the chain theory bf Blom&and-~irgensen dominated; this was based, on the one hand, on the conceDts of variable valency, and on the other hand, on the theory of chemical structure. Aceordine to Blomstrand and Jomensen. Dentavalent nitrogen atoms, like tetravalent carbon atoms in organic compounds are capable of connecting to each other to form chains of various lengths. The links of these chains represent ammonia molecules (-NH3--NH,?) analogous to the chains of organic compounds the links of which are methylene radicals (-CH2-CH2-). According t o the chain theory, the structure of Luteo salt CoClyGNH3 was depicted by the following formula 494 / Journal of Chemical Education
and that of Gros'salt, PtCIc4NHa by the formula
D. I. Mendeleev subjected the chain theory to justified criticism. Having pointed out the fact that in platinum ammoniates the "change in specific properties of ammonia and platinum" occurred, the Russian scientist explained this by "their mutural binding" whereas according to Blomstrand and Jorgenson the ammonia molecules in ammoniates are bound notonly to the metal but to each other as well forming, as was mentioned above, a chain of a certain length.
. . . It seems that the chain of ammoniates can he infinite but I think that the most essential ~hortcomineof such a eoncention i s thnt it ~ignurci mmplrtr.) the numher 1.1 m ~ n w ~ i a lthat r s m , ~be held by p l s t i r u n Rexidrs. one van h.mIly ~ C I I I O bindmg M between twc, atom; uf niinwn in SMII .itable h d i e c ;I, p Ito(il~j. ~
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Mendeleev did not limit himself to the criticism of the views of others; he presented his point of view on the structure of ammoniates and other complex compounds. Mendeleev pointed out first of all that ammoniates cannot be considered in isolation since along with crystalline hydrates and double salts they represent one of the tvues .. of comulex com~ounds. Here is what he wrote W e . . .do not hold any of the notions about the ammonia-platinum compounds that existed before, but approach them in exactly the same way as double salts and water of crystallization; in other words, we introduce a more general conception of compounds higher than salt-like ones. The PtXrZNHa.. . type of compounds represents a body whose type is identical with that of PtXz.2KX or PtXr2H20, etc. (3,p. 1061).
When studying the hintling
