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Stories About Chemistry

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62. A Few Words About Complex Compounds

There were many great chemists in the nineteenth century. But three of them were peerless. They accomplished more for their science than anyone else. They laid the foundations of modern chemistry.

Two of them were Dmitry Mendeleyev, the creator of the Periodic Law and the Periodic System of Elements, and Alexander Butlerov, the author of the theory of structure of organic compounds.

The third was the German chemist Alfred Werner. His discovery is covered by the two words “coordination theory,” but it was epoch-making for inorganic chemistry.

...It all started when chemists took up the study of reactions between metals and ammonia. To a solution of an ordinary salt, such as copper chloride, they added spirits of ammonia. Evaporation of the solution resulted in beautiful blue-green crystals. Analysis showed these crystals to be of a simple composition, but it was a puzzling simplicity.

The formula of copper chloride is CuCl2. The copper is divalent, and everything is quite clear.

Nor were the crystals of the “ammonia” compound too complex: Cu(NH3)2Cl2.

But what forces combine the two ammonia molecules so stably with the copper atom? Both valences of this atom are already used up in the bond with the chlorine atoms. It appears that copper must be tetravalent in this compound.

Another example is the analogous cobalt compound Co(NH3)6Cl3. Cobalt is a typical trivalent element, but in this compound it seems to exhibit nonavalence!

Multitudes of such compounds were synthesized, and each of them was like a delayed-action mine nested in the foundation of the theory of valence.

The situation defied logical explanation. Many metals displayed quite preposterous valences.

Alfred Werner succeeded in accounting for this strange phenomenon.

His idea was that after saturating their ordinary, legitimate valences, atoms can still display additional valence. For instance, after copper has spent its two main valences on chlorine atoms, it can find two additional ones to combine with ammonia.

Compounds such as Cu(NH3)2Cl2 are called complex. In this compound the cation [Cu(NH3)2]2+ is complex. There are many substances in which the anion is of complex structure; for example, K2[PtCl6] contains the complex anion [PtCl6]2–.

But how many secondary valences can a metal display? This depends on its coordination number. The smallest value of the latter is 2, and the largest 12. In the copper-ammonia compound mentioned above it is 2, showing how many molecules of ammonia are combined with the copper atom.

Thus was solved the riddle of unusual valences.

A new branch of inorganic chemistry sprang up, the chemistry of complex compounds

Over a hundred thousand complex compounds are known at present. They are studied in chemical institutions and laboratories all over the world. They are of interest not only to theoretical chemists who try to find how all things are built and why they are built so.

Without complex compounds there would be no life. Both haemoglobin, an important component of the blood, and chlorophyll, the basis of all plant life, are complex compounds. Many ferments and enzymes are of complex constitution.

Analysts use complex compounds for carrying out very complicated analyses of a wide range of substances.

Many metals can be obtained in a very pure state with the aid of complexes. They find application as valuable dyes, and for softening water. In a word, complex compounds are omnipresent.


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