Luteo cobaltic chloride

Hexammino - cobaltic Chloride, or Luteo-cobaltic Chloride, [Co(NH3)8]Cl3.—Several methods of preparation have been described. The best method is that of Jorgensen,1 whereby the salt is prepared by warming chloro-pentammino-cobaltic chloride, [Co(NH3)5C l]Cl2. in a pressure flask with 20 per cent, aqueous ammonia and ammonium chloride for several hours with constant shaking. After cooling, the mixture is removed from the flask and ammonia allowed to evaporate. The liquid is then diluted, hydrochloric acid added, and the whole heated on the water-bath, thus changing any aquo-pentammino-chloride into ehloro-pentammino-ehloride. More concentrated hydrochloric acid is added and the mixture cooled and filtered. The residue on the filter consists of ammonium chloride, chloro-pentammino-chloride, and hexammino-cobaltic chloride. Ammonium chloride is removed by treating with a 20 per cent, solution of hydrochloric acid, and the residue is then treated on a filter with cold water in which chloro-pentammino-cobaltic chloride is insoluble and hexammino-cobaltic chloride soluble. The salt is precipitated from its warm solution by the addition of half its volume of concentrated hydrochloric acid. 

Another procedure that was used in the early days of coordination chemistry was to refer to the compound by its color. For example, [Co(NH3)6]C13, an intensely yellow compound, was known as luteo cobaltic chloride because the prefix luteo indicates the yellow color. Similarly, [Co(NH3)5C1]C1 was known as purpureo cobaltic chloride because of its purple color, but this is not a satisfactory system of nomenclature to deal with a very large number of compounds. 

In solution, all the chlorine in luteo cobaltic chloride ([Co(NH3)6]Cl3, hexaamminecobalt(III) chloride) is immediately precipitated by sUver nitrate. None of its ammonia is removed by treatment with hydrochloric acid at 100 °C. Sulfiiric acid does not remove any ammonia but yields 02(804)3 I2NH3, that is, the chlorine atoms are replaced by sulfate groups. Some sort of very stable metal ammonia bonding but much less stable metal chlorine bonding is obviously indicated. 

Heating removes one-sixth of the ammonia, and only two-thirds of the chlorine in the resulting purpureo cobaltic chloride ([CoCl(NH3)5]Cl2) is precipitable by silver nitrate. Considering that halogen atoms bonded to the metal atom through other groups such as ammonia are precipitable by silver nitrate, while those bonded directly to the metal atom are not, Jorgensen proposed formula (2) for the luteo salt. 

At the very end of the eighteenth century, Tassaert—a French chemist so obscure in the history of chemistry that his first name remains unknown—observed that ammonia combined with a cobalt ore to yield a reddish-brown product. This was most likely the first known coordination compound. Throughout the first half of the nineteenth century, many other, often beautifully crystalline examples of various cobalt ammonates were prepared. These compounds were strikingly colored, and the names given to them—for example, roseo-, luteo- (from the Latin luteus, meaning deep yellow ), and purpureocobaltic chlorides—reflected these colors. 

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