July 7, 1892] 



NA TURE 



This view seems plausible, and in accordance with the chemical 

 properties of the substance, and I should have no hesitation in 

 accepting it if we had not, in the further pursuit of our work on 

 metallic carbonyls, met with another substance — a liquid com- 

 pound of iron with carbonic oxide — which in its properties bears 

 so much resemblance to the nickel compound that one cannot 

 assign to it a different constitution, whilst its composition makes 

 the adoption of a similar structural formula next to impossible. 

 It contains, for one equivalent of iron, five equivalents of car- 

 bonyl. To assign to it a similar constitution, one would, there- 

 fore, have to assume that iron did exercise ten valencies, or two 

 more than any other known element, a view which very few 

 chemists would be prepared to countenance. The atomic re- 

 fraction of iron in this compound, which Dr. Gladstone has had 

 the kindness to determine, is as unusual as that of the nickel in 

 the nickel compound, and bears about the same ratio to the 

 atomic refraction of iron in other compounds. We have, there- 

 fore, to find another explanation for the extraordinarily high 

 atomic refraction of these metals in their compounds with carbon 

 monoxide, which may possibly modify our present view on this 

 subject. As to the structure of these compounds themselves, we 

 are almost bound to assume that they contain the carbonyl atoms 

 in the form of a chain. 



The ferro-carbonyl is prepared in a similar manner to the 

 nickel compound. The iron used is obtained by heating iron 

 oxalate at the very lowest temperature possible. This car- 

 bonyl forms, however, with such very great difficulty, that we 

 overlooked its existence for a long time, and great precautions 

 have to be taken to obtain even a small quantity of it. It forms 

 an amber-coloured liquid, of which I have a small quantity before 

 me. It solidifies below - 21° C. to a mass of needle-shaped 

 crystals. On heating the vapour to 180° C, it is completely de- 

 composed into iron and carbonic oxide. The iron mirrors before 

 me have been obtained in this way. Its chemical composition 

 isFe(CO)5. 



It is interesting that, within a short time after we had made 

 known the existence of this body, Sir Henry Roscoe found it in 

 carbonic oxide gas which had stood compressed in an iron 

 cylinder for a considerable time, and expressed the opinion that 

 the red deposit which sometimes forms in ordinary steatite gas- 

 burners is due to the presence of this substance in ordinary 

 illuminating gas. Its presence in compressed gas used for lime- 

 lights has been noticed by Dr. Thorne, whose attention was 

 called to the face that this gas sometime-; will not give a proper 

 light because the incandescent lime becomes covered with oxide 

 of iron. 



M. Garnier, in a paper communicated to the French Academy 

 of Sciences, supposes even that this gas is sometimes formed in 

 large quantities in blast-furnaces when they are working too 

 cold, and refers to some in tances in which he found large 

 deposits of oxide of iron in the tubes leading away the gas from 

 these furnaces. I find it ditificult to believe that the temperature 

 of a blast-furnace could ever be sufficiently reduced as to give 

 rise to the formation of this compound. On the other hand, 

 it is highly probable that the formation of this compound of 

 iron and carbonic oxide may play an important role in that 

 mysterious process by which we are still making, and have been 

 making for ages, the finest qualities of steel, called the cementa- 

 tion process. 



The chemical behaviour of the substance towards acids ahd 

 oxidizinr agents is exactly the same as that of the nickel com- 

 pound, but to alkalies it behaves diflferently. The liquid dissolves 

 without evolution of gas. After a while a greenish precipitate 

 is formed, which contains chiefly hydrated-ferrous oxide, and the 

 solution becomes brown. On exposure to the air, it takes up 

 oxygen ; the colour changes to a dark red, whilst hydrated ferric 

 oxide separates out. 



We have so far not been able to obtain from this solution any 

 compound fit for analysis, and are still engaged upon unravelling, 

 the nature of the reaction that takes place, and of the compounds 

 that are formed. 



Although the solution resembles in appearance to some extent 

 the solutions obtained by treating potassium carbonyl with water, 

 it does not give any of the characteristic reactions of the latter. 



When speaking of potassium carbonyl, I mentioned that, by 

 its treatment with water, croconate of potassium was obtained, 

 which has the formula K^CjOg. We have transformed this by 

 double decomposition into ferrous croconate (FeCsOj), a salt 

 forming dark crystals of metallic lustre resembling iodine, 

 which is not volatile, and dissolves readily in water, the 



NO. I 184, VOL. 46] 



solution giving all the well-known reactions of iron and of 

 croconic acid. You will note how entirely different the 

 properties of this substance are from those of iron carbonyl, 

 which I have described to you ; yet, on reference to its 

 composition, you will find that it contains exactly the same 

 number of atoms of iron, carbon, and oxygen as the latter. 

 This is a very interesting case of isomerism, considering that 

 both compounds contain only iron, carbon, and oxygen. 



The difference in the properties of these two bodies becomes 

 explainable by comparing the structural formula of the two 

 substances. 



I would now call your attention to the great difference in the 

 constitution of the potassium carbonyl and that of the nickel 

 and ferro carbonyl. In the former the metal potassium is com- 

 bined with the oxygen in the carbon>l ; in the latter the metals 

 nickel and iron are combined with the carbon of carbonyl. In 

 the first case we have a benzole ring with its three single and 

 three double bonds ; in the second a closed chain with only 

 single bonds. It is evident that the chemical properties of these 

 substances must be widely different. 



The ferro-penta-carbonyl remains perfectly unchanged in the 

 dark, but if it is exposed to sunlight it is transformed into a solid 

 body of remarkably fine appearance, of gold colour and lustre, 

 as shown by the sample in this tube. 



This solid body is not volatile, but on heating it in the absence 

 of air, iron separates out and liquid ferro-carbonyl distils over. 

 If, however, it is heated carefully in a current of carbonic oxide 

 it is reconverted in'o the ferro-penta-carbonyl, and completely 

 volatilized. We have so far found no solvent for this substance, 

 so that we have no means as yet of obtaining it in a perfectly 

 pure state. Several determinations of the iron in different 

 samples of the substance have led to fairly concordant figures, 

 which agree with the formula Feo(C0)7, or di ferro-hepta- 

 carbonyl. 



The interesting properties of the substances described have 

 naturally led us "to try," as Lord Kelvin once put it to me so 

 prettily, " to give wings to other heavy metals." We have tried 

 all the well-known and a very large number of the rarer metals ; 

 but with the exception of nickel and iron we have so fir been 

 entirely unsuccessful. Even cobalt, which is so very like nickel, 

 has not yielded the smallest trace of a carbonyl. Ttiis led me to 

 study the question whether, by means of the action of carbonic 

 oxide, the separation on a large scale of nickel from cobalt could 

 not be effected, which has so far been a most complicated 

 metallurgical operation ; and subsequently I was led to investigate 

 whether it would not be possible to use carbonic oxide to extract 

 nickel industrially direct from its ores. 



It had been established that pure nickel prepared with very 

 great precautions in a glass tube, could be partly volatilized by 

 carbonic oxide, and that from the gas thus obtained the nickel 

 could be separated again by heating. The questions to be 

 studied were, therefore, whether it would be possible to 

 reduce the ores, on an industrial scale, under such conditions 

 as to obtain the nickel in a sufficiently finely divided and 

 active a state that the carbonic oxide would volatilize it ; 

 whether such action would be sufficiently rapid to allow 

 of its industrial application ; whether it would be sufficiently 

 complete to remove all the nickel from the ore ; and 

 whether none of the other constituents of the ore would 

 pass with the nickel and render it unfit for use ; and further, 

 whether the nickel could be completely sepaiated out of the 

 gas within practical limits ; and whether the recovered car- 

 bonic oxide could be made use of over and over again. 



For solving these problems within the limits of the resources 

 of a laboratory, I have devised apparatus which con-ists 

 of a cylinder divided into many compartments, through 

 which the properly prepared ore is passed veiy slowly by 

 means of stirrers attached to a shaft. On leaving the bottom 

 of this cylinder, the ore pa.sscs through a transporting 

 screw, and from this to an elevator, which returns it to the 

 top of the cylinder, so that it passes many limes through 

 the cylinder, until all the nickel is volatilized. Into the 

 bottom of this cylinder we pass carbonic oxide, which leaves it 

 at the top charged with nickel carbonyl vapour, and passes 

 through the conduits shown here into tubes set in a furnace and 

 heated to 200°. Here the nickel separates out from the nickel 

 carbonyl. The carbonic oxide is regenerated and taken back 

 to the cylinder by means of a fan, so that the same gas is made 

 to carry fresh quantities of nickel out of the ore in the cylinder, 

 and to deposit it in these tubes an infinite number of times. 



