1892.] on Metallic Carhonyls. 675 



atomic refractive powers in different compounds, it enters with 

 a larger number of valencies into the compound which shows a 

 higher refractive power. In accordance with this view, the very much 

 greater refractive power of the nickel in the carbonyl would find an 

 explanation in assuming that this element, which in all its other 

 known combinations is distinctly bivalent, exercises in the carbonyl 

 the limit of its valency, viz. 8, assigned to it by Mendeleeff, who 

 placed it into the eighth group in his Table of Elements. This would 

 mean that one atom of nickel contained in the nickel carbonyl is com- 

 bined directly with each of the four bivalent atoms of carbonyl, each 

 of which would saturate two of the eight valencies of nickel, as is 

 shown by this formula — 



O 



G 



II 



: C = Ni = C : O 



II 







o 



This view seems plausible, and in accordance with the chemical pro- 

 perties 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 compound 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 equi- 

 valents of carbonyl. To assign to it a similar constitution, one would, 

 therefore, 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 refraction of iron in 

 this compound, which Dr. Gladstone has had the kindness to deter- 

 mine, 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, therefore, to find another explanation for the 

 extraordinarily high atomic refraction of these metals in their com- 

 pounds with carbon-monoxide, which may possibly modify our present 

 view on this subject. As to the structure of these compounds them- 

 selves, we are almost bound to assume that they contain the carbonyl 

 atoms in the form of a chain, as I have represented on this dia^^ram. 



Ni 



O O 



C— C d-(5 \ Fe— C-C-'C ^ 



Fe O : O 



\ C— C "^^ C-C / Fe-C— C— ^ 



6 6 



Nickel-tetra- carbonyl, Fcrro-penta-cai bonyl. Di-fcrro-hcpta-carbonyl, 



C:0 



