[38] This compound is known as nickel tetra-carbonyl. It appears to me yet premature to judge of the structure of such an extraordinary compound as Ni(CO)4. It has long been known that potassium combines with CO forming Kn(CO)n (Chapter IX., Note [31]), but this substance is apparently saline and non-volatile, and has as little in common with Ni(CO)4 as Na2H has with SbH3. However, Berthelot observed that when NiC4O4 is kept in air, it oxidises and gives a colourless compound, Ni3C2O3,10H2O, having apparently saline properties. We may add that Schützenberger, on reducing NiCl2 by heating it in a current of hydrogen, observed that a nickel compound partially volatilises with the HCl and gives metallic nickel when heated again. The platinum compound, PtCl2(CO)3 (Chapter XXIII., Note [11]), offers the greatest analogy to Ni(CO)4. This compound was obtained as a volatile substance by Schützenberger by moderately heating (to 235°) metallic platinum in a mixture of chlorine and carbonic oxide. If we designate CO by Y, and an atom of chlorine by X, then taking into account that, according to the periodic system, Ni is an analogue of Pt, a certain degree of correspondence is seen in the composition NiY4 and PtX2Y2. It would be interesting to compare the reactions of the two compounds.
[39] According to its empirical formula oxalate of nickel also contains nickel and carbonic oxide.
[40] The following are the thermo-chemical data (according to Thomsen, and referred to gram weights expressed by the formula, in large calories or thousand units of heat) for the formation of corresponding compounds of Mn, Fe, Co, Ni, and Cu (+ Aq signifies that the reaction proceeds in an excess of water):
| R = Mn | Fe | Co | Ni | Cu | |
| R + Cl2 + Aq | 128 | 100 | 95 | 94 | 63 |
| R + Br2 + Aq | 106 | 78 | 73 | 72 | 41 |
| R + I2 + Aq | 76 | 48 | 43 | 41 | 32 |
| R + O + H2O | 95 | 68 | 63 | 61 | 38 |
| R + O2 + SO2 + nH2O | 193 | 169 | 163 | 163 | 130 |
| RCl2 + Aq | +16 | 18 | 18 | 19 | 11 |
These examples show that for analogous reactions the amount of heat evolved in passing from Mn to Fe, Co, Ni, and Cu varies in regular sequences as the atomic weight increases. A similar difference is to be found in other groups and series, and proves that thermo-chemical phenomena are subject to the periodic law.
CHAPTER XXIII
THE PLATINUM METALS
The six metals: ruthenium, Ru, rhodium, Rh, palladium, Pd, osmium, Os, iridium, Ir, and platinum, Pt, are met with associated together in nature. Platinum always predominates over the others, and hence they are known as the platinum metals. By their chemical character their position in the periodic system is in the eighth group, corresponding with iron, cobalt, and nickel.
The natural transition from titanium and vanadium to copper and zinc by means of the elements of the iron group is demonstrated by all the properties of these elements, and in exactly the same manner a transition from zirconium, niobium, and molybdenum to silver, cadmium, and indium, through ruthenium, rhodium, and palladium, is in perfect accordance with fact and with the magnitude of the atomic weights, as also is the position of osmium, iridium, and platinum between tantalum and tungsten on the one side, and gold and mercury on the other. In all these three cases the elements of smaller atomic weight (chromium, molybdenum, and tungsten) are able, in their higher grades of oxidation, to give acid oxides having the properties of distinct but feebly energetic acids (in the lower oxides they give bases), whilst the elements of greater atomic weight (zinc, cadmium, mercury), even in their higher grades of oxidation, only give bases, although with feebly developed basic properties. The platinum metals present the same intermediate properties such as we have already seen in iron and the elements of the eighth group.