Note 10 — Chapter XXIII · THE PLATINUM METALS

Although palladium gives the same types of combination (with chlorine) as platinum, its reduction to RX2 is incomparably easier than that of platinic chloride, and in the case of iridium it is also very easy. Iridic chloride, IrCl4, acts as an oxidising agent, readily parts with a fourth of its chlorine to a number of substances, readily evolves chlorine when heated, and it is only at low temperatures that chlorine and aqua regia convert iridium into iridic chloride. In disengaging chlorine iridium more often and easily gives the very stable iridious chloride, IrCl3 (perhaps this substance is Ir2Cl6 = IrCl2,IrCl4, insoluble in water, but soluble in potassium chloride, because it forms the double salt K3IrCl6), than the dichloride, IrCl_2. This compound, corresponding to IrX2, is very stable, and corresponds with the basic oxide, Ir2O3, resembling the oxides Fe2O3, Co2O3. To this form there correspond ammoniacal compounds similar to those given by cobaltic oxide. Although iridium also gives an acid in the form of the salt K2Ir2O7, it does not, like iron (and chromium), form the corresponding chloride, IrCl6. In general, in this as in the other elements, it is impossible to predict the chlorine compounds from those of oxygen. Just as there is no chloride SCl6, but only SCl2, so also, although IrO3 exists, IrCl6 is wanting, the only chloride being IrCl4, and this is unstable, like SCl2, and easily parts with its chlorine. In this respect rhodium is very much like iridium (as platinum is like palladium). For RhCl4 decomposes with extreme ease, whilst rhodium chloride, RhCl3, is very stable, like many of the salts of the type RhX3, although like the platinum elements these salts are easily reduced to metal by the action of heat and powerful reagents. There is as close a resemblance between osmium and ruthenium. Osmium when submitted to the action of dry chlorine gives osmic chloride, OsCl4, but the latter is converted by water (as is osmium by moist chlorine) into osmic anhydride, although the greater portion is then decomposed into Os(HO)4 and 4HCl, like a chloranhydride of an acid. In general this acid character is more developed in osmium than in platinum and iridium. Having parted with chlorine, osmic chloride, OsCl4, gives the unstable trichloride, OsCl3, and the stable soluble dichloride, OsCl2, which corresponds with platinous chloride in its properties and reactions. The relation of ruthenium to the halogens is of the same nature.