The triad and dyad ions in the case of these metals exhibit remarkable differences of colour. Thus chromous ions are blue, chromic, green ; basic ferric ions are orange-yellow, ferrous, pale green; manganic, brown, manganous, pale pink ; cobaltous, red, and nickelous, grass-green. Hence a change in the number of electrons attached to the metallic ion is accompanied by a striking colour-change.

The halides of the palladium and platinum groups of metals closely resemble in character those of gold, which have already been described. The dihalides of the palladium group are all soluble, save Pdl2, which is prepared by precipitation with potassium iodide. Nitro-hydro-chloric acid yields the higher chloride ; it remains on evaporation. These form wi|th chlorides of the alkalies double salts, e.g. R11CI3.2HCI, RI1CI3.2HCI, and PdCl4.2HCl; the latter are probably ionised as KK and =PdCl6, etc. Chlorine also acts directly on red-hot metals of the platinum group, forming a mixture of chlorides ; these, on heating, lose chlorine, giving lower chlorides. Solutions of the halides can also be prepared by the action of the halogen acid on the respective oxides. On heating to a high temperature, all these halides are decomposed into the metal and halogen. The compounds K2PtCl6 and (NH4)2PtCl6 require special mention; they are orange salts, nearly insoluble in water, and are used as tests for potassium and ammonium, and also as a precipitant in estimating these ions. Their existence is probably to be ascribed to the power possessed by chlorine of sometimes acting as a triad, and the structural formula is believed to beColour Of Ions 32

Halides of certain complex groups are also known. When these contain oxygen or hydroxyl, (OH), they are generally termed basic salts or halo-acids; they will be considered later. The others may be divided into two classes : those like ammonium halides, and those derived from hydrocarbons.