Palladium and platinum
Both of these metals are silvery white metals which are very inert. Palldium gas a very weak yellowish hue.
Palladium only is attacked by strong oxidizing acids (e.g. concentrated nitric acid), platinum is not even attacked by aqua regia.
As with the other four members of the so called 'platinum metals' (ruthenium, rhodium, osmium and iridium) the metals palladium and platinum differ a lot from their lighter group member nickel.
The picture shows a sample of the metal palladium. It is a small sample, with a diameter of approximately 4 mm.
The metals can be purchased on eBay in the massive form and sometimes as thin foil, but they are very expensive, with prices ranging from $10 per gram to $50 per gram. Because of their high price, they are not really interesting for the home chemist, although chemically they are interesting and have a rich aqueous chemistry.
In their compounds, palladium and platinum can be in the +2 oxidation state and in the +4 oxidation state. The free aqueous metal ions hardly play any role in the aqueous chemistry of the metals. The aqueous chemistry of these metals is confined to coordination complexes, the most common ones being PtCl42- and PdCl42-. The platinum complex is red/brown, the palladium complex is brown with a somewhat yellowish tinge.
Both platinum compounds and palladium compounds can be obtained remarkably easily, due to their use in photography and alternative processes. The following compounds are available at photography raw chemical suppliers, albeit at high prices:
Palladium chloride is a brown powder, almost insoluble in water. It, however, easily can be dissolved in a solution of sodium chloride, forming a brown solution of sodium tetrachloropalladate (II). Such solutions are easily reduced to metallic palladium. This also is the basis of their use in photography and alternative processes.
Potassium tetrachloroplatinate (II) is a red/brown powder, which dissolves in water easily. Solutions of this compound are red/brown. Like solutions of tetrachloropalladate (II), the metal can be separated easily by means of mild reductors. This compound sometimes is called potassium chloroplatinite.
A company, specialized in precious metal salts for alternative and antique processes is Bostick and Sullivan. They have quite some palladium and platinum salts, besides the two mentioned above.
Both the palladium and the platinum salts are quite interesting, due to their coordination chemistry and the possibility to form interesting colloids, but their high price probably makes the salts less interesting for most home chemists.