Group VI.: 0, usually divalent, but tetravalent and possibly hexavalent in oxonium and other salts; S, Se, Te, di-, tetraand hexa-valent; Cr, di-, triand hexa-valent; Mo, W, di-, tri-, tetra-, pentaand hexa-valent.
Group VIII.: Fe, Co, divalent and trivalent; Ni, divalent; Os, Ru, hexavalent and octavalent; Pd, Pt, divalent and tetravalent; Ir, tri-, tetraand hexa-valent.
Perkin has himself contributed largely to our knowledge of such compounds; pentaand hexa-methylene derivatives have also received considerable attention (see P Olymethylenes) .
In the same way it may be shown that three tri-substitution, three tetra-substitution, one penta-substitution, and one hexa-substitution derivative are possible.
Another hexa-substituted benzene compound capable of direct synthesis is mellitic acid or benzene carboxylic acid, C6(000H)6.
If we accept Kekule's formula for the benzene nucleus, then we may expect the double linkages to be opened up partially, either by oxidation or reduction, with the formation of di-, tetra-, or hexa-hydro derivatives, or entirely, with the production of open chain compounds.
Hydriodic acid reduces it to hexamethylene" (cyclo-hexane or hexa-hydro-benzene); chlorine and bromine form substitution and addition products, but the action is slow unless some carrier such as iodine, molybdenum chloride or ferric chloride for chlorine, and aluminium bromide for bromine, be present.
A mono-, di-, and hexa-hydrate; the latter being the form usually obtained by the solution of the oxide or carbonate in hydrochloric acid.
The tetrachloride, WC1 41 is obtained by partial reduction of the higher chlorides with hydrogen; a mixture of the pentaand hexa-chloride is distilled in a stream of hydrogen or carbon dioxide, and the pentachloride which volatilizes returned to the flask several times.
Piperidine or hexa-hydropyridine, C 5 H 11 N, was first obtained in 1848 by distilling piperine with lime.