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.
Baeyer (Ber., 1902, 35, p. 1201) regards them as oxonium salts containing tetravalent oxygen (C 2 H 5) 2 :0:(MgR) (X), whilst W.
OH, oxonium hydroxide (compare sulphonium salts).
OH towards bases, and as NH 3 :0 towards acids, the salts in the latter case being of the oxonium type.