_Metallic Diazo Derivatives._--Benzene diazonium chloride is decomposed by silver oxide in aqueous solution, with the formation of _benzene diazonium hydroxide_, C6H5·N(OH):·N. This hydroxide, although possessing powerful basic properties, is unstable in the presence of alkalis and neutralizes them, being converted first into the isomeric benzene-diazotic acid, the potassium salt of which is obtained when the diazonium chloride is added to an excess of cold concentrated potash (A. Hantzsch and W. B. Davidson, _Ber._, 1898, 31, p. 1612). _Potassium benzene diazotate_, C6H5N2·OK, crystallizes in colourless silky needles. The free acid is not known; by the addition of the potassium salt to 50% acetic acid at -20° C., the acid anhydride, _benzene diazo oxide_, (C6H5N2)2O, is obtained as a very unstable, yellow, insoluble compound, exploding spontaneously at 0° C. Strong acids convert it into a diazonium salt, and potash converts it into the diazotate. On the constitution, of these anhydrides see E. Bamberger, _Ber._, 1896, 29, p. 446, and A. Hantzsch, _Ber._, 1896, 29, p. 1067; 1898, 31, p. 636. By the addition of the diazonium salts to a hot concentrated solution of a caustic alkali, C. Schraube and C. Schmidt (_Ber._, 1894, 27, p. 520) obtained an isomer of potassium benzene diazotate. These _iso-_diazotates are formed much more readily when the aromatic nucleus in the diazonium salt contains negative radicals. _Potassium benzene iso-diazotate_ resembles the normal salt, but is more stable, and is more highly ionized. Carbon dioxide converts it into _phenyl nitrosamine_, C6H5NH·NO (A. Hantzsch). The potassium salt of the iso-diazo hydroxide yields on methylation a nitrogen ether, R·N(CH3)·NO, whilst the silver salt yields an oxygen ether, R·N:N·OCH3. These results point to the conclusion that the iso-diazo hydroxide is a tautomeric substance. The same oxygen ether is formed by the methylation of the silver salt of the normal diazo hydroxide; this points to the conclusion that the isomeric hydroxides, corresponding with the silver derivatives, have the same structural formulae, namely, R·N:N·OH. These oxygen ethers contain the grouping -N:N-, since they couple very readily with the phenols in alkaline solution to form azo compounds (q.v.) (E. Bamberger, _Ber._, 1895, 28, p. 225); they are also explosive.