to the inorganic acids;
whilst A. Kekule (_Zeit. f. Chemie_, 1866, 2, p. 308), on account of
their ready condensation to form azo compounds and their easy
reduction to hydrazines, assumed that they were substances of the type
R.N:N.Cl. The constitution of the diazonium group -N2.X, may be
inferred from the following facts:--The group C6H5N2- behaves in many
respects similarly to an alkali metal, and even more so to the
ammonium group, since it is capable of forming colourless neutral
salts with mineral acids, which in dilute aqueous solution are
strongly ionized, but do not show any trace of hydrolytic dissociation
(A. Hantzsch, _Ber._, 1895, 28, p. 1734). Again, the diazonium
chlorides combine with platinic chloride to form difficultly soluble
double platinum salts, such as (C6H5N2Cl)2.PtCl4; similar gold salts,
C6H5N2Cl.AuCl3, are known. Determinations of the electrical
conductivity of the diazonium chloride and nitrate also show that the
diazonium radical is strictly comparable with other quaternary
ammonium ions. For these reasons, one must assume the existence of
pentavalent nitrogen in the diazonium salts, in order to account for
their basic properties.

The constitution of the isomeric diazo hydroxides has given rise to
much discussion. E. Bamberger (_Ber._, 1895, 28, pp. 444 et seq.) and
C. W. Blomstrand (_Journ. prakt. Chem._, 1896, 53, pp. 169 et seq.)
hold that the compounds are structurally different, the normal
diazo-hydroxide being a diazonium derivative of the type
R.N([3:]N).OH. The recent work of A. Hantzsch and his pupils seems to
invalidate this view (_Ber._, 1894, 27, pp. 1702 et seq.; see also A.
Hantzsch, _Die Diazoverbindungen_). According to Hantzsch the isomeric
diazo hydroxides are structurally identical, and the differences in
behaviour are due to stereo-chemical relations, the isomerism being
comparable with that of the oximes (q.v.). On such a hypothesis, the
relatively unstable normal diazo hydroxides would be the
_syn-_compounds, since here the nitrogen atoms would be more easily
eliminated, whilst the stable iso-diazo derivatives would be the
_anti-_compounds, thus:

R . N R . N
.. ..
HO . N N . OH
Normal hydroxide Iso hydroxide
(Syn-compound) (Anti-compound)

In support of this theory, Hantzsch has succeeded in isolating a