Hydrazinium

Structures of two protonated derivatives of hydrazine: N₂H+
5 and N₂H2+
6.

Hydrazinium is the cation with the formula N₂H+
5. In some cases, "hydrazine" implies N₂H+
5, e.g. the salt hydrazinium sulfate. Hydrazinium is derived from hydrazine by protonation (treatment with a strong acid). Salts of hydrazinium are common reagents in chemistry and are often used in certain industrial processes. Both are water-soluble, colorless salts.^[1] Hydrazinium is a weak acid with pK_a = 8.1.

Hydrazinium azide

One of the most unusual (and more unstable) salts of hydrazinium is the azide. Hydrazine azide (N₅H₅), the salt of hydrazine and hydrazoic acid, is of scientific interest, because of its high nitrogen content and explosive properties. Structurally, it is [N
2H
5]+
[N
3]−
.^[2] It decomposes explosively into hydrazine, ammonia, and nitrogen gas:^[3]

12 N
5H
5 → 3 N
2H
4 + 16 NH
3 + 19 N
2

Hydrazinediium

Hydrazinium may be protonated to give a dication known as hydrazinediium or hydrazinium(2+); with the formula N₂H2+
6. Reaction of hydrazinium azide with sulfuric acid gives quantitative yields of pure hydrazinediium sulfate and hydrazoic acid:^[4]

[N₂H+
5][N−
3] + H₂SO₄ → [N₂H2+
6][SO2−
4] + HN₃

Hydrazinediium adopts an ethane-like structure.

References

1. Jean-Pierre Schirmann, Paul Bourdauducq "Hydrazine" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2002. doi:10.1002/14356007.a13_177.
2. Chiglien, G.; Etienne, J.; Jaulmes, S.; Laruelle, P. (15 September 1974). "Structure cristalline de l'azoture d'hydrazinium, N5H5". Acta Crystallographica Section B. 30 (9): 2229–2233. doi:10.1107/S0567740874006790.
3. G. B. Manelis (2003). Thermal decomposition and combustion of explosives and propellants. CRC Press. p. 235. ISBN 0-415-29984-5.
4. Klapötke, T.; Peter S. White; Inis C. Tornieporth-Oetting (1996). "Reaction of hydrazinium azide with sulfuric acid: the X-ray structure of [N
   2H
   6][SO
   4]". Polyhedron. 15 (15): 2579–2582. doi:10.1016/0277-5387(95)00527-7.