Hydrides E₂H₄ (E=N, P, As)

   Hydrazine, N2H4, is a colourless liquid (mp 275 K, bp 386 K), miscible with water and with a range of organic solvents, and is corrosive and toxic; its vapour forms explosive mixtures with air. Although Δ_fH^o(N₂H₄, 298 K)= ‏50.6 kJ mol^-1, N₂H₄ at ambient temperatures is kinetically stable with respect to N₂ and H₂. Alkyl derivatives of hydrazine  have been used as rocket fuels, e.g. combined with N₂O₄ in the Apollo missions.  N₂H₄ has uses in the agricultural and plastics industries, and in the removal of O₂ from industrial water boilers to minimize corrosion (the reaction gives N₂ and H₂O). Hydrazine is obtained by the Raschig reaction (the basis for the industrial synthesis) which involves the partial oxidation of NH₃ .

    Hydrazine is obtained from the Raschig process as the monohydrate and is used in this form for many purposes. Dehydration is difficult, and direct methods to produce anhydrous N₂H₄ include reaction below.

   In aqueous solution, N₂H₄ usually forms [N₂H₅]⁺‏ (hydrazinium) salts, but some salts of [N₂H₆]²⁺‏ have been isolated, e.g. [N₂H₆][SO₄]. The pKb values for hydrazine are given in equations below and the first step shows N₂H₄ to be a weaker base than NH₃ .

   Both N₂H₄ and [N₂H₅]‏ are reducing agents, and used for the determination of hydrazine.

  We have alreadymentioned the use of N₂H₄ in rocket fuels.  The stored energy in explosives and propellants (‘high energy density materials’) usually arises either from oxidation of an organic framework, or from an inherent high positive enthalpy of formation. For the hydrazinium salt [N₂H₅]₂[14.15]  Δ_fH^o(s,298 K)= 858 kJ mol^-1 (or 3.7 kJ g^-1), making [N₂H₅]₂[14.15] a spectacular example of a high energy density material.

Figure 1.1a shows the structure of N₂H₄, and the gauche conformation is also adopted by P₂H₄ in the gas phase.

   In the solid state, P₂H₄ has a staggered conformation (Figure 1.1c) while the related N₂F₄ exhibits both conformers. The eclipsed conformation (which would maximize lone pair–lone pair repulsions) is

not observed. Diphosphane, P₂H₄, is a colourless liquid (mp 174 K, bp 329 K), and is toxic and spontaneously inflammable; when heated, it forms higher phosphanes. Diphosphane is formed as a minor product in several reactions in which PH₃ is prepared and may be separated

from PH₃ by condensation in a freezing mixture. It exhibits no basic properties.  The [P₃H₃]^2- ion is formed in reaction 14.38 and is stabilized by coordination to the sodium centre in [Na(NH₃)₃(P₃H₃)]^-. In the solid state, the H atoms in [P₃H₃]^2- are in an all-trans configuration .