Pnictogen hydrides or hydrogen pnictides are binary compounds of hydrogen with pnictogen
atoms (elements of group 15: nitrogen, phosphorus, arsenic, antimony, and bismuth) covalently
bonded to hydrogen.
== Pnictogen trihydrides == The simplest series has the chemical formula
XH3 (less commonly H3X), with X representing any of the pnictogens. They take on the pyramidal
structure (as opposed to the trigonal planar arrangement of the group 13 hydrides), and
therefore are polar. These pnictogen trihydrides are generally increasingly unstable and poisonous
with heavier elements. Like the simple hydrogen halides and chalcogenides,
the pnictogen hydrides are water-soluble. Unlike other hydrides such as hydrogen sulfide
and hydrogen fluoride, which form acidic aqueous solutions, ammonia dissolves in water to make
ammonium hydroxide which is basic (by forming a hydroxide ion as opposed to hydronium).
Phosphine is also water-soluble. Arsine and stibine are less so, and it is not known if
bismuthine is.
Some properties of the pnictogen trihydrides follow:
These gases have no smell in pure form, instead gaining it when in contact with air. Ammonia
has an infamous, intense odour resembling urine and/or fish, commonly the result of
the decomposition of urea. Phosphine smells like fish or garlic, and stibine like rotten
eggs, similar to hydrogen sulfide and selenide.
== Dipnictogen tetrahydrides ==
== Properties ==
All straight-chain saturated hydrogen pnictides follow the formula XnHn+2.
Ammonia is produced industrially on the largest scale among all compounds. Like water, hydrogen
bonding results in a high melting and boiling point compared to the other pnictogen hydrides,
although 26% is lost on melting, another 7% as the liquid is heated to boiling, and the
remaining 67% upon boiling. Other effects of hydrogen bonding are a high dielectric
constant as well as low values of density, viscosity, and electrical conductivity. Like
water, it is an excellent and often-used ionising solvent. Over twenty other hydrides of nitrogen
are known, the most important being hydrazine (N2H4) and hydrogen azide (HN3). Hydrazine
has physical properties that are remarkably similar to those of water: its melting and
boiling points are 2.0 °C and 113.5 °C, the density of the solid at −5 °C is 1.146
g/cm3, while that of the liquid at 25 °C is 1.00 g/cm3. The azanes are a series which
include ammonia, hydrazine and triazane. Phosphine, a toxic, colourless gas, is the
most stable phosphorus hydride and is the first of the homologous straight-chain polyphosphane
series PnHn+2 (n = 1–9) that become increasingly thermally unstable as n increases. Other cyclic
and condensed polyphosphane series are known, from PnHn to PnHn−18, amounting to 85 known
phosphanes in 1997. Insoluble in water but soluble in organic liquids (as well as carbon
disulfide and trichloroacetic acid), phosphine is a strong reducing agent.Arsine, stibine
and bismuthine are highly toxic, thermally unstable, and colourless gases. No appreciable
hydrogen bonding is found in phosphine, arsine, stibine or bismuthine, and there is no appreciable
tendency to dissociate like ammonia to MH+4 and MH−2 (M = P, As, Sb, Bi). The pnictogen
hydrides become denser down the group and the M–H bond lengths increase, while the
H–M–H bond angle decreases slightly. The standard enthalpies of formation reflect the
increasing thermal instability going down the group. Arsine decomposes to arsenic and
hydrogen at 250–300 °C, stibine to antimony and hydrogen at room temperature, and bismuthine
to bismuth and hydrogen above −45 °C. Arsine and stibine are very easily oxidised to arsenic
or antimony trioxide and water; a similar reaction happens with sulfur or selenium.
Reaction with metals at elevated temperatures leads to arsenides and antimonides. A few
lower hydrides are known, such as As2H4, but they are even more unstable and their properties
are unknown.Imidogen, a radical composed of one hydrogen atom and one nitrogen atom (NH),
can be classed as a pnictogen hydride.
== See also == Ammonium
Hydrazoic acid