Pnictogen hydrides or hydrogen pnictides are binary compounds of hydrogen with

pnictogen , - ! colspan=2 style="text-align:left;" , ↓ Period , - ! 2 , , - ! 3 , , - ! 4 , , - ! 5 , , - ! 6 , , - ! 7 , , - , colspan="2", ---- ''Legend'' A pnictogen ( or ; from "to choke" and -gen, "generator") is any ...

( or ; from "to choke" and -gen, "generator") atoms (elements of group 15:

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

arsenic Arsenic is a chemical element; it has Symbol (chemistry), symbol As and atomic number 33. It is a metalloid and one of the pnictogens, and therefore shares many properties with its group 15 neighbors phosphorus and antimony. Arsenic is not ...

antimony Antimony is a chemical element; it has chemical symbol, symbol Sb () and atomic number 51. A lustrous grey metal or metalloid, it is found in nature mainly as the sulfide mineral stibnite (). Antimony compounds have been known since ancient t ...

bismuth Bismuth is a chemical element; it has symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs nat ...

, and

moscovium Moscovium is a synthetic element, synthetic chemical element; it has Chemical symbol, symbol Mc and atomic number 115. It was first synthesized in 2003 by a joint team of Russian and American scientists at the Joint Institute for Nuclear Resea ...

) covalently bonded to

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

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. 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 Urine is a liquid by-product of metabolism in humans and many other animals. In placental mammals, urine flows from the Kidney (vertebrates), kidneys through the ureters to the urinary bladder and exits the urethra through the penile meatus (mal ...

and/or

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

, commonly the result of the decomposition of

urea Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...

. Phosphine smells like fish or garlic, and stibine like rotten eggs, similar to

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

and selenide.

Dipnictogen tetrahydrides

Dipnictogen tetrahydrides have the chemical formula . These are generally less stable than the trihydrides, commonly decomposing to the trihydride and the pnictogen involved.

Higher derivatives

Polyphosphanes exist with the formula P_''n''H_''n''+2 (''n'' = 1–9). Linear and branch isomers of have been detected. Other cyclic and condensed polyphosphane series are known, from P_''n''H_''n'' to P_''n''H_''n''−18, amounting to 85 known phosphanes in 1997.Greenwood and Earnshaw, pp. 492–3

Properties

Noncyclic hydrogen pnictides follow the formula X_nH_n+2. Ammonia is produced industrially on the largest scale among all compounds. Like water,

hydrogen bonding In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...

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.Greenwood and Earnshaw, pp. 420–4 Over twenty other hydrides of

are known, the most important being

hydrazine Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly hazardous unless handled in solution as, for example, hydraz ...

(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.Greenwood and Earnshaw, pp. 426–7 The azanes are a series which include ammonia, hydrazine and triazane. Phosphine, a toxic, colourless gas, is the most stable phosphorus hydride. It is insoluble in water but soluble in organic liquids (as well as carbon disulfide and trichloroacetic acid). Phosphine is a 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 and (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

and

at 250–300 °C, stibine to

and hydrogen at room temperature, and bismuthine to

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 Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

selenium Selenium is a chemical element; it has symbol (chemistry), symbol Se and atomic number 34. It has various physical appearances, including a brick-red powder, a vitreous black solid, and a grey metallic-looking form. It seldom occurs in this elem ...

. 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.Greenwood and Earnshaw, pp. 557–8 Imidogen, a radical composed of one hydrogen atom and one nitrogen atom (NH), can be classed as a pnictogen hydride.

References

Bibliography