The pentazenium cation (also known as pentanitrogen) is a positively charged

Electric charge

Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

 homoleptic polynitrogen ion of the chemical formula

Chemical formula

A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 . Together with dinitrogen and azide

Azide

Azide is the anion with the formula N3−. It is the conjugate base of hydrazoic acid. N3− is a linear anion that is isoelectronic with CO2 and N2O. Per valence bond theory, azide can be described by several resonance structures, an important one being N−=N+=N−...

 anion, it is one of the only three polynitrogen species obtained in bulk quantities.

History

Within High Energy Density Matter research program, ran by U.S. Air Force since 1986, systematic attempts to approach polynitrogen compounds began in 1998, when Air Force Research Laboratory

Air Force Research Laboratory

The Air Force Research Laboratory is a scientific research organization operated by the United States Air Force Materiel Command dedicated to leading the discovery, development, and integration of affordable aerospace warfighting technologies; planning and executing the Air Force science and...

 at Edwards AFB

Edwards Air Force Base

Edwards Air Force Base is a United States Air Force base located on the border of Kern County, Los Angeles County, and San Bernardino County, California, in the Antelope Valley. It is southwest of the central business district of North Edwards, California and due east of Rosamond.It is named in...

 became interested in researching alternatives to the highly toxic hydrazine

Hydrazine

Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...

-based rocket fuel

Rocket propellant

Rocket propellant is mass that is stored in some form of propellant tank, prior to being used as the propulsive mass that is ejected from a rocket engine in the form of a fluid jet to produce thrust. A fuel propellant is often burned with an oxidizer propellant to produce large volumes of very hot...

 and simultaneously funded several such proposals. Karl O. Christe, a senior investigator at AFRL, chose to attempt building linear N out of and , based on the proposed bond structure:

[F-N≡N]⁺ + HN=N=N → [N≡N-N=N=N]⁺ + HF

The reaction succeeded and was created in sufficient quantities to be fully characterized by NMR, IR and Raman spectroscopy in 1999. The salt was highly explosive, but when was replaced by , a stronger Lewis acid, much more stable was produced, shock-resistant and thermally stable up to 60-70°C. This made bulk quantities, easy handling, and X-ray crystal structure analysis possible.

Preparation

Reaction of and in dry at -78°C is the only known method so far:

cis- + →

+ → + HF

Chemistry

is capable of oxidizing water, NO, and , but not or ; its electron affinity is 10.44 eV (1018.4 kJ/mol). For this reason, must be prepared and handled in dry environment:

4 + → 4HF + 4 + 10 +

2 + 2 → 2 + 5

Due to stability of the fluoroantimonate, it is used as the precursor for all other known salts, typically accomplished by metathesis reactions in non-aqueous solvents such as HF, , , or

Acetonitrile

Acetonitrile is the chemical compound with formula . This colourless liquid is the simplest organic nitrile. It is produced mainly as a byproduct of acrylonitrile manufacture...

, where suitable hexafluoroantimonates are insoluble:

2 + → +

The most stable salts of decompose when heated to 50 - 60°C: , , and , while the most unstable salts that were obtained and studied, and were extremely shock and temperature sensitive, exploding in solutions as dilute as 0.5 mmol. A number of salts, such as fluoride, azide, nitrate, or perchlorate, cannot be formed.

Structure and bonding

In valence bond theory

Valence bond theory

In chemistry, valence bond theory is one of two basic theories, along with molecular orbital theory, that were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of the dissociated atoms combine to give individual chemical bonds...

, pentazenium can be described by six resonance structures:

[N≡N⁺-N^--N⁺≡N]⁺ [N^-=N⁺=N-N⁺≡N]⁺ [N≡N⁺-N=N⁺=N^-]⁺ [N⁺=N-N^--N=N⁺]⁺ [N⁺=N-N^--N⁺≡N]⁺ [N≡N⁺-N^--N=N⁺]⁺

According to both ab initio calculations and the experimental X-ray structure, the cation is planar, symmetric, and approximately V-shaped, with bond angles 111° at the central atom (angle N2-N3-N4) and 168° at the second and fourth atoms (angles N1-N2-N3 and N3-N4-N5). The bond lengths for N1-N2 and N4-N5 equal 1.10A and the bond lengths N2-N3 and N3-N4 are 1.30A