Pentazenium
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| Template:Longitem | Template:Chem2 |
| Molar mass | 70.0335 g/mol |
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In chemistry, the pentazenium cation (also known as pentanitrogen) is a positively-charged polyatomic ion with the chemical formula Template:Chem2 and structure Template:Chem2. Together with solid nitrogen polymers and the azide anion, it is one of only three poly-nitrogen species obtained in bulk quantities.
History
Within the High Energy Density Matter research program, run by the U.S. Air Force since 1986, systematic attempts to approach polynitrogen compounds began in 1998, when Air Force Research Laboratory at Edwards AFB became interested in researching alternatives to the highly toxic hydrazine-based rocket fuel and simultaneously funded several such proposals. Karl O. Christe, then, a senior investigator at AFRL, chose to attempt building linear Template:Chem2 out of Template:Chem2 and Template:Chem2, based on the proposed bond structure:[1]
The reaction succeeded, and Template:Chem2 was created in sufficient quantities to be fully characterized by NMR, IR and Raman spectroscopy in 1999.[2] The salt was highly explosive, but when Template:Chem2 was replaced by Template:Chem2, a stronger Lewis acid, much more stable Template:Chem2 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.[3]
Actually N5+ had been predicted by ab initio calculations as a member of the dicyanamide isoelectronic series by Pyykkö and Runeberg in 1991 and this was quoted as ref. [10] of Christe [2] in 1999.
Preparation
Reaction of Template:Chem2 and [[hydrazoic acid|Template:Chem2]] in dry HF at −78 °C is the only known method so far:
Chemistry
Template:Chem2 is capable of oxidizing water, NO, Template:Chem2 and Template:Chem2, but not Template:Chem2 or Template:Chem2; its electron affinity is 10.44 eV (1018.4 kJ/mol). For this reason, Template:Chem2 must be prepared and handled in a dry environment:
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, Template:Chem2, Template:Chem2, or [[acetonitrile|Template:Chem2]], where suitable hexafluoroantimonates are insoluble:
The most stable salts of Template:Chem2 decompose when heated to 50–60 °C: Template:Chem2, Template:Chem2, and Template:Chem2, while the most unstable salts that were obtained and studied, Template:Chem2 and [[Pentazenium tetraazidoborate|Template:Chem2]] 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.[1]
Structure and bonding
In valence bond theory, pentazenium can be described by six resonance structures:
where the last three structures have smaller contributions to the overall structure because they have less favorable formal charge states than the first three.[4]Template:Better source needed
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 are 1.10 Å and the bond lengths N2–N3 and N3–N4 are 1.30 Å.[3]
See also
References
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