Perxenate
In chemistry, perxenates are salts of the yellowxenon-containing anion XeO4−
6. This anion has octahedral molecular geometry, as determined by Raman spectroscopy, having O–Xe–O bond angles varying between 87° and 93°. The Xe–O bond length was determined by X-ray crystallography to be 1.875 Å.
Perxenates are synthesized by the disproportionation of xenon trioxide when dissolved in strong alkali:
When Ba(OH)
2 is used as the alkali, barium perxenate can be crystallized from the resulting solution.
Perxenic acid is the unstable conjugate acid of the perxenate anion, formed by the solution of xenon tetroxide in water. It has not been isolated as a free acid, because under acidic conditions it rapidly decomposes into xenon trioxide and oxygen gas:
Its extrapolated formula, H
4XeO
6, is inferred from the octahedral geometry of the perxenate ion (XeO4−
6) in its alkali metal salts.
The pKa of aqueous perxenic acid, (pKH
4XeO
6), has been indirectly calculated to be < 0, making it a very strong acid. Its first ionization yields the anion H
3XeO−
6, which has a pK value of 4.29, still relatively acidic. The twice deprotonated species H
2XeO2−
6 has a pK value of 10.81. Due to its rapid decomposition under acidic conditions as described above, however, it is most commonly encountered as perxenate salts, bearing the anion XeO4−
6.
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