A few stable nitronium salts with anions of weak nucleophilicity can be isolated. These include nitronium perchlorate , nitronium tetrafluoroborate , nitronium hexafluorophosphate , nitronium hexafluoroarsenate , and nitronium hexafluoroantimonate . These are all very hygroscopic compounds.
The solid form of dinitrogen pentoxide, , actually consists of nitronium and nitrate ions, so it is an ionic compound, nitronium nitrate , not a molecular solid. However, dinitrogen pentoxide in liquid or gaseous state is molecular and does not contain nitronium ions.