In　a　previous　paper　[Inorg.　Chem.,　2010,　49,　1245],　we　studied　the　reaction　of　F2NO+　with　an　excess　of　HN3　which　led　to　the　quantitative　formation　of　N5+　and　N2O.　Based　on　15N-labeling　experiments　and　theoretical　calculations,　the　formation　of　a　4-oxo-N7O+　intermediate　with　a　decomposition　energy　barrier　of　about　40　kcal　mol-1　was　proposed.　Since　this　relatively　high　barrier　disagreed　with　our　failure　to　experimentally　observe　this　cation,　a　thorough　theoretical　study　of　the　isomerization,　dissociation　and　formation　pathways　of　N4FO+　and　N　7O+　was　carried　out　at　the　B3LYP　and　G3B3　levels　at　240　K.　It　was　found　that　the　self-decomposition　of　4-oxo-N7O+　to　NO+　and　N2　has　a　considerably　lower　barrier　of　only　19.6　kcal　mol-1　and,　therefore,　would　be　more　likely　than　a　self-decomposition　to　N5+　and　N2O.　Additional　calculations　also　showed　that　alternate　reaction　pathways　between　the　stable　and　well-characterized　z-N4FO+　intermediate　product　and　HN3　involving　7-　or　9-membered　cyclic　transition　states,　can　lead　to　the　observed　N5+　and　N2O　products　with　the　observed　15N　distribution　and　barriers　as　low　as　20.7　kcal　mol　-1.　The　transition　states　for　these　reactions　contain　a　1-oxo-N　7O+　component　which　can　decompose　without　a　barrier　to　N5+　and　N2O.　These　alternative　pathways　involving　an　unstable　1-oxo-N7O+　cation　are　in　better　agreement　with　experiment　than　the　one　involving　4-oxo-N7O　+.　The　correctness　of　this　re-interpretation　was　experimentally　verified　by　a　15N-labeling　experiment　between　α-　and　γ-15N-labeled　HN3　and　unlabeled　N4FO　+　which　resulted　exclusively　in　unlabeled　N2O　and　α-　and　γ-15N-labeled　N5+.　Therefore,　we　conclude　that　in　the　reaction　of　NF2O+　with　excess　HN　3　the　experimental　and　theoretical　evidence　supports　only　the　formation　of　an　unstable　1-oxo-N7O+　cation,　and　that　for　the　preparation　of　the　symmetric　4-oxo-N7O+　cation　different　synthetic　approaches　will　be　required.　©　2014　the　Partner　Organisations.