The possible existence of FN5 was studied by ab initio electronic structure theory. Calculations were carried out at the MP2/6-31+G(d) and CCSD(T)/aug-cc-pVDZ levels of theory for the N5+AsF6- ion pair and its decomposition to FN5 and AsF5. Six different vibrationally stable isomers of FN5 were identified. Intrinsic reaction coordinate (IRC) and dynamic reaction path (DRP) calculations were used to study the isomerization of FN5 and its decomposition to FN3 and N-2. A Rice-Ramsperger-Kassel-Marcus (RRKM) analysis was performed, indicating upper limits to the lifetimes of the FN5 isomers in the nanosecond range. These theoretical predictions were confirmed by an experimental study of the thermolyses of N5AsF6 and [N-5](2)SnF6 and the displacement of FN5 from N5SbF6 with CsF, using FT-IR spectroscopy. In accord with the theoretical predictions, the primary reaction product FN5 could not be observed, but its decomposition products FN3, F2N2, and NF3 were identified.