An isomeric study for N-5, N-5(+), and N-5(-) has been carried out using the Gaussian-3 (G3) method. At the MP2(FU)/6-31G(d) level, six N-5, five N-5(+) and four N-5(-) isomers are identified. Of these 15 species, 12 of them have not been reported previously. The most stable N-5 isomer is a weak N-2...N-3 complex with C-2v symmetry, while the most stable N-5(-) isomer is also a weak N-2...N-3(-) complex with the same symmetry. The most stable species for N-5(+) has an open-chain structure with C-2v symmetry, which has been synthesized and characterized spectroscopically. Natural bond orbital (NBO) analysis suggests that the stability of this cation is enhanced by hyperconjugation.