We investigate the stability of N5+N5- as an isolated species and in a potential periodic structure. Its barriers toward loss of N-3(.) from the N-5(+) ion or addition to form N-10 are 25 and 15 kcal/mol. The barrier toward loss of N-2 from the N-5(-) ion is 15 kcal/mol. Increasingly positive charges of the N-5 species in N-5(.), "N5+N3-", N5+N5-, and N-5(+) are correlated with increased dissociation barriers. Preliminary calculations on quasi-periodic (N5+N5-)(2) clusters and periodic DFT calculations agree on a density of similar to1.9 g/cm(3) and a lattice energy of similar to130 kcal/mol. If a crystalline form of N5+N5- could be stabilized, it would be an ideal monopropellant since its density is nearly twice that of hydrazine while the energy densities (heat of reaction per gram) are comparable.