Ab initio quantum-chemical calculations have been applied to study the structure, energies and properties of the pentanitrogen cation (N-5(+)) recently prepared experimentally by Christe and co-workers. The cation has an open C-2v form and a closed-shell singlet ground state N5s whose calculated vibrational wavenumbers agree well with the observed IR-Raman values. The triplet structure N5t ((3)A") corresponds to a weak complex between linear triplet N-3(+) and N-2 and occurs at similar to 26 kJ/mol above N5s. In both states, the cyclic structure does not correspond to an energy minimum. The heat of formation is Delta H-f,H-0(N-5(+),(1)A(1)) = 1470 +/- 15 kJ/mol. Decomposition of the pentanitrogen cation is endothermic by similar to 44 kJ/mol relative to N-2 + N-3(+)((3)Sigma(g)(-)) and 181 kJ/mol relative to N-2 + N-3(+) ((1)Sigma(g)(+)). The N-5 radical also has an open C-2v form and its ionization energy is IEa(N-5) = 7.4 +/- 0.2 eV. (C) 2000 Elsevier Science B.V. All rights reserved.