Accurate ab initio calculations have been performed in order to investigate both the stable isomers and the reactivity of the N2H2+ cation. In addition to the trans-HNNH+ isomer already observed in the photoelectron studies, a formaldehyde type (isodiazene cation) and H2O2-like isomers are found. At the coupled cluster level of theory, the isodiazene cation is calculated to be as stable as trans-HNNH+. We have also studied the reactivity of N2H2+ and its implication on the reactive processes involving N-2/N-2(+) and H-2(+)/H-2, H/H+ and HN2+/HN2, and HN and HN+ by performing suitable one-dimensional cuts of the six-dimensional potential energy functions of the lowest electronic states of H2N2+. We have pointed out the crucial role of this tetratomic intermediate cation and the importance of the short range internuclear distances during these processes. In the case of N-2/N-2(+) and H-2(+)/H-2 reactions, we have shown that the initial orientation of the reactants may influence the N2H2+ tetratomic intermediate: One can expect to form the trans isomer preferentially if the internuclear axes of the H-2/H-2(+) and the N-2(+)/N-2 molecules are parallel to each other when these diatoms are colliding and after intramolecular isomerization process. However, if the internuclear axes of the diatomics are perpendicular to each other, the isodiazene cation is formed preferentially. Different branching ratios are expected for each collision scheme. These reactive processes are found to involve vibronic, Renner-Teller and spin-orbit couplings between the electronic states of N2H2+. These interactions mix these electronic states, leading to the formation of atomic, diatomic, and triatomic species via the decomposition of the N2H2+ intermediate complex. (C) 2004 American Institute of Physics.