The recent successful preparation of the cyclo-pentazolate N-5(-) anion (cyclo-N-5(-)) by the dissociation of the p-pentazolylphenolate anion using high-energy collisions is accounted for by considering the electronic structure of the system. It is shown that a symmetry-allowed conical intersection is involved, leading directly from an electronically excited state of the precursor to ground-state cyclo-N-5(-). The presence of the conical intersection is manifested by the structure of the thermal transition state of the C-N bond dissociation reaction, which is shown to be bent. A similar mechanism is proposed for the formation of cyclo-N-5(-) from the dimethylaminophenylpentazole anion radical. High-level model calculations on the dissociation of these precursors and of the HN5.-anion radical, which is the parent molecule of the larger aromatic pentazolates, support the proposed model.