Can the pentazole anion (N5−) be isolated and/or trapped in metal complexes?

The 1,3-dipolar cycloreversion pathway of the pentazole anion (N₅^?) to the azide anion (N₃^?) plus dinitrogen (N₂) has been investigated using ab initio methods. At the MP4SDQ/6–31 + G* level of theory plus zero-point energy contributions, the pentazole anion is predicted to lie at 31 kcal mol^?1 above the N₃^? + N₂ system but the energy barrier for decomposition is 22 kcal mol^?1. This indicates that the pentazole anion could be isolated in an inert matrix at low temperature. Comparison between extended Hückel calculations on the (N₅)M(CO)₃ and (C₅H₅)M(CO)₃ complexes (with M = Fe²⁺, Mn⁺ and Cr) suggests that the N₅^? complexes would be formed if the fragments could be brought together. Predicted vibrational frequencies of the N₅^? anion are also reported.