The structures, energies, and rearrangements of imidoylnitrenes H–C­(NH)–N, H2N–C­(NH)–N, Ph–C­(NH)–N, H–C­(NPh)–N, and MeO–C­(NCN)–N (10a–e) are investigated at DFT and CASPT2 levels of theory. Imidoylnitrenes are potentially formed by pyrolysis or photolysis of azides, tetrazoles (6, 6′), or sydnones. Unlike most acylnitrenes, the imidoylnitrenes 10 have triplet ground states. The first excited states are the open-shell singlets (OSSs), lying between ca. 4 and 20 kcal mol–1 above the triplets at the CASPT2 level. The second excited states are the closed-shell singlets (CSSs), lying >50 kcal mol–1 higher in energy. The OSS imidoylnitrenes can ring-close to 1H-diazirines 9 with very low activation energies (2–12 kcal mol–1), and the 1H-diazirines can then rearrange to nitrile imines 8 with activation energies of 37–48 kcal mol–1. Conversely, nitrile imines generated directly by pyrolysis or photolysis of 2,5-substituted tetrazoles 6 can rearrange to 1H-diazirines 9 and imidoylnitrenes 10 with activation energies of 37–60 kcal mol–1. Finally, the imidoylnitrenes 10 can rearrange to carbodiimides 11 with modest activation barriers of 12–20 kcal mol–1. Calculated vibrational data, UV–vis spectra, and spin densities in the triplet states are also reported, and zero-field splitting parameters |D/hc| in the range 0.9–1 cm–1 and nonzero |E/hc| values are predicted.