Abstract1,3,5‐Triazapentadienes 1, prepared from amidines and imidoyl chlorides 2, have been used as nucleophilic building blocks for the reactions with various electrophilic reagents. With aldehydes 4 1,2‐dihydrotriazines 3 were obtained, whereas ketones 5 gave 3 or 1,3,5‐triazahexa‐1,3,5‐trienes 6, depending on the substitution pattern of 1. Acyl chlorides 7 reacted with 1 to give 1‐oxa‐3,5,7‐triazahepta‐1,3,5‐trienes 8; in a similar manner imidoyl chlorides 2 gave rise to the formation of the new 1,3,5,7‐tetraazahepta‐1,3,5‐trienes 9. Treatment of N‐benzoylbenzimidoyl chloride 10 with 1 afforded the 1‐oxa‐3,5,7,9‐tetraazanona‐1,3,5,7‐tetraenes 11, or, depending on the substitution pattern, their ring‐tautomeric compounds 12 with 1,2‐dihydrotriazine structures.1‐Oxa‐3,5‐diazinium salt 13 as cyclic electrophile produced, when treated with 1g, 1‐oxa‐3,5,7,9,11‐pentaazaundeca‐1,3,5,7,9‐pentaene (14), a long oligonitrile with push‐pull substitution pattern. All new compounds were completely characterized including X‐ray diffraction for each type, indicating rather twisted three‐dimensional structures for the open‐chain compounds. The relative energies of 1,3,5‐triazapentadiene isomers were studied by DFT calculations [level: B3LYP/6‐31+G(d,p)], as well as the ring‐chain tautomerism of selected examples. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)