P-N bond activation of 2H-azaphosphirene complexes 1 and 2 by using triflic acid led to ring expansion in the presence of nitriles. In the absence of nitriles, the reaction surprisingly afforded two haptomeric N-protonated 1-aza-3-phospha-butadiene complexes in the case of complex 1, whereas the N-protonated 2H-azaphosphirene complex [H-2](+) was characterized by NMR spectroscopy.Protonation of 2H-azaphosphirene complexes 1 and 2 using trifluoromethanesulfonic acid in the presence and in the absence of nitriles was studied. Reactions with nitrile derivatives of different nucleophilicity and steric demand (3 a-f) led to highly bond- and regioselective ring-expansion reactions of complexes 1 and 2 and yielded 2H-1,4,2-diazaphosphole complexes 4 a-f and 5 a after deprotonation with NEt(3). Upon protonation of complex 1 in the absence of nitriles a mixture of two haptomeric N-protonated 1-aza-3-phosphabuta-1,3-diene complexes 13 and 14 was obtained, which upon addition of Me(2)NCN (3 a) and NEt(3) afforded 2H-1,4,2-diazaphosphole complex 16 a bearing a P-substituent with only one SiMe(3) group. Protonation of complex 2 led to the 2H-azaphosphirenium complex [H-2](+), unambiguously identified by NMR spectroscopy at low temperature. All isolated products were characterized by multinuclear NMR spectroscopy, IR, MS, and single-crystal X-ray crystallography in the cases of complexes 4 a, 4 e, 6 a, and 16 a. Furthermore, ESI-MS/MS studies on primarily formed reactive intermediates and DFT studies on the reaction mechanism are presented.