Abstract For the purpose of comparing the reaction of various aminocarbene complexes of chromium with alkynes and to ascertain several points of the mechanism of their interaction, a series of complexes derived from large cyclic amines, HN(CH 2) n ( n≥6) and from a small cyclic amine ( n=2) was synthesized. In the case of the larger amines, all the complexes examined herein, led to the expected bridgehead lactams 12 as the major product, providing strong evidence for a concerted rearrangement of an intermediate nitrogen-ylid complex such as 4. The X-ray structure of the lactam complex 12d ( n=12) has been established in order to confirm the ring opening and the migration of the twelve carbon-atom alkyl chain from nitrogen to the γ-carbon. Interestingly, the last possibility, the migration from nitrogen to oxygen in 4, which had so far not been observed but which according to calculations should also be possible, took place in the case of complex 10b ( n=7), giving rise, yet in low yield, to an alkoxypyrrole 14. Minor products resulting from annulation reactions without CO insertions, were also observed. For aminocarbene complexes derived from methylaziridine ( n=2), important results, which substantiate previous observations, have been obtained especially as far as the mechanism of the insertion reaction is concerned: the regioselectivity of the ring-opening reaction could be established by X-ray crystallography on two isomeric complexes 25 and 26, the timing of the various steps could be determined by the examination of the reactivity of vinyl–aziridinyl carbene complexes 31 and 35 which led surprisingly to aziridinyl phenols 33 and 36. Finally, an unexpected product, the structure of which could also be established by X-ray crystallography as 27, and resulting from the oxidation at the α position of the carbonyl in complexes 25 or 26 was isolated during the interaction of complex 23 with diphenylacetylene.