The cycloaddition reactions of various C-heteroarylimines with alpha,beta-unsaturated Fischer carbene complexes have been studied. Either [3 + 3] carbocyclization or [3 + 3] and [4 + 3] heterocyclization reactions were accomplished, depending on the structure of the imine and on the type of unsaturation of the carbene complex. Imines derived from furan-, 4, benzofuran-, 5, and N-methylindole-2-carboxaldehyde, 7, gave C3 + C3 cycloadducts 10-12 on reaction with alkynyl carbene complexes 2a and 3. The reaction of N-unsubstituted pyrrole imine 8a with alkenyl carbene complexes 1 involved the ring nitrogen atom of the pyrrole unit leading to C2N + C3 heterocycloadducts 14 and 15. Moreover, N-unsubstituted imines 8 and 9 underwent C2N2 + C3 heterocyclization to alkynyl carbene complexes 2 and 3 to furnish zwitterionic metalpentacarbonyl containing heterocycles 17 and 18. We have Studied the thermal behavior of heterocycles 17, finding that they led to bispyrrole 21 or indolizine 20 when heated at 120 degrees C in the presence and in the absence of carbon monoxide, respectively. The results obtained show that the [3 + 3] cyclizations are promoted by a [1,2]-M(CO)(5) shift, while in the case of the [4 + 3] cyclization, a [1,3]-M(CO)(5) shift follows the ring-closure step. The X-ray analysis of compound 17b, resulting from 1,3-pentacarbonyltungsten migration, is provided.