Striking differences in the reactions of alkoxycarbene and thiocarbene complexes of chromium and tungsten are observed. Thus, (β-imino)ethoxycarbene complexes 10a−e, generated in situ from [(OC)5WC(OEt)CH2R] (7a−c; R = n-Pr, Me, c-C7H7) and imidoyl chlorides R1ClCNCHR2R3 (9a−f; R1 = t-Bu, Ph, 2-furyl; R2 = H, Me; R3 = Me, Et, Ph), undergo a metalla(di-π-methane) rearrangement to (N-enamino)ethoxycarbene complexes 12a−e, while the corresponding (β-imino)thiocarbene complexes 11a−l, derived from [(OC)5MC(SEt)CH2R] (8a−e; M = W, Cr; R = n-Pr, Me, c-C7H7, c-C6H7Fe(CO)3) and imidoyl chlorides under similar conditions, form pyrroles 16a−h and 17k,l by α-cyclization. On the basis of the calculated DFT/BP86 potential energy surfaces of the particular reaction channels it is shown that (β-imino)alkoxycarbene compounds 10 prefer a metalla(di-π-methane) rearrangement due to the kinetic stability of the (N-enamino)ethoxycarbene products, while formation of pyrroles is not favored due to the presence of high energetic stationary structures in the α-cyclization pathway. For (β-imino)thiocarbene compounds 11, on the other hand, rearranged products are kinetically unstable, and α-cyclization reactions are strongly favored on thermodynamic grounds.