Three sets of 1,3-and 1,2-derivatives of tricarbonylcyclobutadieneiron, [Fe(CO) 3 -η 4 -CbFc 2 R 2 ] [Cb =cyclobutadiene, Fc = ferrocenyl, R = -CHO: 1a (1,3-isomer), 1b (1,2-isomer); R = -CHNOH: 2a (1,3-isomer), 2b (1,2-isomer); R = -CN: 3a (1,3isomer), 3b (1,2-isomer)] have been synthesized under facile reaction conditions starting from FcC≡C-CHO and Fe(CO) 5 . Ef-[a]