Reactions between Iron(0) Alkynylcarbene Complexes and Cyclopentadiene: Formation of Iron(0) η¹-Vinylcarbene Complexes and Their Facile Thermal Transformation into the Corresponding (η³:η¹-Allylacyl)iron(II) Complexes

[(3-tert-Butylnorbornadienyl)ethoxymethylene]Fe(CO)₄, the iron(0) η¹-vinylcarbene complex 2, is obtained from the reaction of [(3,3-dimethylbutynyl)ethoxymethylene]Fe(CO)₄ (1a) with cyclopentadiene at 25 °C. Complex 2 transforms into the η³:η¹-allylacyl complex 4a via a CO insertion reaction at the original carbene carbon by heating at 50 °C for 1 h. However, direct formation of η³:η¹-allylacyl complexes is observed in reactions of cyclopentadiene with other alkynylcarbene complexes, 1b (R = SiMe₃), 1c (R = cyclohexyl), 1d (R = n-propyl), and 1e (R = phenyl), at 25 °C.