Reaction of allene with [Fe₃(CO)₁₂] at 85–90° gives a yellow, binuclear complex of formula [Fe₂(CO)₆(C₉H₁₂)](I); this can also be prepared from the allene dimer complex, [Fe₂(CO)₆(C₆H₈)]. At 120° the reaction affords a second isomer (II), which is orange-yellow and can be alternatively derived by thermal isomerisation of (I). A third isomer (III), which is lemon-yellow is obtained by thermal isomerisation of (I) or (II). The stepwise transformation (I)→(II)→(III) demonstrates the relative thermal stability. The molecular structures for the three diamagnetic complexes were deduced primarily on the basis of ¹H n.m.r. spectra and the structure of (II) was confirmed by X-ray analysis.