The novel cationic diiron μ-allenyl complexes [Fe2Cp2(CO)2(μ-CO){μ-η1:η2α,β-Cα(H)CβCγ(R)2}]+ (R = Me, 4a; R = Ph, 4b) have been obtained in good yields by a two-step reaction starting from [Fe2Cp2(CO)4]. The solid state structures of [4a][CF3SO3] and of the diruthenium analogues [Ru2Cp2(CO)2(μ-CO){μ-η1:η2α,β-Cα(H)CβCγ(R)2}][BPh4] (R = Me, [2a][BPh4]; R = Ph, [2c][BPh4]) have been ascertained by X-ray diffraction studies. The reactions of 2c and 4a with Brønsted bases result in formation of the μ-allenylidene compound [Ru2Cp2(CO)2(μ-CO){μ-η1:η1-CαCβCγ(Ph)2}] (5) and of the dimetallacyclopentenone [Fe2Cp2(CO)(μ-CO){μ-η1:η3-Cα(H)Cβ(Cγ(Me)CH2)C(O)}] (6), respectively. The nitrile adducts [Ru2Cp2(CO)(NCMe)(μ-CO){μ-η1:η2-Cα(H)CβCγ(R)2}]+ (R = Me, 7a; R = Ph, 7b), prepared by treatment of 2a,c with MeCN/Me3NO, react with N2CHCO2Et/NEt3 at room temperature, affording the butenolide-substituted carbene complexes [Ru2Cp2(CO)(μ-CO){μ-η1:η3-Cα(H)CβCγ(R)2OC(O)C(H)] (R = Me, 10a; R = Ph, 10b). The intermediate cationic compound [Ru2Cp2(CO)(μ-CO){μ-η1:η3-Cα(H)CβCγ(Me)2OC(OEt)C(H)]+ (9) has been detected in the course of the reaction leading to 10a. The addition of N2CHCO2Et/NHEt2 to 7a gives the 2-furaniminium-carbene [Ru2Cp2(CO)(μ-CO){μ-η1:η3-Cα(H)CβCγ(Me)2OC(OEt)C(H)]+ (11). The X-ray structures of 10a, 10b and [11][BF4] have been determined. The reactions of 4a,b with MeCN/Me3NO result in prevalent decomposition to mononuclear iron species.