Reaction of [Cp′Fe(CO)2]2 (1b (Cp′ = MeC5H4) with the phosphorus ylide Ph3PCH2 yields two products. {μ-CH2)(μ-CO)[Cp′Fe(CO)]2} (2b) and Cp′(CO)(PPh3)Fe(COCH3) (4). The crystal structure analysis of 2b is reported. It is shown, that with different phosphorus ylides the reaction of [CpFe(CO)2]2 (1a) (CP = C5H5) proceeds differently depending also on the amount of n-butyllithium present. So a threefold excess of the LiX adducts (X = Cl, Br) of the ylides Ph3PCHR in the presence of a large excess of n-butyllithium gives metallacyclic compounds of composition Cp(CO)Fe{HRCP(C6H4)Ph2} (R = H (3a), R = CH3 (3b), R = C2H5 (3c)) in the case of alkylidene ylides and Cp(CO)(PPh3)FeCH2Ph (5) with the benzylidene ylide (R = C6H5). In the presence of a large excess of n-butyllithium, a twofold excess of Ph3PCHPh reacts with 1a to give the compound C32H23FeO2P (6) characterized by spectroscopic data and an X-ray diffraction study. In 6 an η3-cyclopentyl ring is coordinated to the iron atom together with two CO ligands, a phenyl ring and an ylidic carbon atom. Also characterized by an X-ray diffraction study was 3c, which was found to have a metallacycle containing an iron and a phosphorous atom, the ylidic carbon and two carbon atoms from one "phenyl" ring of the PPh3 group as ring members. Reaction of (CH3)2PhPCH2·LiI with 1a gives extremely air-sensitive monophosphonioalkyl complex {(ηCo)2[CpFe(CO)][CpFeL]} (8) (L = Me3PCH2). Some mechanistic considerations on the various reaction types of 1a based on the data obtained are presented. © 1989.