Tertiary phosphines react, at −80 °C, with the pyruvoyl-substituted iron complex (CO)4Fe[C(O)C(O)CH3](CO2CH3) (1) to give rise to phosphonium-substituted metallalactones fac-(CO)3tiebar above startFe[C(O)C(CH3)(PR3)Otiebar above endC(O)](CO2CH3) (2). These zwitterionic compounds are formed by an initial addition of the phosphine to the noncoordinated carbonyl of the pyruvoyl unit followed by the addition of the oxygen of this C═O on a terminal carbonyl. They display an anionic metal center substituted by three organic ligands and a positive charge located on a phosphonium group. Attempts to extend the reaction to the related cationic pentacarbonyl pyruvoyl-substituted iron complex failed, as no reaction was observed with the same reagents. However, the expected products of these reactions, cationic phosphonium metallalactones {(CO)4tiebar above startFe[C(O)C(CH3)(PR3)Otiebar above endC(O)]}+, were obtained by acidic dissociation of the alkoxycarbonyl ligand of the relevant zwitterionic metallalactones.