Thermal evolution at 4 °C of the structurally characterized cis(CO)4Fe[C(O)C(O)CH3][C(O)CH3] (1(2)) gives rise to the cis(CO)4Fe[C(O)CH3]2 (1(3)) which, probably owing to synthetic problems, has never been described in the literature. By reaction with anionic nucleophiles (Nu−), 1(2) affords anionic trifunctionalized metallalactones {(CO)3Fe[C(O)CH3][C(O)C(CH3)(Nu)OC7(O);(Fe–C7)]}− (3) formed by addition of the nucleophile reagent on the β carbon of the pyruvoyl moiety followed by the cyclization of this ligand on a terminal carbonyl of the complex. Anions 3 are characterized by 1H and 13C NMR and by X-ray diffraction for the complex with Nu = C(H)(CO2C2H5)2. Complexes 3 are also prepared by reaction of CH3Li with the neutral metallalactones (CO)4Fe[C(O)C(CH3)(Nu)OC7(O);Fe–7C] (2). The results of this study shed light on the reaction of cyclization of a pyruvoyl ligand as they clearly show that the presence of a second ligand (for example CO2R) with a labile OR group is not required to perform the formation of the metallalactone ring and then that the observed reaction has no connection with organic chain-ring transformations.