Anionic nucleophilic reagents Nu- = CH3O-, C2H5O-, t-BuO-, CH 3S-, and P(C6H5)2 - were found to react with (CO)4Fe(CO2CH 3)[C(O)C(O)CH3] to afford two isomers of anionic trifunctionalized metallalactones, (CO)3Fe[C(O)C(CH 3)(Nu)OC(O)](CO2CH3)-, whose formation resulted from an addition of the nucleophile to the β carbonyl of the pyruvoyl ligand. In this reaction, the metallacycle formation was found to occur by a further addition of the oxygen of the same β carbonyl on a terminal carbonyl. In acidic medium, these anionic complexes evolved into neutral metallalactones (CO)4Fe[C(O)C(CH3)(Nu)OC(O)] already obtained by performing the same reaction with pronucleophiles (Nu-H type reagents). Achievement of (CO)4Fe[C(O)C(=CH2)OC(O)] when the reaction was performed with t-BuO- could result from an intermediate formation of the enolate form of the pyruvoyl ligand [Fe]-C(O)-C(=CH2)-O- under basic conditions. © 2005 American Chemical Society.