Lithiated 1,3-diene-iron complexes undergo a CO insertion in the presence of trimethyl phosphite via a ferrate intermediate to give 3-formyl or 3-acetyl-1,3-diene-Fe(CO)(2)P(OMe)(3) complexes. Formylation proceeds when the reaction is quenched by a strong Brønsted acid, whereas the ketone is obtained after the treatment with iodomethane.