In acetonitrile the reaction of RFe(CO)2(π-C5H5)(R = Me, Et, or Pri) with phosphorus ligands leads to the formation of the acyl complexes RCOFe(CO)(L)(π-C5H5). Examination of the 1H n.m.r. spectra of the complexes shows evidence of diastereotopic shielding by an asymmetric iron atom. Kinetic measurements are interpreted in terms of an intermediate, which is not specifically solvated. The acyl complexes RCOFe(CO)(L)(π-C5H5)[R = Et or Pri; L = PPh3, PPhMe2, PPh2Me, or P(OPh)3] on reaction with Ph3C+BF4– give the cationic complexes [(π-C5H5)Fe(CO)2L]+BF4–.