Conformational study of π-tetracarbonyl(mono-olefin)iron complexes by infrared spectra and dipole moment measurements

The electric dipole moments of the complexes π-LFe(CO)4, where L =trans-PhCHCHCOCH3, trans-PhCHCHCOPh, trans-PhCHCHCHO, CH2HCHO, and CH2CHCN, have been determined. It is found that the ligand plane is never parallel to the axis of the trigonal bipyramid. The ligand CO group in the carbonyl derivatives is slightly out of the CCC plane. The structure of the acrylonitrile complex in solution is the same as that found for the solid from X-ray studies. The ketone iigands have the same conformation in the complex as in the free ligand. The aldehyde ligands. present only as the s-trans-conformer in the free state, form both isomers when bonded to the metal. The enthalpy variation for the conformational equilibrium s-cis⇄s-trans for the aldehyde complexes has been ca culated from i.r. data, and an assignment of the CO stretchings has been proposed.