Vibrational spectra of π-olefin transition metal complexes : II. N-methylmaleimide and (π-methylmaleimide)iron tetracarbonyl

Infrared and Raman spectra of N-methylmaleimide (solid, solution, gas) and of (π-N-methylmaleimide)iron tetracarbonyl (solid, solution) have been obtained. An assignment of the normal modes of both molecules is offered and compared with the data for (π-maleic anhydride)iron tetracarbonyl. The change of the ligand modes after coordination to the metal atom is discussed. The CC stretching vibration in the complex is assigned to the band at 1370 cm^-1 (1585 cm^-1 in the free ligand). The decrease in the IR intensity of the out-of-plane CH modes of the ligand after coordination is explained by the lowering of the effective positive charge on olefinic protons due to the electron back-donation from metal to ligand. The upfield shift of the signal of CH protons in PMR spectrum of the complex supports this conclusion.