(Monooxooctaethylchlorinato)iron(III) chloride, (oxo-OEC)FeCl, 1, has been investigated by X-ray crystallography and by H-1 NMR spectroscopy. Its bis(imidazole-d(4)) complex has been studied by multidimensional 1H NMR and EPR spectroscopies, and the results are compared to those for the bis(Im-d(4)) complex of (octaethylchlorinato)-iron(III) chloride, (OEC)FeCl, 2. EPR and NMR results show that both [(oxo-OEC)Fe(Im-d(4))(2)]Cl and [(OEC)Fe(Im-d(4))(2)]Cl are low-spin Fe(III) complexes with (d(xy))(2)(d(xz),d(yz))(3) electronic ground states, both at 4.2 K (EPR spectra) and at ambient temperatures utilized for solution NMR studies. The pattern of chemical shifts of the pyrrole-CH2 and meso protons are similar, with the 8,17-carbons having the largest and the 12,13-carbons having the smallest spin densities in each case, except that [(OEC)Fe(Im-d(4))(2)]Cl has a slightly wider range of pyrrole-CH2 chemical shifts and more resonances are observed for [(oxo-OEC)Fe(Im-d(4))(2)]Cl due to its lower symmetry. Full proton resonance assignments for both complexes have been made from COSY, NOESY, and NOE difference experiments.