We have investigated the Fe-57 Mossbauer quadrupole splittings in the following compounds by using density functional theory, and in some cases via experiment : Fe(CO)(3)(cyclo-butadiene), Fe(CO)(5), Fe(CO)(3)(1,4-butadiene), CpFe(CO)(2)Me, Fe(CO)(3)(propenal), CpFe(CO)(2)Cl, (CO)(pyridine)(DMGBPh(2))(2)Fe(II) (DMG = dimethylglyoximato), (CO) (pyridine)(DMGBBN)(2)Fe(II) (BBN = 9-borabicyclo [3.3.1]nonane), (CO)(1-methylimidazole)(5,10,15,20-tetraphenporphinato)Fe(II), (CO)(pyridine)(5,10,15,20-tetraphenyl-porphinato)- Fe(II), (nitrosobenzene)(pyridine)(5,10,15,20-tetraphenylporphinato)Fe(II), (pyridine)(2)(5,10,15,20-tetraphenylporphinato)Fe(II), (1-methylimidazole)(2)(5,10,15,20-tetramesitylporphinato)Fe(II) and (trimethylphosphine)(2)(2,3,7,8,12,13,17,18-octaethylporphinato)Fe(II). The electric field gradients at iron were evaluated by using a locally dense basis approach : a Wachters＇ all electron representation for iron, a 6-311++G(2d) basis for all atoms directly bonded to iron, and either a 6-31G* basis for all other atoms or, in the case of the metalloporphyrins, a 6-31G*/3-21G* or 4-31G* basis, with the smaller basis being used on the peripheral atoms. Using a value of 0.16 x 10(-28) m(2) for the quadrupole moment of Fe-57(m), we find good agreement between theoretical and experimental quadrupole splittings : a slope of 1.04, an R-2 value of 0.975, and a root-mean-square error of 0.18 mm s(-1), for the 14 compounds examined. We have also investigated the effects of the CO ligand tilt and bend on the Fe-57 quadrupole splittings in several heme models. The theoretical results provide no support for the very large (40 degrees) Fe-C-O bond angles suggested by several diffraction studies on Physeter catodon carbonmonoxymyoglobin (P2(1) crystals). In contrast, the experimental results for (CO)(1-MeIm)(5,10,15,20-tetraphenylporphinato)Fe(II), which contains a linear and untilted Fe-CO, are in very close accord with the experimental values for CO-myoglobin : 0.35 mm s(-1) for the model system versus 0.363-0.373 mm s(-1) for MbCO, with V-zz oriented perpendicular to the porphyrin plane, as found experimentally. Calculations on metalloporphyrins at the more distorted X-ray geometries yield quadrupole splittings around 2 mm s(-1), inconsistent with experiment.