We have investigated the 57Fe Mössbauer 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)2Me, Fe(CO)3(propenal), CpFe(CO)2Cl, (CO)(pyridine)(DMGBPh2)2Fe(II) (DMG = dimethylglyoximato), (CO)(pyridine)(DMGBBN)2Fe(II) (BBN = 9-borabicyclo[3.3.1]nonane), (CO)(1-methylimidazole)(5,10,15,20-tetraphenylporphinato)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++G2d 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 × 10-28 m2 for the quadrupole moment of 57Fem, we find good agreement between theoretical and experimental quadrupole splittings: a slope of 1.04, an R2 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 57Fe quadrupole splittings in several heme models. The theoretical results provide no support for the very large (40°) Fe−C−O bond angles suggested by several diffraction studies on Physeter catodon carbonmonoxymyoglobin (P21 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 Vzz 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.