The Fe-57 chemical shifts of a series of 94.5% Fe-57 enriched model heme complexes (those of tetraphenylporphyrin, TPP, and two of its p-phenyl substituted derivatives, tetramesitylporphyrin, TMP, and octaethylporphyrin, OEP), for which trimethylphosphine serves as at least one of the axial ligands, have been measured by recording the P-31 NMR spectrum using double resonance at the appropriate Fe-57 frequency. The discovery of an approximate correlation between P-31 and Fe-57 chemical shifts makes it easy to predict the Fe-57 chemical shift of new complexes, thus simplifying the search for the proper decoupling frequency. The Fe-57 chemical shifts of substituted phenyl TPP complexes increase as the electron-donating nature of the substituents increases, and most OEP complexes have chemical shifts that are 200-250 ppm larger than their TPP counterparts; both of these trends are attributed to increased pi donor characteristics that increase the magnitude of the paramagnetic screening constant, sigma(para). Mixed axial ligand complexes where trimethylphosphine is one of the ligands have larger Fe-57 chemical shifts when the other ligand (L) is a strong sigma donor/weak pi acceptor, but within a group of closely related ligands such as 4-substituted pyridines, the weakest sigma donor/strongest pi acceptor ligand produces the largest Fe-57 chemical shift (4-CNPy > Py > 4-NMe(2)Py), in line with the predictions of the Ramsey formula for sigma(para). Solvent effects on the Fe-57 chemical shift of [TMPFe(PMe(3))(2)] are small and, in distinction to 6-coordinate low-spin cobalt(III) porphyrinates, do not follow any common measures of solvent polarity or donor strength (dielectric constant, Gutmann’s donor number, Reichardt’s E(T)) P-31 and Fe-57 chemical shifts both show linear temperature dependence, but with opposite slopes. The structures of two (PMe(3))(2) complexes of iron(II) porphyrinates, [(p-OCH3)(4)TPPFe(PMe(3))(2)]. 2C(6)D(6) and [OEPFe(PMe(3))(2)], have also been determined. In both cases the porphyrinate core is essentially planar and the axial trimethylphosphine ligands have their methyl groups in staggered conformation.