We have synthesized and characterized via single-crystal X-ray diffraction methods iron(II), ruthenium(II), and osmium(II) carbonyl derivatives of (1-methylimidazole)(5, 10, 15, 20-tetraphenylporphyrinate) [(5,10,15,20-tetraphenylporphyrinate = TPP)], Fe(TPP)(CO)(1-MeIm).toluene; Ru(TPP)(CO)(1-MeIm).chloroform, and Os(TPP)(CO)(1-MeIm).chloroform, together with the osmium(II) pyridine adduct Os(TPP)(CO)(py) 2benzene. The crystallographic results permit a detailed structural comparison between all of the six carbonyl metalloporphyrins which can be prepared from TPP, Fe, Ru, Os, and the two axial bases 1-methylimidazole and pyridine. The structures of all three (Fe, Ru, Os) 1-methylimidazole complexes display major saddle distortions, with the extent of the distortions being Fe > Ru similar to Os. For the pyridine complexes, deviations from planarity of the porphyrin ring are about an order of magnitude smaller than those for the 1-methylimidazole species. The M-C-O bond angles in all complexes are in the range 176.8-179.3 degrees. We also determined the C-13 and O-17 NMR isotropic chemical shifts, the C-13 NMR chemical shift tensor elements, and, for the three 1-MeIm adducts, the O-17 nuclear quadrupole coupling constants. We then used density functional theory (DFT) to relate the experimental spectroscopic results to the experimental structures. For the C-13 and O-17 isotropic shifts, there are excellent correlations between theory and experiment (C-13, R-2 value = similar to 0.99; O-17, R-2 value = similar to 0.99), although the slopes (C-13, similar to-0.97; O-17, similar to-1.27) deviate somewhat from the ideal values. For the O-17 nuclear quadrupole coupling constant, our results indicate an rms error between theory and experiment of 0.20 MHz, for experimental values ranging from (+)1.0 to (-)0.40 MHz, where the; signs are deduced from the calculations. The ability to Predict spectroscopic observables in metalloporphyrin systems having relatively well characterized structures by using density functional theory provides additional confidence in the application of these theoretical methods to systems where structures are much less certain, such as heme proteins.