The results of a13C and17O NMR and IR spectroscopic study of a series of carbonyl(4-substituted pyridine) (meso-tetra-phenylporphinato)iron(II) (Fe(TPP)(CO)(py-4-X)) complexes are presented. Good to excellent linear correlations between theI3C and17O NMR chemical shifts and the IR stretching frequencies of the carbonyl ligand are observed as the pyridine substituent is varied. Good to excellent linear correlations are also observed between these NMR chemical shifts and IR stretching frequencies and the NMR chemical shifts and IR stretching force constants for the trans carbonyls of a series of c/J-Mo(CO)4(py-4“X)2complexes as the pyridine substituent is varied. The relationship between the donor ability of the pyridine ligands and the13C and170 NMR chemical shifts and IR stretching frequencies of the carbonyl ligands in the Fe(TPP)(CO)(py-4-X) complexes has been quantitated by Fitting the spectroscopic data to the single and the dual Taft substituent parameters of the pyridine substituent. Good to excellent correlations are observed. The upfield shift in theI3C NMR resonance of the carbonyl ligand as the electron-donor ability of the pyridine increases is unique. This has been rationalized by using the Buchner and Schenk description of metal carbonyl13C NMR chemical shifts. © 1987, American Chemical Society. All rights reserved.