Poor correlations are normally observed between the chemical shifts of the various NMR‐active nuclei in Mo carbonyl complexes of group 15 ligands as the group 15 atom substituents are varied. It is possible to observe good chemical shift correlations and to develop relationships between chemical shift and changes in electron density due to variations in the group 15 atom substituents, provided that this variation does not change the cone angles of these ligands. This is the case for complexes of the type cis‐Mo(CO)4(py‐R)2 (py‐R = 3‐ or 4‐substituted pyridine). Good to excellent linear correlations are observed between the chemical shifts of the cis and trans CO 13C and 17O resonances and the 95Mo resonances in these complexes, and between these chemical shifts and the CO infrared stretching force constants of the complexes. Good to excellent correlations also occur between either the chemical shifts or the CO infrared stretching force constants and either the Taft σm/σp or σR and σ1 parameters. These correlations provide significant insights into the relationship between the electron donor/acceptor abilities of the pyridine ligands and the NMR and infrared spectra exhibited by the cis‐Mo(CO)4(py‐R)2 complexes. Copyright © 1986 John Wiley & Sons, Ltd.