The synthesis and characterization of a new family of luminescent cis-(4,4′-X25,5′-Y2-2,2′-bipyridine)2Os(CO)Cl(PF6) complexes (X = NEt2, OMe, Me, H, Cl, Y = H; X = H, Y = Me; X = Y = Me) are described. Conventional 1H NMR, spin-decoupling, and nuclear Overhauser effect (NOE) experiments are utilized for structure determinations of the complexes and complete assignment of proton resonances for the complexes and free ligands in CD3CN solution (supplementary material). Nonradiative deactivation of the luminescent metal-to-ligand charge-transfer (MLCT) excited states is dominant for all complexes and is governed by the energy gap law. Linear correlations of electrochemical, IR (CO stretching frequency), and absorption/emission properties of the complexes with Hammett σ constants of the bipyridyl substituent groups are established and provide a convenient method for the prediction and control of ground- and excited-state properties within the family. © 1990, American Chemical Society. All rights reserved.