Infrared data in the nu(CO) region (1800-2150 cm(-1), in acetonitrile at 298 K) are reported for the ground ((ν) over bar (gs)) and polypyridyl-based, metal-to-ligand charge-transfer (MLCT) excited ((ν) over tilde (es)) states of cis-[Os(pp)(2)(CO)(L)](n+) (pp = 1,10-phenanthroline (phen) or 2,2'-bipyridine (bpy); L = PPh3, CH3CN, pyridine, Cl, or H) and tac-[Re(PP)(CO)(3)(4-Etpy)](+) (pp = phen, bpy, 4,4'-(CH3)(2)bpy, 4,4'-(CH3O)(2)bpy, or 4,4'-(CO2Et)(2)bpy; 4-Etpy = 4-ethylpyridine). Systematic variations in (ν) over bar (gs), (ν) over bar (es) and Delta(ν) over bar (Delta(ν) over bar = (ν) over bar (es) - (ν) over bar (gs)) are observed with the excited-to-ground-state energy gap (E-0) derived by a Franck-Condon analysis of emission spectra. These variations can be explained qualitatively by invoking a series of electronic interactions. Variations in dpi(M)-pi*(CO) back-bonding are important in the ground state. In the excited state, the important interactions are (1) loss of back-bonding and a(M-CO) bond polarization, (2) pi*(pp.(-))-pi*(CO) mixing, which provides the orbital basis for mixing pi*(CO)- and pi*(4,4'-X(2)bpy)-based MLCT excited states, and (3) dpi(M)-pi(pp) mixing, which provides the orbital basis for mixing pipi*- and pi*(4,4'-X-2-bpy.(-))-based MLCT states. The results of density functional theory (DFT) calculations on the ground and excited states of fac-[Re-1(bpy)(CO)(3)(4-Etpy)](+) provide assignments for the v(CO) modes in the MLCT excited state. They also support the importance of pi*(4,4'-X(2)bpy.(-))-pi*(CO) mixing, provide an explanation for the relative intensities of the A'(2) and A" excited-state bands, and provide an explanation for the large excited-to-ground-state nu(CO) shift for the A'(2) mode and its relative insensitivity to variations in X.