The structure-properties relationship in a series of carbonyl rhenium(I) complexes based on substituted terpyridine ligands of general formula [Re(kappa N-x-Rtpy)(CO)(gamma)L](n+) is explored by both experimental and theoretical methods. In these compounds, the terpyridine ligands adopt both bidentate (kappa N-2) and terdentate (kappa N-3) coordination modes associated with three or two carbonyls, respectively. Conversion from the kappa N-2 to the ON coordination mode leads to large changes in the absorption spectra and oxidation potentials due to destabilization of the HOMO level of each complex. The absorption profiles of the kappa N-3 complexes cover the whole visible spectra with lower maxima around 700 nm, tailing out to 800 nm, while no emission is observed with Br- as the axial ligand L. When the axial ligand is modified from the native halide to pyridine or triphenylphosphine, the lowest absorption band is blue-shifted by 60 and 90 nm, respectively. These cationic complexes are near-infrared emitters with emission maxima between 840 and 950 nm for the pyridine compounds and 780-800 nm for the triphenylphosphine compounds.