The structure−properties relationship in a series of carbonyl rhenium(I) complexes based on substituted terpyridine ligands of general formula [Re(κ x N-Rtpy)(CO) y L] n+ is explored by both experimental and theoretical methods. In these compounds, the terpyridine ligands adopt both bidentate (κ 2 N) and terdentate (κ 3 N) coordination modes associated with three or two carbonyls, respectively. Conversion from the κ 2 N to the κ 3 N 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 κ 3 N 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.