Density functional theory calculations were performed on a series of six ruthenium complexes possessing tridentate ligands [Ru(tpy)(2)](2+) (1, tpy = [2,2`,6`,2 '']-terpyridine), [Ru(tpy)(pydppx)](2+) (2 pydppx = 3-(pyrid-2`-yl)-11,12-dimethyldipyrido[3,2-a 2`,3`-c]phenazine), [Ru(pydppx)(2)](2+) (3), [Ru(tpy)(pydppn)](2+) (4, pydppn = 3-(pyrid-2`-yl)-4,5 9,16-tetraazadibenzo[a,c]naphthacene), [Ru(pydppn)(2)](2+) (5) and [Ru(tpy)(pydbn)](+) (6, pyHdbn = 3-pyrid-2`-yl-4,9 16-triazadibenzo[a c]naphthacene) The calculations were compared to experimental data, including electrochemistry and electronic absorption spectra The theoretical results reveal that the lowest-lying singlet and triplet states in 4 and 5 are pydppn-based pi pi* in character which are remarkably different from the lowest-lying metal-to-ligand charge transfer (MLCT) states in 1-3 The calculated lowest triplet states in 4 and 5 are consistent with the (3)pi pi* states observed experimentally However, although the extended pi-system of pydbn(-) is similar to that of pydppn, the HOMO of 6 lies above those of 4 and 5, resulting in strikingly different spectroscopic properties Calculations show that the lowest triplet excited state of 6 is a combination of (MLCT)-M-3 and (3)pi pi* This work demonstrates that the electronic structure of the tridentate ligand has a pronounced effect on the photophysical properties of ruthenium(II) complexes and that DFT and TD-DFT methods are a useful tool that can be used to predict photophysical and redox properties of transition metal complexes