The electronic, optical, and photophysical properties of [Re(im)(CO)(3)(phen)](+) and [Ru(bpy)(2)(im)(2)](2+) (im = imidazole; phen = 1,10-phenanthroline; bpy = 2,2'-bipyridine) in water, including spinorbit coupling (SOC) effects, were studied by means of density functional theory (DFT) and time-dependent DFT. The main features of the visible experimental absorption spectra of both molecules are well-reproduced. Whereas the theoretical spectrum of the Re(I) complex is characterized by one metal-to-ligand charge transfer (MLCTphen) state of low intensity at 394 nm and a strongly absorbing MLCTphen state calculated at 370 nm, the spectrum of the Ru(II) complex presents a high density of singlet MLCTbpy excited states with significant oscillator strengths that contribute to the two broad bands centered at 490 and 340 nm. The absorption spectrum of [Re(im) (CO)(3)(phen)](+) is perturbed by SOC with non-negligible mixing between the low-lying triplet and singlet absorbing states, while SOC has no effect on the absorption spectrum of [Ru(bpy)(2)(im)(2)](2+). A detailed structural investigation of the two lowest singlet and four lowest triplet excited states of the Re(I) complex point to MLCTphen (S-1, S-2, T-1, T-2) and intra-ligand ILphen (T-3) localized spin-densities characterized by small contractions from both ReN and phen CC central bonds in the MLCT states and nearly no deformation in the IL state. A mechanism of luminescent decay of [Re(im) (CO)(3)(phen)](+) is proposed on the basis of the calculated energy minima and wavelengths of emission for the interpretation of the three frequency/time-scale signals put in evidence by ultrafast experiments. The long-lived emissive properties of [Ru(bpy)(2)(im)(2)](2+) are analyzed on the basis of the relative energies of the two lowest (MLCTbpy)-M-3 and metal-centered (MC)-M-3 excited states. The minimum corresponding to the (MC)-M-3 spin density shows a significant structural rearrangement with an increase of the RuN bond distance of 0.33 angstrom and a closure of the NRuN bond angle of 20 degrees inducing a large distortion of the octahedral motif. The spin-density associated with the lowest (MLCTbpy)-M-3 localized on one bpy ligand suggests the presence of a second degenerate (MLCTbpy)-M-3 minimum. The luminescence of the Ru(II) complex calculated at 669 nm is partially quenched by the presence of the low (MC)-M-3 nonradiative state at 1064 nm. When interacting with modified metal-based proteins the two complexes will behave differently because of these distinctive photophysical properties.