The UV-visible absorption spectra of [Ru(E)(E')(CO)(2)(iPr-DAB)] (E = E' = SnPh3 or Cl, E = SnPh3 or Cl, E' = CH3; iPr-DAB = N,N'-di-isopropyl-1,4-diaza-1,3-butadiene) are investigated using CASSCF/ CASPT2 and TD-DFT calculations on model complexes [Ru(E)(E')(CO)(2)(Me-DAB)] (E = E' = SnH3 or Cl; E = SnH3 or Cl, E' = CH3; Me-DAB = N,N'-dimethyl-1,4-diaza-1,3-butadiene). The calculated transition energies and oscillator strengths allow an unambiguous assignment of the spectra of the nonhalide complexes [Ru(SnPh3)(2)(CO)(2)(iPr-DAB)] and [Ru(SnPh3)(Me)(CO)(2)(iPr-DAB)]. The agreement between the CASSCF/ CASPT2 and TD-DFT approaches is remarkably good in the case of these nonhalide complexes. The lowest-energy part of the spectrum (visible absorption) originates in electronic transitions that correspond to excitations from the axial E-Ru-E'sigma (2) orbital into the low-lying pi*(DAB) orbital (sigma -bond-to-ligand charge transfer, SBLCT, transitions). while the absorption between 25 000 and 35 000 cm(-1) is due to metal-to-ligand charge transfer (MLCT) excitations from the 4d(Ru) orbitals to pi*(DAB) (MLCT). Above 35 000 cm(-1), the transitions mostly correspond to MLCT and SBLCT excitations into pi *co orbitals. Analysis of the occupied sigma orbitals involved in electronic transitions of the nonhalide complexes shows that the Kohn-Sham orbitals are generally more delocalized than their CASSCF/CASPT2 counterparts. The CASSCF/CASPT2 and TD-DFT approaches lead to different descriptions of electronic transitions of the halide complexes [Ru(Cl)2(CO) 2(Me -DAB) I and [Ru(Cl)(Me)(CO)2(Me-DAB)]. CASSCF/CASPT2 reproduces well the observed blue-shift of the lowest absorption band on going from the nonhalide to halide complexes. TD-DFT systematically underestimates the transition energies of these complexes, although it reproduces the general spectral features. The CASSCF/ CASPT2 and TD-DFT techniques differ significantly in their assessment of the chloride contribution. Thus, CASSCF/CASPT2 assigns the lowest-energy absorption to predominantly Ru --> DAB MLCT transitions, while TD-DFT predicts a mixed XLCT/MLCT character, with the XLCT component being predominant. (XLCT stands for halide (X)-to-ligand-charge transfer.) Analysis of Kohn-Sham orbitals shows a very important 3p(CI) admixture into the high-lying occupied orbitals, in contrast to the CASSCF/CASSPT2 molecular orbitals which are nearly pure 4d(Ru) with the usual contribution of the back-donation to pi*(CO) orbitals. Further dramatic differences were found between characters of the occupied sigma orbitals, as calculated by CASSCF/CASPT2 and DFT. They differ even in their bonding character with respect to the axial E-Ru and Cl-Ru bonds. These differences are attributed to a drawback of the DFT technique with respect to the dynamical correlation effects which become very important in complexes with a polar Ru-Cl bond. Similar differences in the CASSCF/ CASPT2 and TD-DFT descriptions of the lowest allowed transition of [Ru(Cl)(2)(CO)(2)(Me-DAB)] and [Ru(Cl)(Me)(CO)(2)(Me-DAB)] were found by comparing the changes of Mulliken population upon excitation. This comparison also reveals that CASSCF/CASPT2 generally predicts smaller electron density redistribution upon excitation than TD-DFT, despite the more localized character of CASSCF/CASPT2 molecular orbitals.