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):  Combination of CASSCF/CASPT2 and TD-DFT Calculations

• Published in: Journal of the American Chemical Society 2001-11, Vol.123 (46), p.11431-11440
• Main Authors: Turki, Mohamed, Daniel, Chantal, Záliš, Stanislav, Vlček, Antonín, van Slageren, Joris, Stufkens, Derk J
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja010782b

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‘ σ2 orbital into the low-lying π*DAB orbital (σ-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 4dRu orbitals to π*DAB (MLCT). Above 35 000 cm-1, the transitions mostly correspond to MLCT and SBLCT excitations into π*CO orbitals. Analysis of the occupied σ 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)] 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 3pCl admixture into the high-lying occupied orbitals, in contrast to the CASSCF/CASSPT2 molecular orbitals which are nearly pure 4dRu with the usual contribution of the back-donation to π*CO orbitals. Further dramatic differences were found be