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

Abstract

The UV-visible absorption spectra of [Ru(E)(E')(CO)(2)(iPr-DAB)] (E = E' = SnPh(3) or Cl; E = SnPh(3) or Cl, E' = CH(3); 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' = SnH(3) or Cl; E = SnH(3) or Cl, E' = CH(3); 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(SnPh(3))(2)(CO)(2)(iPr-DAB)] and [Ru(SnPh(3))(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)] 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(Cl) 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)...

Substituent Effect on the Structure and Biological Property of99mTc-Labeled Diphosphonates: Theoretical Studies

“…This is not surprising given that crystal packing and hydrogen bonds can induce conformational features that are not present in the theoretical calculation of the isolated molecule in the gas phase at 0 K. Additionally, there is also inherent drawback in the DFT calculations arising from the dynamical correlation effects. 31 On the other hand, the calculated bond lengths and bond angles within the MDP ligand are in general agreement with the experimental studies of the free acid H 4 MDP 32 and related diphosphonates.…”

Asymptotic correction of the exchange–correlation kernel of time-dependent density functional theory for long-range charge-transfer excitations

“…It has been noted 21 that the anomalously low excitation energy sometimes observed in TDDFT calculations on transition-metal complexes, 22 seems to be related to exactly this type of charge-transfer between spatially separated ligands, and might therefore find a natural explanation in the asymptotic error of the ALDA f xc , and hopefully be remedied with the asymptotic correction proposed in this work. In practice, the asymptotical correction ͑9͒ should be added only to the diagonal elements K ib,ib of the coupling matrix ͑3͒.…”

Relativistic effects in spectroscopy and photophysics of heavy-metal complexes illustrated by spin–orbit calculations of [Re(imidazole)(CO)3(phen)]+

“…The "appropriateness of assigning spin labels to excited states of inorganic complexes" was questioned [4] already in 1974, shortly after the discovery of [Ru(bpy) 3 ] 2+ emission and excited-state electron-transfer reactivity. Distinct "triplet-state" photophysics and photochemistry are especially eminent in the case of closed-shell pseudo-octahedral d 6 , square-planar d 8 and flattened-tetrahedral d 10 metal complexes with electron-accepting ␣-diimine (N ∧ N) or cyclometallated (C ∧ N) ligands that introduce low-lying metal to ligand charge transfer (MLCT, d*) and intraligand (IL, *) excited states, while the metal atom (usually Re I , Os II , Ir III , Pt II or Au I ) is responsible for the relativistic SO effects.…”

“…Quantum chemical calculations of excited states also often neglect SOC. Nevertheless, spin-free CASSCF/CASPT2-type [5][6][7][8] and (TD)DFT [8][9][10] techniques are still instrumental in interpreting photophysical behavior and spectra. DFT techniques with hybrid functionals are successfully used to visualize singlet and triplet CT states and reveal their characters by displaying differences of electron density distribution upon excitation and/or excited-state spin-density distributions [9][10][11][12][13].…”

“…[9][10][11][12][13]21,22,60,61]. For example, the low-lying excited-states of [Re(I)(CO) 3 (bpy)] contain about 50% of I → bpy LLCT and 50% Re(CO) 3 → bpy MLCT characters [10] and are best viewed as delocalized Re(I)(CO) 3 → bpy CT. Mixing between metal d and ligand (L) p orbitals is well described (sometimes even exaggerated) by DFT with hybrid functionals [8,9,13,62], which calculates the lowest two MLCT/LLCT transitions as occurring from occupied (d-p) Kohn-Sham molecular orbitals, which are -antibonding with respect to the M-L bond, to a *(N ∧ N) orbital. Corresponding two transitions from the (d + p) -bonding orbitals lie at higher energies depending on L. (Note that d orbitals are, in addition, mixed with *(CO).)…”