sigma-pi* electronic transition of the di- and trinuclear complexes Ru(E)(E')(CO)(2)(iPr-DAB): Resonance Raman, electronic absorption, emission, and density functional study (E=Me, SnPh3, M(CO)(5); E'=M(CO)(5); M=Mn, Re; iPr-DAB=N,N'-diisopropyl-1,4-diaza-1,3-butadiene)

Di- and trinuclear complexes Ru(E)(E')(CO)(2)(iPr-DAB) (E = Me, SnPh3, Mn(CO)(5), E' = Mn(CO)(5); E = Me, SnPh3, Re(CO)(5), E' = Re(CO)(5); iPr-DAB E N,N'-diisopropyl-1,4-diazabutadiene), which contain Ru-Mn or Ru-Re sigma bon;is, absorb strongly in the visible spectral region. A combined spectroscopic (absorption and resonance Raman) and theoretical (DFT-MO) study shows that the lowest allowed electronic transition has a sigma((Ru-E)) --> pi((DAB))* or sigma(n(E-Ru-E')) --> pi((DAB))* character. The latter transition originates in a sigma orbital, which is only weakly bonding or even nonbonding (sigma(n)) with respect to the a;dal E-Ru-E' bonds. The second visible absorption band that occurs at higher energies for Ru(M(CO)(5))(2)(CO)(2)(iPr-DAB) (M = Mn, Re) is attributed to a sigma(b) --> pi* transition, the electron being excited from a strongly E-Ru-E' sigma-bonding orbital. Both sigma --> pi* transitions of the trinuclear complexes have surprisingly high oscillator strengths (0.1-0.2). An emission study of Ru(E)(E')(CO)(2)(iPr-DAB) shows that their lowest excited state has a (3) sigma pi* character. The emission is rather long lived (4-100 mu s at 80 K, depending on the complex composition) because of slow nonradiative decay to the ground state.