σ−π* 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 = <i>N</i>,<i>N‘</i>-Diisopropyl-1,4-diaza-1,3-butadiene)

Abstract

Di- and trinuclear complexes Ru(E)(E‘)(CO) 2 (iPr-DAB) (E = Me, SnPh 3, Mn(CO) 5, E‘ = Mn(CO) 5; E = Me, SnPh 3, Re(CO) 5, E‘ = Re(CO) 5; iPr-DAB = N, N‘ -diisopropyl-1,4-diazabutadiene), which contain Ru−Mn or Ru−Re σ bonds, 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 σ (Ru-E) → π* (DAB) or σ n(E-Ru-E‘) → π* (DAB) character. The latter transition originates in a σ orbital, which is only weakly bonding or even nonbonding (σ n ) with respect to the axial 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 σ b → π* transition, the electron being excited from a strongly E−Ru−E‘ σ-bonding orbital. Both σ→π* 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 σπ* character. The emission is rather long lived (4−100 μs at 80 K, depending on the complex composition) because of slow nonradiative decay to the ground state.