Time-Resolved Absorption, Infrared, and Resonance Raman Spectra of the Complexes [Ru(X)(R)(CO)₂(α-Diimine)] (X = Halide; R = Alkyl): Influence of X on the Charge Transfer Character of the Lowest Excited State

Nanosecond time-resolved absorption (TA), resonance Raman (TR³), and infrared (TRIR) spectra are reported for several complexes [Ru(X)(R)(CO)₂(α-diimine)] (X = Cl, Br, I; R = Me, Et; α-diimine = N, N'-diisopropyl-1, 4-diaza-1, 3-butadiene (iPr-DAB), pyridine-2-carbaldehyde-N-isopropylimine (iPr-PyCa), 2, 2'-bipyridine (bpy)). This is the first instance in which the TA, TR³, and TRIR techniques have been used to probe excited states in the same series of complexes. The TA spectra of the iodide complexes show a transient absorption between 550 and 700 nm, which does not depend on the solvent but shifts to lower energy in the order iPr-DAB > bpy > iPr-PyCa. This band is assigned to an intraligand transition. For the corresponding chloride and bromide complexes this band occurs at higher energy, most probably because of a change of character of the lowest excited state from XLCT to MLCT. The TRIR spectra show an increase in ν(CO) (and k(CO)) on promotion to the excited state; however, the shifts Δν(CO) show a decrease in the order Cl^- > Br^-> I^-. The TR³ spectra of the excited complexes [Ru(X)-(R)(CO)₂(iPr-DAB)] show ν_s(CN) of the iPr-DAB ligand 50-80 cm^-1 lower in frequency than for the complexes in their ground state. This frequency shift decreases in the order Cl^-> Br^-> I^-, indicating a decrease of CT character of the lowest excited state in this order. However, going from X = Br to I, the effect on Δν(CO) is much larger than the decrease of Δν_s(CN). This different effect on the CO- and CN-stretching frequencies is assigned to a gradual change in character of the lowest excited state from MLCT to XLCT when Cl^- is replaced by Br^- and I^-. This result confirms a similar conclusion derived from previous resonance Raman and emission experiments on these complexes.