TIME-RESOLVED ABSORPTION, INFRARED, AND RESONANCE RAMAN-SPECTRA OF THE COMPLEXES [RU(X)(R)(CO)(2)(ALPHA-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 (TR(3)), and infrared (TRIR) spectra are reported for several complexes [Ru(X)(R)( CO)(2)(alpha-diimine)] (X = Cl, Br, I; R = Me, Et; alpha-diimine = N,N '-diisopropyl-1,4-diaza-1,3-butadiene (iPr-DAB), pyridine-2-carbaldehy de-N-isopropylimine (iPr-PyCa), 2,2'-bipyridine (bpy)). This is the fi rst instance in which the TA, TR(3), and TRIR techniques have been use d to probe excited states in the same series of complexes. The TA spec tra of the iodide complexes show a transient absorption between 550 an d 700 nm, which does not depend on the solvent but shifts to lower ene rgy in the order iPr-DAB > bpy > iPr-PyCa. This band is assigned to an intraligand transition. For the corresponding chloride and bromide co mplexes 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 v(CO) (and k(CO)) on promotion to th e excited state; however, the shifts Delta v(CO) show a decrease in th e order Cl- > Br- > I-. The TR(3) spectra of the excited complexes [Ru (X)(R)(Co)(2)(iPr-DAB)] show v(s)(CN) of the iPr-DAB ligand 50-80 cm(- 1) lower in frequency than for the complexes in their ground state. Th is frequency shift decreases in the order Cl- > Br- > I-, indicating a decrease of CT character of the lowest excited state in this order. H owever, going from X = Br to I, the effect on Delta v(CO) is much larg er than the decrease of Delta v(s)(CN). This different effect on the C O- and CN-stretching frequencies is assigned to a gradual change in ch aracter of the lowest excited state from MLCT to XLCT when Cl- is repl aced by Br- and I-. This result confirms a similar conclusion derived from previous resonance Raman and emission experiments on these comple xes.