Topics in Current Chemistry

Electronic Spectroscopy and Photoreactivity of Transition Metal Complexes 145
ately followed in energy by MLCT (5d W!p * CO) states instead of MC states.
Similar trends have been observed for the bidentate substituted complexes
W(CO) 4L (L=ethylenediamine, N,N 0 -di-alkyl-1,4-diazabutadiene) on the basis
of TD-DFT and CASPT2 calculations [125].
3.2
Metal-to-Ligand-Charge-Transfer a-Diimine Complexes
In a series of structurally related transition metal a-diimine compounds, or
even within the same molecule several different types of Charge Transfer
(CT) transitions (from the metal M, from an halide ligand X or an axial sM-L
bond to the electron accepting a-diimine group) may be present in a limited
domain of the absorption spectrum (visible-near UV). This gives to this
class of molecules unconventional photo-chemical/physical properties which
may be tailored acting on the electronic degree of delocalization between the
metal centre, the a-diimine group and the heteroligands. In order to understand
the differences of spectroscopy and photo-reactivity observed in a variety
of molecules [126–135] a systematic quantum chemical study has been
performed for several MLCT a-diimine model complexes [94, 136, 137]. A
few examples are reported in the next following sub-sections.
3.2.1
Near UV-visible Absorption Spectra of [Ru(E)(E 0 )(CO) 2(iPr-DAB)]
(E=E 0 =SnPh 3 or Cl; E=SnPh 3 or Cl, E 0 =CH 3;
iPr-DAB=N,N 0 -diisopropyl-1,4-diaza-1,3-butadiene)
The near-UV/visible spectra of a series of [Ru(E)(E 0 )(CO) 2(iPr-DAB)]
(E=E 0 =SnPh 3 or Cl; E=SnPh 3 or Cl, E 0 =CH 3; iPr-DAB=N,N 0 -diisopropyl-1,4diaza-1,3-butadiene)
have been investigated on the basis of theoretical and
experimental analysis of model and real molecules, respectively [94].
The lowest-energy part of the absorption spectra of both [Ru(SnH 3) 2
(CO) 2(Me-DAB)] and [Ru(SnH 3)(CH 3)(CO) 2(Me-DAB)] model complexes
originates in an electronic transition that corresponds to excitation from an
axial E-Ru-SnH 3 (E=SnH 3 or Me) s E-Ru-SnH3 orbital into the low-lying p * DAB
orbital localized predominantly on Me-DAB (SBLCT transition) (Scheme 6).
The first intense band observed at 2.36 eV and 2.32 eV for [Ru(SnPh 3) 2
(CO) 2(iPr-DAB)] and [Ru(SnPh 3)(Me)(CO) 2(iPr-DAB)], respectively have
been assigned to SBLCT transitions calculated at 2.66 eV (b 1 A 1) and 2.55 eV
(b 1 A 0 ) at the CASPT2 level (Table 3). The near-UV region dominated by
weak shoulders at 3.12 eV [Ru(SnPh 3) 2(CO) 2(iPr-DAB)] and at ~2.75 eV and
3.17 eV [Ru(SnPh 3)(Me)(CO) 2(iPr-DAB)] consists mainly of MLCT
(4d Ru!p * DAB) transitions calculated at 3.08 eV (a 1 B 2) and 2.60 eV (a 1 A 00 )/
3.21 eV (c 1 A 0 ) for [Ru(SnH 3) 2(CO) 2(Me-DAB)] and [Ru(SnH 3)(CH 3)(CO) 2
(Me-DAB)], respectively. This assignment is confirmed by the TD-DFT