Topics in Current Chemistry

Electronic Spectroscopy and Photoreactivity of Transition Metal Complexes 149
3.3
Cyclometalated Complexes
3.3.1
Electronic Spectra of M(2-thienylpyridine)2 (M=Pd, Pt) Complexes:
a CASSCF/CASPT2 Study
Photo-chemical and photo-physical investigations based on highly resolved
emission and excitation spectra of cyclometalated complexes [139, 141] revealed
that the lowest excited states are Ligand Centred (LC) corresponding
to p!p* excitations with a large degree of MLCT admixture in contrast to
the compounds discussed in the previous sub-sections which are characterized
by low-lying MLCT, SBLCT and MC states.
A careful theoretical analysis based on CASSCF/CASPT2 calculations of
transition energies and oscillator strengths of two cyclometalated complexes
Pd(thpy) 2 and Pt(thpy) 2 containing either Pd(II) or Pt(II) as the central metal
coordinated to two cis-conjugated C-protonated 2-(2-thienyl) pyridine
(thpy Ÿ ) ligands (Scheme 8) has been reported [142]. The purpose of this
Scheme 8
study was an analysis of the excitation and emission spectra and the search
for some trends on modifying the metal centre. The calculated excitation energies
to the low-lying triplet and singlet states are reported in Table 5. The
a 3 A 1 and a 3 B 2 states, almost purely LC with very limited MLCT contribution
in Pd(thpy) 2 are about 1500 cm Ÿ1 lower for Pt(thpy) 2 (18,170 cm Ÿ1 ,
18,830 cm Ÿ1 ) than for Pd(thpy) 2 (19,690 cm Ÿ1 , 19,960 cm Ÿ1 ), both being considerably
lower than the calculated vertical excitation energies for the free
thpy ligands (~23,000 cm Ÿ1 ). The calculated values are in perfect agreement
with the experimental data which establish these states at 17,160 cm Ÿ1
(Pt(thpy) 2) and 18,420 cm Ÿ1 (Pd(thpy) 2) [143, 144]. However, one interesting
question which illustrates the complementarity s between experiments and