Topics in Current Chemistry

Electronic Spectroscopy and Photoreactivity of Transition Metal Complexes 153
ening and vibrational progression. Furthermore the 1 A 1g! 1 T 1g and
1 A1g! 1 T 2g transitions are more intense. In diffuse reflectance measurements
transitions with small extinction coefficients appear enhanced compared to
more intense transitions with a lot of fine structure in the region below
20,000 cm Ÿ1 [156]. Three bands are observed at 20,000 cm Ÿ1 , 24,800 cm Ÿ1
and 14,000 cm Ÿ1 which can be assigned to T 1g ( 1 T 1g), T 2g ( 1 T 2g) and E g
( 3 T 1g) components calculated at the CASPT2/SO level at 19,140 cm Ÿ1 ,
23,790 cm Ÿ1 and 13,390 cm Ÿ1 , respectively.
3.4.2
Trischelated Diketonato Complexes of Trivalent Chromium
The tris(acetylacetonato)chromium(III) complex Cr(acac) 3 has been the subject
of a number of spectroscopic and theoretical studies [157]. Of special interest
are the trigonal splittings of the 4 T 2g and 4 T 1g states. For the lowest
4 T2g state the experimental splitting amounts to 800 cm Ÿ1 with the 4 A 1 component
below the 4 E component. The ground and excited states of the neutral
tris (1,3-propanedionato) chromium (III) d 3 complex (Cr(PDO) 3) model for
Cr(acac) 3 have been investigated with CASSCF/CASPT2 methods [158] the
spin-orbit being included in a perturbative approach [155].
The transition energies to the quartet and doublet metal centred states
calculated at the CASPT2 level are compared to the experimental values in
Table 8.
The absolute positions of the doublet states are overestimated by about
2500 cm Ÿ1 . The spin-orbit coupled emitting doublet states is predicted to be
the 2A(3/2) component of the 2 E g state in agreement with spectral assignment
but in conflict with excited state EPR experiments. Despite the quality of the
calculations the large zero-field splitting (ZFS) of the 2 E g (220–290 cm Ÿ1 )has
not been reproduced with theoretical values less than 100 cm Ÿ1 .
Table 8 CASPT2 calculated excitation energies (in cm Ÿ1 ) to the low-lying doublet and quartet
states of Cr(PDO) 3 including the spin-orbit interactions
Transition Electronic states Excitation energies Experiment a
4 A2g! 2 E g
4 A2g! 2 T 1g
4 A2g! 2 T 2g
4 A2g! 4 T 2g
4 A2g! 4 T 1g
a From [159–161]
2
E 15540
2
12940 ( E)
13200 ( 2 E)
2
A2 16270
2
E 16660
2
A1 22170
2
E 21470
4
A1 16090 17700 ( 4 4
E 17410
A1) 4
18,500 ( E)
4
A2 25760
4
E 24760
4
22700 ( E)