as the energy caused by dependence of the solvent viscosity on
temperature, and therefore, it should not be attributed toDG6¼
of the thermal activation from the^3 MLCT state to the^3 LF state.
The reasons why these complexes are not active in the photo-
chemical ligand substitution reaction can be classified into two
cases. One is that although the thermal activation from^3 MLCT
to^3 LF is preferable, the energy surface of^3 LF is not repulsive
and a large activation energy (DG 2 6¼) is required to eliminate
the CO ligand at room temperature (Scheme 3b). The other case
is that the energy gap from^3 MLCT to^3 LF (DG 1 6¼) is too large to
produce the^3 LF state at room temperature (Scheme 3c).
As the pyridine ligand has a relatively weak LF compared to
the phosphorous ligands, the d–d splitting of the pyridine com-
plex should be smaller. Therefore, the^3 MLCT state energies
(E 00 ) of the pyridine complexes are relatively low, for example,
17,490 cm^1 for2a. The energy was as low as 17,270 cm^1 for
fac-[Re{(CF 3 ) 2 bpy}(CO) 3 {P(OEt) 3 }]þ (3c), which has the strong
electron-withdrawing CF 3 groups at the 4,4^0 -positions of the
bpy ligand. These serve to stabilize thep* state. The similar sit-
uation can be seen in the case of the rhenium(I) complexes with a
Clligand where LF is even smaller than complexes with pyri-
dine as a ligand. For example, the^3 MLCT state energy offac-
Re{(MeO) 2 bpy}(CO) 3 Cl (1c), which features two electron-donat-
ing substituents on the bpy ligand, was 16,320 cm^1 , which is
slightly lower energy than that of 3c. The^3 LF excited state
energies of both1c and2ashould be smaller than that of 3c
owing to the smaller LFs than the phosphorous ligands. There-
fore, the^3 MLCT–^3 LF energy gaps of1cand2ashould be similar
or less than that of3c. Although3cwas photoreactive under
366-nm irradiation,2aand1cwere not at all reactive, as men-
tioned above. This result clearly indicates that the photo-
stabilities of2a and 1c are not caused by a large DG6¼ that
prevents thermal activation to the reactive^3 LF state from the
(^3) MLCT state (Table V).
Based on the above discussion, the following interpretation has
been proposed usingScheme 3b. Although the potential curves of
the^3 LF state and the^3 MLCT state cross each other in a rela-
tively low-energy region, it is necessary to overcome another
higher-potential barrier for ReCO bond scission. The Re CO
bond of3cshould be weakened due to thep-acidity of the phos-
phorous ligand in the trans-position to the released CO ligand.
However, it should be reasonable that the ReCO bonds of1c
and2ashould be stronger than3cbecause the pyridine ligand
is a weakp-acid and the Clligand is ap-base (Scheme 4).
RHENIUM(I) DIIMINE COMPLEXES 159