inorganic chemistry

(OEt) 3 }(CH 3 CN)]þ, in which an MeCN ligand occupies a position
trans to the P(OEt) 3 ligand. This product was obtained quantita-
tively, and no other isomers were produced (Eq. 4). The same
photoinduced ligand substitution reaction was observed for
complexes with other phosphorous ligands.

N N

N N

Re Re

CO

CO

CO

hn CO

CO

+CO

NCCH 3

CH 3 CN

P(OEt) + +

3 P(OEt)^3

ð 4 Þ

Generally, there are two reaction mechanisms for photochemical
ligand substitution reactions on metal complexes, that is,“disso-
ciative”and“associative”mechanisms. The photochemical reac-
tion of3ain CH 2 Cl 2 solution was investigated in the presence
of molecules with different coordinative abilities (CH 3 CN, pyri-
dine, P(OEt) 3 ). In all the cases, the photochemical ligand substi-
tution reactions proceeded quantitatively and the reaction rates
did not depend on the identity of the incoming ligand. This
clearly shows that photochemical reaction proceeds through the
dissociative mechanism.
A CDCl 3 solution containing3awas irradiated under a^13 CO
atmosphere. The^13 C NMR measurement revealed that only the
CO ligand in the trans-position to P(OEt) 3 was substituted
(Fig. 9). This result indicates that the primary process of the pho-
tochemical ligand substitution reaction is the elimination of the
CO ligand in the trans-position to P(OEt) 3 ligand, and a new
ligand should be promptly introduced at the same position. The
coordinatively unsaturated species do not undergo any structural
rearrangements which cause isomerization of the complex.
The CO stretching vibrational bands of rhenium carbonyl
complexes are useful spectroscopic probes for studying their elec-
tronic excited states. As these IR absorption bands reside in the fre-
quency region where few absorption bands exist, their absorption
cross section are large and the band frequencies are greatly
influenced by the amount of electron density on the central rhe-
nium(I). Therefore, transient IR absorption (TR-IR) measurements
should be one of the ultimate tools for the study of the photophysics
and photochemistry of the rhenium carbonyl complexes.
TR-IR spectra of the complex3awere measured in an CH 3 CN
solution following irradiation with 355-nm laser light (Fig. 10).
Immediately after laser pulse, the three absorption peaks (at
1928, 1962, 2047 cm
^1 ) attributed to the three ground-state CO

RHENIUM(I) DIIMINE COMPLEXES 151