inorganic chemistry

C. EFFECT OFWEAKINTERACTIONSBETWEENLIGANDS ON
ELECTRONICSTRUCTURE

Tricarbonyl monophosphine rhenium(I) complexes fac-[Re
(diimine)(CO) 3 (PR 3 )]þ can be readily converted into the
corresponding biscarbonyl bisphosphine rhenium(I) complexescis,
trans-[Re(diimine)(CO) 2 (PR 3 )(PR^03 )]þby means of a photochemical
ligand substitution reaction (seeSection III.D) (26,31). Due to the
strongp-acidity of the carbonyl ligand, substitution by a phosphine
ligand causes a destabilization of the HOMO level (dporbital of the
Re center, see Section III) and bathochromic shifts in the absorption
and emission spectra. For example, cis,trans-[Re(bpy)(CO) 2 {P
(OEt) 3 } 2 ]þ(11a) has an MLCT absorption band and an emission
maxima in a longer wavelength region compared tofac-[Re(bpy)
(CO) 3 P(OEt) 3 ]þ(3a)(Fig. 5). Moreover, the emission quantum yield
of the11a(1.7%) was much smaller than that of3a(8.8%) ( 32 ),
which obeys the Energy Gap Law (see Section II.B).
However, the nature of the phosphine ligands is another critical
factor for controlling the photophysical properties of this family of
complexes. In particular, the introduction of triarylphosphine
ligand(s) was found to bring about drastic changes in the
photophysical and electrochemical properties(33,34). For exam-
ple, a Re(I) complex with two tris(p-chlorophenyl)phosphine lig-
ands (11b) exhibited a red-shifted MLCT absorption band but
blue-shifted emission compared to the bis(iso-propylphosphite)
Re(I) complex (11c)(Fig. 6). As these two ligands have comparable
electron-withdrawing abilities estimated from Tolman'swvalues

300

abs.

350

(a) (b)

wavelength (nm) wavelength (nm)

400 450 500 450 700 750

Emission intensity / a.u.

500 550 600 650

FIG. 5. UV/Vis absorption (a) and emission (b) spectra ofcis,trans-
[Re(bpy)(CO) 2 {P(OEt) 3 } 2 ]þ (11a, solid line) and fac-[Re(bpy)(CO) 3 (P
(OEt) 3 )]þ(3a, dotted line).

144 HIROYUKI TAKEDAet al.