The coordination bonds between the rhenium(I) and the CO
ligands consist of two electron-donating mechanisms:s-donation
from the lone-pair electrons of the CO ligand to the rhenium(I),
and thep-back donation from the occupied t2gorbital electron
of the rhenium(I) to the emptyp orbital of the CO ligands.
The photoexcitation process of this complex is attributed to two
types of electronic transition, that is, an LF transition (Re
t2g!Re eg) in a lower-energy region and an MLCT transition
(Re dp!COp) in a higher region. Although the molar extinc-
tion coefficient of the LF absorption band is very small because
it is a forbidden transition, it causes a photochemical reaction.
The LF transition weakens the ReCO bond, and one CO ligand
is eliminated (Eq. 1) because an electron located on the bonding
t2gorbital is excited to an antibonding egorbital. The rate con-
stant of CO elimination from the LF excited state is accelerated
by a factor of 10^16 compared to the ground state, and the reaction
quantum yield is close to unity. For the presence of the good lig-
ands (L), a photoinduced ligand substitution reaction can proceed
to give Re(CO) 5 L(Eq. 2).
CO
CO hnCO
COOCOC
Reı + COCOCO
COOCOC
Reı ð 1 ÞCO
LCO
COOCOC
ReıCO+L
CO
COOCOC
Reı ð 2 ÞB. RERE,REM,ANDRECBONDHOMOLYSISREACTIONS
For Re(I) dimers with a ReRe bond, photoexcitation induces
homolysis of the Re Re bond. The two rhenium(0) metal centers
in Re 2 (CO) 10 each donate one electron from their dz 2 orbitals to
make asM M bond. Upon photoirradiation, the bondings-
orbital (sb) electron is excited into the antibonding s-orbital
(sz*), which leads to metal–metal bond cleavage. It has been also
reported that a CO ligand elimination proceeds competitively
with the metal–metal bond homolysis reaction. When a cyclohex-
ane solution of Re 2 (CO) 10 was irradiated by the 355-nm laser
light, the ratio of the reaction quantum yields of ReRe bond
RHENIUM(I) DIIMINE COMPLEXES 147