Excitation by high-energy light, such as that of 313 nm wavelength, induces a photochemical ligand substitution (PLS) reaction of fac-[Re(bpy)(CO)(3)Cl] (1a) to give the solvento complexes (OC-6-34)-and (OC-6-44)-[Re(bpy)(CO)(2)(MeCN)Cl] (2 and 3) in good yields. The disappearance quantum yield of 1a was 0.01 +/- 0.001 at 313 nm. The products were isolated, and X-ray crystallographic analysis was successfully performed for 2. Time-resolved IR measurements clearly indicated that the CO ligand dissociates with subpicosecond rates after excitation, leading to vibrationally hot photoproducts, which relax within 50-100 ps. Detailed studies of the reaction mechanism show that the PLS reaction of 1a does not proceed via the lowest vibrational level in the (MLCT)-M-3 excited state. The PLS reaction gives 2 and (OC-6-24)-[Re(bpy)(CO)(2)(MeCN)Cl] (5) as primary products, and one of the products, 5, isomerizes to 3. This type of PLS reaction is more general, occurring in various fac-rhenium(I) diimine tricarbonyl complexes such as fac-[Re(X(2)bpy)(CO)(3)Cl] (X(2)bpy = 4,4'-X-2-bpy; X = MeO, NH2, CF3), fac-[Re(bpy)(CO)(3)(pyridine)](+), and fac-[Re(bpy)(CO)(3)(MeCN)](+). The stable photoproducts (OC-6-44)- and (OC-6-43)-[Re(bpy)(CO)(2)(MeCN)(pyridine)](+) and (OC-6-32)- and (OC-6-33)-[Re(bpy)(CO)(2)(MeCN)(2)](+) were isolated. The PLS reaction of rhenium tricarbonyl-diimine complexes is therefore applicable as a general synthetic method for novel dicarbonyls.