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The photochemical properties of Ru(II) mono(bipyridine) complexes Ru(L)(CO) 2 Cl 2 [L ) 2,2′-bipyridine (1, 2) or 4,4′-dimethyl-2,2′-bipyridine (3)] were studied in CH 3 CN and CH 2 Cl 2 . Photochemical ligand substitution reactions occurring in CH 3 CN were followed by infrared and 1 H NMR spectroscopies and cyclic voltammetry. The structures of the photolysis products Ru(bpy)(CO)(CH 3 CN)Cl 2 (4), mer-(CH 3 CN) Ru(bpy)(CH 3 CN) 3 Cl + (6), and fac-(CH 3 CN) Ru(dmbpy)(CH 3 CN) 3 Cl + (8) were determined by single-crystal X-ray crystallography. Photosubstitution of the trans (1) and cis (2) Cl isomers of Ru(bpy)(CO) 2 Cl 2 (bpy ) 2,2′-bipyridine) led to the same monosubstituted Ru(bpy)(CO)(CH 3 CN)Cl 2 complex (4). Probably some isomerization occurs from the trans Cl isomer. Irradiation in CH 2 Cl 2 led mainly to the dichloro-bridged [Ru(bpy)(CO)Cl 2 ] 2 (5) dimer starting from either the trans (1) or cis (2) isomer of Ru(bpy)(CO) 2 Cl 2 .