Ligand selective monosubstitution with complete enantiomeric retention in Delta and Lambda -cis-[Ru(bpy)(2)(DMSO)(Cl)]PF6 has been achieved photochemically. Irradiation in the presence of various ligands, including 4,4'-bipyridine, results in the replacement of the DMSO by the new ligand with complete enantioretention. This is revealed by the retention of the sign of the LC region CD Cotton effects, with the corresponding thermal reaction resulting in the racemic product. Importantly, 4,4'-bipyridine only coordinates through one pyridyl group, leaving the other for for subsequent reactions. [GRAPHICS]