A general and versatile method for the site-specific incorporation of polypyridine Ru(II) and Os(II) complexes into DNA oligonucleotides using solid-phase phosphoramidite chemistry is reported. Novel nucleosides containing a [(bpy)(2)M(3-ethynyl-1,10-phenanthroline)](2+) (M = Ru, Os) metal center covalently attached to the 5-position in 2'-deoxyuridine are synthesized, and their electrochemical as well as photophysical properties are studied. The Ru(II) nucleoside exhibits a rather long-lived excited state in phosphate buffer pH 7.0 (tau = 1.08 micros) associated with a relatively high emission quantum efficiency (phi = 0.051). The solvent dependence of the absorption and emission spectra is consistent with an emissive MLCT state where charge localization takes place on the extended heterocycle-linked phenanthroline. In contrast, the Os(II)-containing nucleoside is quite nonemissive in aqueous environment (tau = 0.027 micros, phi = 1 x 10(-4)). The metal-containing nucleosides are converted into their phosphoramidites and are utilized for the high-yield preparation of modified oligonucleotides. The novel oligonucleotides, characterized by absorption and emission spectroscopy, enzymatic digestion, and electrophoresis, form stable duplexes. Circular dichroism spectra confirm that the global conformation of the double helix is not altered by the presence of these polypyridyl complexes in the major groove. Metal-containing phosphoramidites with predetermined absolute configuration at the octahedral coordination center are synthesized and utilized for the synthesis of diasteromerically pure metal-containing DNA oligonucleotides. Emission spectroscopy suggests a higher protection of the Delta metal center from the bulk solvent and better accommodation within the major groove.