The reactions of arene-metal complexes (arene = p-cymene, benzene or pentamethylcyclopentadienyl, metal = Ru, Rh or Os), including 1,3,5-triaza-7-phosphatricyclo-[18.104.22.168]decanephosphine (pta) and chloro co-ligands, with 9-methylguanine, adenine, and a series of nucleosides were studied in water to ascertain the binding modes. The products were characterized by NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS). Tandem mass spectrometry was found to provide excellent information on preferential binding sites. In general, the N7 position on guanine (the most basic site) was the preferred donor atom for coordination to the metal complexes. The x-ray structures of the precursor complexes, [(h5-C10H15)RhCl(pta-Me)2]Cl2, [(h6-C10H14)OsCl(pta)2]Cl, and [(h6-C6H6)OsCl2(CH3CN)], are also reported.