Compounds of the type [Ru(tpy)(L2)(dmso)](z+) (tpy is 2,2':6',2"-terpyridine; L2 can be 2,2'-bipyridine (bpy), N',N',N',N',-tetramethylethylenediamine (tmen), 2-pyridine carboxylate (pic), acetylacetonate (acac), malonate (mal), or oxalate (ox)) have been studied by X-ray crystallography, electrochemistry, NMR, IR, and UV-vis spectroscopy. When L2 is bpy, tmen, or pic, the dmso ligand can be intramolecularly isomerized either electrochemically or photochemically. lsomerization is not observed when L2 is acac, mal, or ox. lsomerization results in a drastic change in the absorption spectrum, as well as in the voltammetry. Absorption maxima shift by 3470 (419-490 nm), 4775 (421-527 nm), and 4440 cm(-1) (429-530 nm) for the bpy, pic, and tmen complexes, respectively. Reduction potentials for S-bonded and O-bonded complexes differ by 0.57, 0.75, and 0.62 V for the bpy, pic, and tmen complexes, respectively. Quantum yields of isomerization (phi s -> o) were determined for the bpy (0.024 +/- 1), pic (0.25 +/- 1), and tmen (0.007 +/- 1) complexes. In comparison of these data to photosubstitution quantum yields, it appears that the isomerization mechanism does not involve the ligand field states. This result is surprising given the importance of these states in the photochemistry of ruthenium and osmium polypyridine complexes. These results and details of the mechanism are discussed.