Two new oxorhenium(V) compounds were prepared and characterized: MeReO(mtp)(Me(2)Bpy) and MeReO-(mtp)(dppb), where mtpH(2) is 2-(mercaptomethyl)thiophenol, Me(2)Bpy is 4,4'-dimethyl-2,2'-bipyridine, and dppb is 1,2-(Ph2P)(2)C6H4. The more stable geometric isomer of MeReO(mtp)X forms MeReO(mtp)Y (X, Y = PR3, NC5H4R) in two steps, both of which show a first-order dependence on [Y], proceeding through the metastable geometric isomer MeReO(mtp)Y*. When Y = PR3, no MeReO(mtp)Y* was detected at equilibrium; with NC5H4R, however, both isomers were detected. The values of K-Py*Py were 8.5-9.8, largely irrespective of R; for NC5H5, DeltaH degrees = -4.47 +/- 0.29 kJ and DeltaS degrees = 3.9 +/- 1.0 J K-1. For the more symmetric edt ligand, geometric isomers do not exist, but enantiomers do. The rate of racemization of MeReO(edt)(NC5H4R) was proportional to [Py]. Values of k(rac) for 16 compounds span the range 135-370 L mol(-1) s(-1) in C6H6 at 25 degreesC (p = -0.39 +/- 0.07). In toluene-d(8), k(rac) for 4-picoline has DeltaH(double dagger) = 28.9 +/- 0.4 kJ, DeltaS(double dagger) = -103.6 +/- 0.9 J K-1. A common mechanism applies to ligand substitution (mtp) and racemization (edt). MeReO(dithiolate)Py complexes react with Bpy, Me(2)Bpy, Phen, and Me(2)Phen to form six-coordinate chelates, with rate constants 0.024-0.74 L mol(-1) s(-1) at 25 degreesC, some 10(3) times smaller than with pyridines, no doubt owing to the bulk of the bidentates. Values of DeltaS(double dagger) are -86 to -138 J K-1, reflecting substantial orientational barriers as well as the inherent contribution of the associative mechanism. The product is MeReO(mtp)(Me(2)Bpy)*. The formation of the metastable isomer is consistent with the mechanism assigned to the ligand substitution and racemization reactions. Such compounds, once formed, no longer participate in ligand substitution reactions at reasonable rates. The formation of the metastable isomer is consistent with the mechanism assigned to the ligand substitution and racemization reactions.