When t/'iZZM-Cr(C204)2(H20)2-is added to acidic solutions that contain H2C2O2 and HC2O4-, rapid isomerization of the trans complex occurs. This step is followed by the slower anation of the resulting cis complex by oxalate species. Anation proceeds to an equilibrium mixture of species described by the equilibrium quotient Ki = [Cr(C2CU)33-] [H+]2/ [cis-Cr(C204)2(H20)2-][H2C204]. At 50°C and µ = 1.00 M, maintained with potassium nitrate, Ks = 0.890 ± 0.012 M. The kinetics of the two-step reaction at µ = 1.00 M, between 35 and 55°C, are reported. Both steps of the reaction obey simple first-order kinetics. Observed pseudo-first-order rate constants for the trans-cis isomerization in sodium perchlorate and potassium nitrate were fit to the form /cobsd = kn2o + £m[M+] + &h[H+], where M is sodium or potassium. Activation enthalpies for the various constants are 15.1 ± 0.5 (fcNa), 14.7 ± 0.8 (kt0, and 18.5 ± 0.3 (ku) kcal/mol. Activation entropies are -23.9 ± 1.6 (kNa), -26.0 ± 2.6 (/ck), and -8.5 ± 0.8 (kn) cal/(mol deg). Data interpretations are based on AH* = 17.9 kcal/mol and AS* = -14.7 ca!/(mole deg) for Zch20. Hydrogen ion and ligand dependencies of the observed pseudo-first-order anation rate constants are medium-dependent and slower rates are observed in perchlorate media. In perchlorate media, Zcobsd = /cb[HC204"] + Zc4[H+], while in nitrate media, Acobsd = {/ci[H2C2O4] + (/:2/:4//c3)[H+]2|/|l + (k2//£3)[H+]|. At 50°C in potassium nitrate, k\ = 2.84 X 10-2 AH sec-1, ki/ki = 73.8 ± 11.3 M-1, and ¿4 = 4.38 X 10-4 AH sec-1. Activation enthalpies are 20.7 ± 0.4 and 21.9 ± 0.4 kcal/mol, and activation entropies are -1.9 ± 1.0 and -6.5 ± 1.4 cal/(mol deg), for k] and /c4, respectively. A steady-state mechanism, which describes k 1 and ki as anation steps and ki and ¿4 as aquation steps, is proposed for the anation reaction in both media. This mechanism identifies kb as a composite constant equal to ktks/kiKt, where K\ is the first dissociation constant of oxalic acid. The anation rate constant, k\, is limited by kn for the trans-cis isomerization. Other mechanistic similarities between the isomerization process and aquation and anation processes are described, and a new conformation for the bidentate oxalate ligand is proposed as a precursor to each process.