and Rh(C204)~~-react with Ce(IV) in aqueous sulfuric acid with the stepwise oxidation of coordinated oxalate, the initial reactions proceeding to produce bis-oxalato complexes. For the Cr(C20&3--Ce(IV) system evaluation of rate constants for the initial direct redox reaction requires consideration of the competing aquation of Cr(Cz0&3-. In 1 M sulfuric acid a t 25" the second-order rate constant for the direct one-electron oxidation of Cr(Ct04)33-is 4.0 (f0.5) X 10-2 M-1 sec-1, while for the corresponding direct one-electron oxidation of Rh(C20&3-the rate constant is 6.1 (&l.O) X 1 0 -4 M-1 sec-1. Rate behavior for several aqueous-acidic sulfate solvents, differing in concentrations of hydrogen ion and sulfate ion, suggests that activated complexes for the initial direct redox reactions of Cr(C~04)~3-and R h ( C~0 4 )~~-are of generally similar but not identical compositions. The measured AH* values for both systems exhibit slight temperature dependences which are probably caused by small changes with temperature in the average compositions of activated complexes. A t corresponding temperatures the AH* value for the rhodium system is significantly higher than that for the chromium system. The initial direct redox step for each complex is interpreted as involving oxidation of a coordinated oxalate to an oxalate radical anion. Factors which are likely to contribute to differences in the redox reactivities of Rh (C204)33-and Cr(C20&3are discussed. , J . A m . Chem. SOC., 90, 4280 (1968), and earlier papers; (k) J. E. Hix, Jr., and M. M. Jones, ibid., 90, 1723 (I968), and earlier papers; (1) J. P. Candlin, K. A. Taylor, and D. T. Thompson, "Reactions of Transition Metal Complexes," Elsevier Publishing Co., Amsterdam, 1968, Chapter 3.(4) In certain cases it may not be clear whether a reaction of a complex with an external redox reagent is best described as a reaction of a ligand or ligands, a reaction of the central metal, or a reaction of the complex as a whole. In the cases of present interest the ligand has, at least by the end of the reaction sequence, undergone an obvious chemical change.(5)Kinetic data for substitution reactions of trans-Pt(P(CzH5)8)z(ms)C1 (ms = mesityl) with various nucleophiles in methanol and dimethyl sulfoxide are reported. The results are discussed in terms of a balance between steric factors and electronic structure in determining the reaction mechanism. Depending on the solvent and nucleophile, the kinetic behavior is similar either to that found for "pseudooctahedral" complexes or to the associative mechanism typical of square-planar substitution reactions.
