ELECTRON SELF-EXCHANGE OF RE2X4(PME2PH)(4)(0 +) (X = CL, BR) BY H-1-NMR LINE BROADENING IN METHYLENE-CHLORIDE/

The kinetics of the electron self-exchange reaction of the redox coupl e Re2X4(PMe2Ph)(4)/[Re2X4(PMe2Ph)PF6 (X = Cl, Br) has been measured as a function of temperature and reactant concentration in methylene chl oride by the H-1 NMR line-broadening method. The self-exchange rate co nstants were obtained from broadening of the methyl singlet on the pho sphine ligand in the fast-exchange limit. Extrapolated to 298 K from l ower temperatures, they are 2.3 x 10(8) M-1 s(-1) for X = Cl and 4.2 x 10(8) M-1 s(-1) for X = Br. The corresponding activation parameters a re Delta H-double dagger = 9.3 +/- 0.6 kcal/mol and Delta S-double dag ger = 10.9 +/- 2.2 cal/(mol K) for X = Cl and Delta H-double dagger = 8.1 +/- 0.7 kcal/mol and Delta S-double dagger = 8.1 +/- 2.9 cal/(mol K) for X = Br. As expected, on the basis of previous cross-reaction st udies, the reactivity of the dirhenium complexes is high, The previous ly calculated apparent self-exchange rate constant from cross-reaction studies for Re2Br4(PMe2Ph)(4) (0/+) was 2.5 x 10(8) M-1 s(-1) (298 K) . The inner- and outer-sphere reorganization energies for Re2Cl4(PMe2P h)(4) are estimated to be 1.3 and 4.4 kcal/mol, respectively, indicati ng a large contribution to the rate constant from solvation. These res ults are consistent with studies of the heterogeneous electron transfe r of these complexes and are compared to studies of other metal comple xes in methylene chloride.