INTRINSIC BARRIERS TO ATOM TRANSFER - SELF-EXCHANGE REACTIONS OF CPM(CO)3X/CPM(CO)3- HALIDE COUPLES

The self-exchange reactions were followed by H-1 NMR spectroscopy: For the X = 1 complexes, standard line width measurements yield (M = Mo) k(298) = 1.5 x 10(4) M-1 s-1 (DELTA-H = 6.4 (+/- 0.4) kcal mol-1, DELTA-S = -18 (+/- 1.5) cal K-1 mol-1) and (M = W) k(298) = 4.5 x 10(3) M-1 s-1 (DELTA-H = 7.5 (+/- 0.1) kcal mol-1, DELTA-S = -16.8 (+/- 0.5) cal K-1 mol-1). For the X = Br complexes, magnetization-transfer experiments yield (M = Mo) k(298) = 1.6 x 10(1) M-1 s-1 (DELTA-H = 12.1 (+/- 4.5) kcal mol-1, DELTA-S = -12 (+/- 15) cal K-1 mol-1) and (M = W) k(298) = 1.5 M-1 s-1 (DELTA-H = 15.1 (+/- 5.2) kcal mol-1, DELTA-S = -7 (+/- 16) cal K-1 mol-1); H-1 NMR longitudinal relaxation times T1 for the Cp groups of the reactants are typically 40 s. The X = Cl systems were studied by conventional techniques, with the rates of "transfer" of Cp-d5 from (Cp-d5)W(CO)3- to CpW(CO)3Cl being monitored; for M = Mo, k(298) = 9.0 x 10(-2) M-1 s-1 (DELTA-H = 18.9 (+/- 1.0) kcal mol-1, DELTA-S = 0 (+/- 4) cal K-1 mol-1) and for M = W, k(298) = 2.1 x 10(-3) M-1 s-1 (DELTA-H = 17.7 (+/- 3.3) kcal mol-1, DELTA-S = -11 (+/- 11) cal K-1 mol-1). For X = CH3, the CpW(CO)3-/CpW(CO)3CH3 self-exchange rate constant, also determined by monitoring rates of "transfer" of Cp-d5 from (Cpd5)W(CO)3- to CpW(CO)3X, is almost-equal-to 1 x 10(-5) M-1 s-1 at 335 K. The latter self-exchange reaction is discussed in terms of the intrinsic barrier for oxidative addition to the anion. For the X = halogen exchanges, the role of the M(I) ("metal radical") state is considered, and it is concluded that the latter isovalent state is not an intermediate for these systems. However, the isovalent state may serve to stabilize the transition state for two-electron transfer between the metal centers through configuration interaction. Our results, taken with those for other X = halogen systems, indicate that effective transfer of X+ may be intrinsically rapid when both reactants are 18-electron species and steric factors are favorable.