Intrinsic barriers to atom transfer: Self-exchange reactions of CpM(CO)3X/CpM(CO)3 - halide couples

Rate constants and activation parameters were measured for the self-exchange of CpM(CO)₃ ^- with CpM(CO)₃X in CD₃CN solvent: CpM(CO)₃ ^- + CpM(CO)₃X ⇌CpM(CO)₃X + CpM(CO)₃ ^- The self-exchange reactions were followed by ¹H NMR spectroscopy: For the X = I complexes, standard line width measurements yield (M = Mo) k(298) = 1.5 × 10⁴ M^-1 s^-1 (ΔH^‡ = 6.4 (±0.4) kcal mol^-1, ΔS^‡ = -18 (±1.5) cal K^-1 mo^-1) and (M = W) k(298) = 4.5 × 10³ M^-1 s^-1 (ΔH^‡ = 7.5 (±0.1) kcal mol^-1, ΔS^‡ = -16.8 (±0.5) cal K^-1 mol^-1). For the X = Br complexes, magnetization-transfer experiments yield (M = Mo) k(298) = 1.6 × 10¹ M^-1 S^-1 (ΔH^‡ = 12.1 (±4.5) kcal mol^-1, ΔS^‡ = -12 (±15) cal K^-1 mol^-1) and (M = W) k(298) = 1.5 M^-1 S^-1 (ΔH^‡ = 15.1 (±5.2) kcal mol^-1, ΔS^‡ = -7 (±16) cal K^-1 mol^-1); ¹H 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 × 10^-2 M^-1 s^-1 (ΔH‡ = 18.9 (±1.0) kcal mol^-1, ΔS^‡ = 0 (±4) cal K^-1 mol^-1) and for M = W, k(298) = 2.1 × 10^-3 M^-1 s^-1 (ΔH^‡ = 17.7 (±3.3) kcal mol^-1, ΔS^‡ = -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 (Cp-d₅)W(CO)₃ ^- to CpW(CO)₃X, is ≈ 1 × 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.