Rate of Reaction of (P(C₆H₁₁)₃)₂W(CO)₃with py, P(OMe)₃, PPh₂Me, and 2,6-Me₂py. Stopped-Flow Kinetic Study of Concerted Reactions at a Sterically Crowded Metal Center

The rate of reaction of the complexes (P(C₆H₁₁)₃)₂W(CO)₃ and (P(C₆D₁₁)₃)₂W(CO)₃ with P(OMe)₃ have been measured by stopped-flow kinetics. At 25°C the hydrogen-substituted complex follows second-order kinetics with k = 5.45 × 10⁴ M^-1 s^-1 and ΔH^≠ = 4.2 kcal/mol. An inverse kinetic isotope effect is observed with k(H)/k(D) = 0.87. The ligand PPh₂Me reacts 3 times more slowly than P(OMe)₃ and shows a larger inverse kinetic isotope effect, k(H)/k(D) = 0.78. Reaction of (P(C₆H₁₁)₃)₂W(CO)₃ with 2,5-dimethylpyridine occurs with the rate constant 8.6 × 10³ M^-1 s^-1 at 257deg;C, 2 orders of magnitude slower than the reaction with pyridine. The unusually high selectivity is ascribed to steric factors at the crowded metal center. New stopped-flow data at low pyridine concentrations are reported for dissociation of pyridine from the complexes (P(C₆H₁₁)₃)₂W(CO)₃(py) and (P-(C₆D₁₁)₃)₂W(CO)₃(py). A primary kinetic isotope effect k₁(H)/k₁(D) = 1.19 is observed in these reactions. These results imply that, in spite of the steric crowding at the metal center, dissociation of pyridine is assisted by partial formation of the W⋯H "agostic" bond in the transition state.