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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry