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

Alberto A. Gonzalez, Kai Zhang, Carl D. Hoff

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Abstract

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.

Original languageEnglish (US)
Pages (from-to)4285-4290
Number of pages6
JournalInorganic Chemistry
Volume28
Issue number23
DOIs
StatePublished - Nov 1 1989

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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