Hydride transfer from transition metal hydrides (MH) to Ph3C+BF4- gives M-FBF3 and Ph3CH. Deuterium kinetic isotope effects were determined for several MH/MD pairs (CH2Cl2 solution, 25°C). For hydride transfer from Cp(*)(CO)3MoH (Cp(*) = η5-C5Me5) to substituted trityl cations containing zero, one, two, or three p-MeO groups [Ph(n)(p-MeOC6H4)3- (n)C+BF4-; n = 3, 2, 1, 0], the isotope effect remains essentially constant at k(MoH)/k(MoD) = 1.7-1.9 as the rate constant decreases from k(H) = 6.5 x 103 to 1.4 M-1 s-1. For hydride transfer to Ph3C+BF4- from five metal hydrides [Cp(CO)3MoH, Cp(*)(CO)3WH, (indenyl)(CO)3WH, Cp(*)(CO)3MoH, and trans-Cp(CO)2(PCy3)MoH; Cp = η5-C5H5] with second- order rate constants k(H)- ≥ 3.8 x 102 M-1 s-1, the kinetic isotope effects are also k(MH)/k(MD) = 1.7-1.8. For a series of five tungsten hydrides with substituted Cp ligands, the kinetic isotope effects decrease from k(WH)k(WD) = 1.8 to 0.47 as the rate constant decreases (from k(H)- = 2.0 x 103 to 0.72 M-1 s-1). The steadily decreasing values of k(MH)/k(MD) with decreasing rate constants of hydride transfer are interpreted as indicating progressively stronger force constants of isotopically sensitive modes of the transition state, as the reaction slows clown in progressing from more electron-donating Cp ligands to less electron- rich Cp ligands. The inverse isotope effect (k(WH)/k(WD) = 0.47) found for the slowest tungsten hydride, (C5H4CO2Me)(CO)3WH, is proposed to be due to a product-like transition state for irreversible hydride transfer.
ASJC Scopus subject areas
- Colloid and Surface Chemistry