Isotope effects on hydride transfer reactions from transition metal hydrides to trityl cation. An inverse isotope effect for a hydride transfer

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.