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-F BF3 and Ph3CH. Deuterium kinetic isotope effects were determined for severa l MH/MD pairs (CH2Cl2 solution, 25 degrees C). For hydride transfer from Cp *(CO)(3)MoH (Cp* = eta(5)-(CMe5)-Me-5) to substituted trityl cations contai ning 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(M oD) = 1.7-1.9 as the rate constant decreases from k(H)- = 6.5 x 10(3) to 1. 4 M-1 s(-1). For hydride transfer to Ph3C+BF4- from five metal hydrides [Cp (CO)(3)MoH, Cp*(CO)(3)WH, (indenyl)(CO)(3)WH, Cp"(CO)(3)MoH, and trans-Cp(C O)(2)(PCy3)MoH; Cp = eta(5)-C5H5] With second-order rate constants k(H)- gr eater than or equal to 3.8 x 10(2) M-1 s(-1), the kinetic isotope effects a re 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(W D) = 1.8 to 0.47 as the rate constant decreases (from k(H)- = 2.0 x 10(3) t o 0.72 M-1 s(-1)). The steadily decreasing values of k(MH)/k(MD), with decr easing rate constants of hydride transfer are interpreted as indicating pro gressively stronger force constants of isotopically sensitive modes of the transition state, as the reaction slows down in progressing from more elect ron-donating Cp ligands to less electron-rich Cp ligands. The inverse isoto pe effect (k(WH)/k(WD) == 0.47) found for the slowest tungsten hydride, (C5 H4CO2Me)(CO)(3)WH, is proposed to be due to a product-like transition state for irreversible hydride transfer.