Hydride Transfer from (η⁵-C₅Me₅)(CO)₂MH (M = Fe, Ru, Os) to Trityl Cation:  Different Products from Different Metals and the Kinetics of Hydride Transfer

Reactions of the metal hydride complexes Cp*(CO) 2 MH (M = Fe, Ru, Os) with Ph 3 C + BF 4 - in CH 2 Cl 2 were studied. Hydride transfer from Cp*(CO) 2 FeH to Ph 3 C + BF 4 - gives Cp*(CO) 2 FeFBF 3 . Hydride transfer from the ruthenium hydride Cp*(CO) 2 RuH to Ph 3 C + BF 4 - produces the bridging hydride complex {[Cp*(CO) 2 Ru] 2 (μ-H)} + BF 4 -, indicating that Cp*(CO) 2 RuH exhibits significant nucleophilicity in addition to hydridic reactivity of the Ru−H bond. The osmium hydride Cp*(CO) 2 OsH reacts with Ph 3 C + BF 4 - in CH 2 Cl 2 to give a mixture of Cp*(CO) 2 OsFBF 3 and [Cp*(CO) 2 Os(ClCH 2 Cl)] + BF 4 - . The kinetics of these hydride transfer reactions were monitored by stopped-flow methods, leading to the second-order rate law −d[Ph 3 C + BF 4 - ]/d t = k [Ph 3 C + BF 4 - ][MH]. The temperature dependence of the rate constants was determined for the iron hydride, the osmium hydride, and the osmium deuteride Cp*(CO) 2 OsD. Activation parameters for hydride transfer from Cp*(CO) 2 FeH are Δ H ⧧ = 2.6 ± 0.1 kcal mol - 1 and Δ S ⧧ = −22.1 ± 0.4 cal K - 1 mol - 1; for Cp*(CO) 2 OsH the activation parameters are Δ H ⧧ = 4.9 ± 0.1 kcal mol - 1 and Δ S ⧧ = −16.8 ± 0.5 cal K - 1 mol - 1 . A kinetic isotope effect ( k H / k D = 1.6 at 0 °C) was found for the reaction of Cp*(CO) 2 OsD. The order of kinetic hydricity is HRu > HFe > HOs. Second-order rate constants (extrapolated to 25 °C from data collected at lower temperatures) are k = 3.2 × 10 5 M - 1 s - 1 for Cp*(CO) 2 OsH and k = 1.1 × 10 6 M - 1 s - 1 for Cp*(CO) 2 FeH; for Cp*(CO) 2 RuH, k > 5 × 10 6 M - 1 s - 1 is estimated at 25 °C. Rate constants were also determined for hydride transfer to Ph 2 ( p -MeOC 6 H 4 )C + at 25 °C: k = 1.5 × 10 5 M - 1 s - 1 for Cp*(CO) 2 RuH, k = 4.1 × 10 4 M - 1 s - 1 for Cp*(CO) 2 FeH, and k = 3.2 × 10 3 M - 1 s - 1 for Cp*(CO) 2 OsH.