Ruthenium and Osmium

Molybdenum and Tungsten Catalysts for Hydrogenation, Hydrosilylation and Hydrolysis

“…Replacement of an electron -withdrawing CO ligand by an electron -donating phosphine ligand diminishes the acidity of the metal hydride considerably, by about 7 p K a units in (CO) 4 CoH versus (CO) 3 (PPh 3 )CoH, and by about 10 p K a units for the more electron -donating PMe 3 ligand in Cp(CO) 3 WH versus Cp(CO) 2 (PMe 3 )WH. The fi rst -row metal hydride (CO) 5 MnH is more acidic than the third row congener (CO) 5 ReH.…”

“…Second row metal hydrides are faster hydride donors than their third row congeners, as shown by HMo > HW and HRu > HOs. As was found for the thermodynamic hydricity, changes of ligands can have a large infl uence on kinetic hydricity, with the rate constant for hydride transfer from trans -HMo(CO) 2 (PMe 3 )Cp exceeding that for HMo(CO) 3 Cp by a factor of about 10 4 . Electronic effects dominate over steric effects, as indicated by the faster rates of hydride transfer for HMo(CO) 3 Cp * compared to HMo(CO) 3 Cp.…”

“…As was found for the thermodynamic hydricity, changes of ligands can have a large infl uence on kinetic hydricity, with the rate constant for hydride transfer from trans -HMo(CO) 2 (PMe 3 )Cp exceeding that for HMo(CO) 3 Cp by a factor of about 10 4 . Electronic effects dominate over steric effects, as indicated by the faster rates of hydride transfer for HMo(CO) 3 Cp * compared to HMo(CO) 3 Cp. The Cp * ligand is much larger sterically than Cp, yet the higher electron -donating ability of this ligand compared to Cp makes its hydride complexes more hydridic.…”

“…The Cp * ligand is much larger sterically than Cp, yet the higher electron -donating ability of this ligand compared to Cp makes its hydride complexes more hydridic. Even one methyl group on a Cp ligand can have a measurable effect, with the hydricity of HW(CO) 3 (C 5 H 4 Me) being about three times that of HW(CO) 3 Cp. The silicon hydride HSiEt 3 , which is used as a hydride donor in the stoichiometric ionic hydrogenations described below, was shown to be among the slower hydride donors compared to transition metal hydrides.…”

“…Noyori has called catalytic hydrogenation a " Core Technology in Synthesis " [2] . The fi eld of homogeneous hydrogenation has become large and diverse, but is dominated by the use of precious metal catalysts, particularly ruthenium [3] and rhodium [4] . Detailed studies of the kinetics have revealed many variants on the mechanism, but Equation 3.1 shows in generalized form a key step in the traditional mechanism, involving insertion of a bound ketone into an M -H bond.…”