Thermodynamic and kinetic studies of hydride transfer for a series of molybdenum and tungsten hydrides

The free energies for hydride donation (Delta G(M)(+)) have been determined in acetonitrile solution for a series of seven molybdenum and tungsten com pounds (1-7) of general formula (C5R5)M(CO)(2)(L)H, which yield the salts [ (C5R5)M(CO)(2)(L)(NCMe)][BF4] in these reactions. These data constitute the first thermodynamic data for hydride transfer by transition metal hydrides , and were gathered from equilibrium studies with carbenium ion salts of kn own hydride ion affinities in acetonitrile. The metal hydride Delta G(M)(do uble dagger) values range from ca. 79 to 89 kcal/mol, and these values may be compared with pK(a)s for related compounds to demonstrate that proton-tr ansfer processes are somewhat more sensitive to changes in co-ligands than are hydride transfer processes. Additionally, kinetic studies of hydride tr ansfer reactions with hydride acceptor [(p-MeOPh)(2)CPh][BF4] exhibit secon d-order rate constants ranging from ca. 200 to 7500 M-1 s(-1). These rates Show a correlation with thermodynamic driving force, and a Bronsted plot yi elds a slope of 0.20. The thermodynamic data may be used in conjuction with the appropriate thermodynamic cycles to calculate energies for various pro cesses in which compounds such as 1-7 are known to function as hydride dono rs.