The free energies for hydride donation (Delta G(M)(+)) have been determined in acetonitrile solution for a series of seven molybdenum and tungsten compounds (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 known hydride ion affinities in acetonitrile. The metal hydride Delta G(M)(double 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-transfer processes are somewhat more sensitive to changes in co-ligands than are hydride transfer processes. Additionally, kinetic studies of hydride transfer reactions with hydride acceptor [(p-MeOPh)(2)CPh][BF4] exhibit second-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 yields a slope of 0.20. The thermodynamic data may be used in conjuction with the appropriate thermodynamic cycles to calculate energies for various processes in which compounds such as 1-7 are known to function as hydride donors.