| Reported herein are the first thermodynamic hydridicities for a collection of metal hydrides of molybdenum, tungsten, niobium, tantalum, and the polyhydrides of tungsten, rhenium, osmium and iridium. The studies were carried out in acetonitrile solution and the compounds show hydride transfer free energies ranging from 74 to over 100 kcal/mol, as determined by bracketing and equilibrium studies involving a known series of hydride acceptors. The data indicate that the free energies of hydride transfer are heavily influenced by co-ligand sets, 4d vs. 5d effects, the intermediate and final product geometries, and the overall charge on the metal hydride species. The osmium compounds OsH 4P3 provide an opportunity to study two sequential hydride transfers and to gather accurate data on the energetic differences associated with hydride transfer from neutral and cationic compounds of the same metal. |
