The thermodynamic hydride donor abilities of [HW(CO)(5)](-) (40 kcal/mol), [HW(CO)(4)P(OMe3)](-) (37 kcal/mol), and [HW(CO)(4)(PPh3)](-) (36 kcal/mol) have been measured in acetonitrile by either equilibrium or calorimetric methods. The hydride donor abilities of these complexes-are compared with other complexes for which similar thermodynamic measurements have been made. [HW(CO)(5)](-), [HW(CO)(4)P(OMe3)](-), and [HW(CO)(4)(PPh3)](-) all react rapidly with [Cp*Re(PMe3)(NO)(CO)](+) to form dinuclear intermediates with bridging formyl ligands. These intermediates slowly form [Cp*Re(PMe3)(NO)(CHO)] and [W(CO)(4)(L)(CH3-CN)]. The structure of cis-[HW(CO)(4)(PPh3)](-) has been determined and has the expected octahedral structure. The hydride ligand bends away from the CO ligand trans to PPh3 and toward PPh3.