The hydricity of the heterobimetallic iridium/ruthenium catalyst [Cp*Ir(H)(-bpm)Ru(bpy)(2)](3+) (1, where Cp* = eta(s)-pentamethylcyclopentadienyl, bpm = 2,2'-bipyrimidine, and bpy = 2,2'-bipyridine) has been determined in both acetonitrile (63.1 kcal mol(1)) and water (29.7 kcal mol(1)). Hydride 1 features a large increase in the hydride donor ability when the solvent is changed from acetonitrile to water. The acidity of 1, in contrast, is essentially solvent-independent because 1 remains strongly acidic in both solvents. On the basis of an X-ray crystallographic study, spectroscopic analysis, and time-dependent density functional theory calculations, the disparate reactivity trends are ascribed to substantial delocalization of the electron density onto both the bpm and bpy ligands in the conjugate base of 1, [Cp*Ir(mu-bpm)Ru(bpy)(2)](2+) (3). The H-2 evolution tendencies of 1 are considered in the context of thermodynamic parameters.