A series of [M(diphosphine)(2)]X-2, [HM(diphosphine)(2)]X, and M(diphosphine)(2) complexes have been prepared for the purpose of determining the relative thermodynamic hydricities of the [HM(diphosphine)(2)]X complexes (M = Ni, Pt; X = BF4, PF6; diphosphine = bis(diphenylphosphino)ethane (dppe), bis(diethylphosphino)ethane (depe), bis(dimethylphosphino)ethane (dmpe), bis(dimethylphosphino)propane (dmpp)). Measurements of the half-wave potentials (E-1/2) for the M(II) and M(O) complexes and pK(a) measurements for the metal hydride complexes have been used in a thermochemical cycle to obtain quantitative thermodynamic information on the relative hydride donor abilities of the metal-hydride complexes. The hydride donor strengths vary by 23 kcal/mol and are influenced by the metal, the ligand substituents, and the size of the chelate bite of the diphosphine Ligand. The best hydride donor of the complexes prepared is [HPt(dmpe)(z)](PF6), a third-row transition metal with basic substituents and a diphosphine ligand with a small chelate bite. The best hydride accepters have the opposite characteristics. X-ray diffraction studies were carried out on eight complexes: [Ni(dmpe)(2)](BF4)(2), [Ni(depe)(2)](BF4)(2), [Ni(dmpp)(2)](BF4)(2), [Pt(dmpp)(2)](PF6)(2), [Ni(dmpe)(2)(CH3CN)] (BF4)(2), [Ni(dmpp)(2)(CH3CN)](BF4)(2), Ni(dmpp)(2), and Pt(dmpp)(2). The cations [Ni(dmpp)(2)](2+) and [Pt(dmpp)(2)](2+) exhibit significant tetrahedral distortions from a square-planar geometry arising from the larger chelate bite of dmpp compared to that of dmpe. This tetrahedral distortion produces a decrease in the energy of the lowest unoccupied molecular orbital of the [M(dmpp)(2)](2+) complexes, stabilizes the +1 oxidation state, and makes the [HM(dmpp)(2)](+) complexes poorer hydride donors than their dmpe analogues. Another interesting structural feature is the shortening of the M-P bond upon reduction from M(II) to M(O).