A series of [M(diphosphine)2]X2, [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(0) 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)2](PF6), a third-row transition metal with basic substituents and a diphosphine ligand with a small chelate bite. The best hydride acceptors 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(0).

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A linear free-energy relationship exists between the half-wave potentials of the (II/I) couples of Ni(diphosphine)2 complexes and the hydride donor ability (ΔG°H − ) of the corresponding [HNi(diphosphine)2]+ complexes. A similar correlation is observed between the half-wave potentials of the (I/0) couples of Ni(diphosphine)2 complexes and the pK a values (or ΔG°H + ) of the corresponding [HNi(diphosphine)2]+ complexes. As a result, it is possible to use the potentials of these two couples to predict the free energies of all three Ni−H bond cleavage reactions, ΔG°H + , ΔG°H • , and ΔG°H − , for this class of nickel complexes. The molecular structures of Ni(Et2PCH2CH2PPh2)2 and Ni(Ph2PCHCHPPh2)2 were determined by single-crystal X-ray diffraction studies.