Relative Hydride, Proton, and Hydrogen Atom Transfer Abilities of HM(diphosphine)2PF6Complexes (M = Pt, Ni)

A series of M(diphosphine)2X2, HM(diphosphine)2X, and M(diphosphine)2 complexes have been prepared for the purpose of determining the relative thermodynamic hydricities of the HM(diphosphine)2X 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 (E1/2) for the M(II) and M(0) complexes and pKa 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)22+ and Pt(dmpp)22+ 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)22+ 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).