Studies of Structural Effects on the Half-Wave Potentials of Mononuclear and Dinuclear Nickel(II) Diphosphine/Dithiolate Complexes

Two series of mononuclear Ni(II) complexes of the formula (PNP)Ni(dithiolate) where PNP = R2PCH2N(CH3)CH2PR2, R = Et and Ph, have been synthesized containing dithiolate ligands that vary from five- to seven-membered
chelate rings. Two series of dinuclear Ni(II) complexes of the formula {[(diphosphine)Ni]2(dithiolate)}(X)2 (X = BF4
or PF6) have been synthesized in which the chelate ring size of the dithiolate and diphosphine ligands have been
systematically varied. The structures of the alkylated mononuclear complex, [(PNPEt)Ni(SC2H4SMe)]OTf, and the
dinuclear complex, [(dppeNi)2(SC3H6S)](BF4)2, have been determined by X-ray diffraction studies. The complexes
have been studied by cyclic voltammetry to determine how the half-wave potentials of the Ni(II/I) couples vary with
chelate ring size of the ligands. For the mononuclear complexes, this potential becomes more positive as the
natural bite angle of the dithiolate ligand increases. However, the potentials of the Ni(II/I) couples of the dinuclear
complexes do not show a dependence on the chelate ring size of the ligands. Other aspects of the reduction
chemistry of these complexes have been explored.