Add time:08/03/2019         Source:sciencedirect.com

All of the data obtained by electrochemical methods involving the mercury electrode, spectrophotometry and conductometry have showed the stepwise formations of a neutral σ-donor-acceptor pentacoordinated NiX2-L3 and a cationic tetracoordinated NiL42+, associated with a shift in the dissociation-coordination equilibrium owing to an excessive concentration of the ligand in solutions of the binary systems containing NiII compounds coordinated with acid ligands (X = Cl−, No3−, acac) and the trivalent phosphorus ligands (L = Pbu3, PPh3, P(OEt)3, P(O-i-Pr)3, P(O-s-Bu)3, P(OC6H11), P(OPh3). The complexes have for the first time been prepared for all the phosphines and phosphites studied. Ni(acac)2 was found to enhance coordination by the phosphines and phosphites in acetonitrile. The value of the semiwave potential shift (ΔE12, V) towards less negative potentials depends on the number of ligands present in the inner coordination sphere of metal, and was found to be 200–300 and 800–1300 mV for NiX2L2 and NiL42+, respectively. The electrochemical reduction of NiL42+ complexes into the zero-valent nickel isostructural complexes is reversible with values similar to that of the nickel formal potential Ni2+/Ni0 (E12 = E0 = −0.25 V). For the first time, the homogeneous formations of catalytically active complexes have been studied electrochemically in situ with the tricomponent Ziegler-Natta catalytic systems to give stable NiII complexes in toluene solutions of Ni(acac)2-{PR3 or P(OR)3}-OAC (Ni/P/Al = 1/4/4/) at T ⩽ −10°C. These complexes can undergo reversible reductions and determine catalytic activity of the system. They are regarded as ionic and are tentatively assigned the structure of bimetallic complexes [RNiL3]+[AIR4]−. The hydride complexes of divalent nickel HNiL4+ were found to be as catalytically active as the Ni(acac)2-P(OR)3{R = Et, i-Pr}-AlEt3 (Ni/P/Al = 1/4/4) systems formed in toluene-isopropanol solvent at T = −10 to +25°C. The activating affect by the organic phosphorus ligand on the electronic state of Ni has been found for the first time to decrease the energy levels of the boundary orbitals as the coordination sphere becomes saturated with the ligand, thus changing the ligand's environment and configuration of the complex, the degree of alkylation and the degree of stabilization of the catalytically active complexes. In the case of the NiL42+ complexes, the ligand-to-metal charge transfer energies permitted the estimation of the affinity to the nickel electron inherent to the nickel LUMO energy level in the given complex, that energy being similar to HOMO energy in Ni0L4 under the conditions of the reversible electronic process.

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