A theoretical analysis of two aspects of the mechanism of reductive elimination is presented—how the choice of central metal and peripheral ligands affects the activation energy for reductive elimination from a four-coordinate MR2(PR3)2 complex and how ligand asymmetry controls cis-trans rearrangements and elimination pathways proceeding through three-coordinate intermediates. The following conclusions emerge: (1) In the four-coordinate complex, the better the σ-donating capability of the leaving groups, the more facile the elimination; (2) Stronger donor ligands trans to the leaving groups will increase the barrier to elimination; (3) The reductive elimination barrier in four-coordinate complexes is controlled by the energy of an antisymmetric b2 orbital, which in turn depends on the energy of the metal levels. The activation energy for such direct reductive elimination should be, and is, substantially lower for Ni than for Pt or Pd; (4) T-shaped trans PdLR2, arising from dissociation of L in PdL2R2, will encounter a substantial barrier to polytopal rearrangement to cis PdLR2, which in turn has an open channel for reductive elimination of R2; (5) If the leaving groups are poor donors, cis-trans isomerization in the three-coordinate manifold should be easier than elimination.

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A series of dialkyl(dipyridy1)nickel complexes R,Ni(dipy) (1, R = CH3, CzHs, n-CSH5, or i-CIHO) was prepared. The thermal stability of 1 decreases in the order CH3 > C2H5 > C3H7 > C4H9. Thermal decomposition of 1 is first order with respect to 1, and the activation energy for pyrolysis of (CzH&Ni(dipy) is 68 kcal/mol. Reactions of 1 with olefins (tetracyanoethylene, maleic anhydride, acrylamide, acrylonitrile, methacrylonitrile, acrolein, methyl vinyl ketone, methyl acrylate, methyl methacrylate, vinyl chloride, vinyl acetate, 2-vinylpyridine, styrene, isobutyl vinyl ether, ethylene, norbornene, and cyclooctadiene) at room temperature led in most cases to cleavage of the R-Ni bond and to formation of olefin-coordinated complexes of the type Ni(dipy)(olefin), (3, n = 1 or 2). The R-Ni bonds are activated through coordination of the olefins to 1; with acrylonitrile and acrolein the complexes R,Ni(dipy)(olefin) (2) were isolated as unstable intermediates. From infrared and electronic spectroscopy, the coordinated olefins in 2 and 3 appear to be T bonded. Based on a kinetic study of the reactions of 1 with olefins at room temperature a mechanism involving the formation of 2 as an intermediate is proposed. Olefins with electronegative substituents form stronger T complexes with 1 and activate the Ni-R bonds more strongly than those with less electronegative substituents. The observation of charge-transfer bands in the complexes allows estimation of the energy levels of the highest occupied d orbitals and a study of the effect of solvent and olefin coordination to the metal. A mechanism for the activation of Ni-R bonds by interaction of 1 with olefins is proposed.he coordination of olefins with alkyl-transition (8) A. Yamamoto, T. Shimizu, and S . Ikeda, P0lj.m. J . , 1, 171 (1970); Makromol. Chem., 136, 297 (1970).(9) Previously we reported that (CnH&Ni(dipy) also initiates the polymerization of acrylonitrile,e but we confirmed later that the initiation requires the presence of a trace of oxygen, whereas this is not the case for ethyliron and -cobalt complexes.

Transition metal compounds including NiCl2(bpy) (bpy=2,2′-bipyridine), NiBr2(PPh3)2 (PPh3=triphenylphosphine), PdCl2(bpy), NiCl2, CoCl2, FeCl2, and FeCl3 catalyze polycondensation of di- and polyhalogenated organic aromatic compounds by dehalogenation with magnesium under mild conditions. Poly(p-phenylene), poly(m-phenylene), poly(oxybiphenylene), and poly(phenylenemethylene) were prepared from p-dihalobenzene, m-dichlorobenzene, bis(p-bromophenyl) ether, and α,p-dichlorotoluene. They have a high degree of polymerization and high thermal stability. Poly(p-phenylene), poly(oxybiphenylene), and poly(phenylenemethylene) have regularly repeated structures as proved by IR spectroscopy and X-ray powder diffraction. Polymerization of polyhalogenated aromatic compounds (1,3,5-trichlorobenzene and hexachlorobiphenyl) in tetrahydrofuran (THF) gave copolymers having aromatic nuclei and THF units. Polymerization of the haloaromatic compounds is considered to proceed through a mechanism in which consecutive cycles of oxidative addition of haloaromatic compounds (R–X) to the transition metal catalyst take place so as to cause alkylation (arylation) by the Grignard reagent formed in the system (R′MgX) and elimination of R–R′ from an active transition metal compound having R and R′ groups.

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The kinetics of the CO formation by electrochemical reduction of carbon dioxide on a gold electrode investigated in a series of potassium phosphate buffer solutions. CO is the principal product under the above condition. Small amounts of hydrocarbons, such as CH4, C2H4, and C2H6, are detected in the solutions of pH 5.2, 6.2, and 6.8. The Tafel slopes of the partial current density for the CO formation, ICO, at different pH are all ca. −120 mV/decade. The reaction mechanism has been proposed on the basis of the relationships between ICO and the electrode potential and between ICO and the reaction order with respect to CO2, which is ca. 1.