Spectroelectrochemistry of nickel complexes. Voltammetric and ESR studies of the redox reactions of phosphine-dithiolate and phosphine-catecholate complexes of nickel

Article abstract of DOI:10.1021/ic00136a056

The redox properties of nickel(II) complexes of the type [Ni(PPh₃)₂L]ⁿ⁺ (L = dithiolate (n = 0) or dithiocarbamate (n = 1)) and Ni(dpe)L (dpe = bis(diphenylphosphino)ethane, L = dithiolate or catecholate) have been studied by cyclic voltammetry at a platinum electrode, and the products of the redox reactions have been identified by electron spin resonance spectroscopy. All of these complexes show reversible or quasi-reversible one-electron reduction processes, and the reduction potentials for the PPh₃ complexes are about 0.5 V higher than those of the corresponding dpe complexes. In the case of triphenylphosphine complexes such as Ni(PPh₃)₂((CN)₂C₂S₂), the voltammetry shows evidence of a dissociation equilibrium involving loss of triphenylphosphine from the nickel species present after the electron-transfer process. The frozen-solution ESR spectra of the reduction products show large, anisotropic hyperfine coupling to two equivalent ³¹P nuclei and anisotropic g values characteristic of d⁹ nickel(I) species. The PPh₃ complexes have smaller ³¹P hyperfine coupling constants than the corresponding dpe complexes. The ³¹P hyperfine coupling parameters have been analyzed for some representative complexes, and the amount of spin density transferred from the metal to the phosphine ligands has been estimated. In addition to the reduction process, the catecholate complexes undergo a reversible one-electron oxidation. The ESR spectra of the products of such oxidations show only a small ³¹P hyperfine coupling, hyperfine coupling to nuclei in the catecholate ligand, and almost isotropic g values. These species are therefore formulated as nickel(II) complexes containing coordinated semiquinone radical anions.