Binuclear Robson type Ni(ii) complex as a reactant supplementing our knowledge of the orientation effects in electrochemical kinetics

Abstract

The multistep reduction of a binuclear Ni(II) Robson-type complex with a multidentate template-like organic ligand (formed from 4-tert-butyl-2,6-diformylphenol and 1,3-diaminopropane), Ni₂L, is studied using the electron photoemission technique. The number of transferred electrons corresponding to a single reduction wave is found to be 8 per complex species. This value is attributed to both complete Ni(II) reduction (with Ni metal formation) and ligand reduction. Contributions of Ni(II) and ligand to acceptor orbital were estimated. Three initial subsequent steps correspond to electron transfer to mixed metal–ligand orbital with comparable contributions. For more deep reduction, ligand contribution predominates. The first single-electron step is evidenced to be rate-determining, with the rate constant of 0.03 cm² s⁻¹. The latter value is discussed in the framework of a semiquantitative analysis of the rate constants estimated in the framework of quantum-mechanical electron transfer theory for different orientations of Ni₂L in the reaction layer. The analysis includes estimations of key kinetic parameters (electronic transmission coefficient, solvent- and intramolecular contributions to the total reorganization energy) which strongly rest on the results of quantum chemical modeling. The transmission coefficients at realistic electrode-reactant distances of the closest approach are below 0.001. This means that despite of the noticeable delocalization of Ni₂L acceptor orbital, the electron transfer is diabatic. Predominating contribution to reorganization energy results from solvent and does not exceed 0.5 eV for any reactant orientation. The highest reactivity is predicted for a planar orientation located mostly outside the compact part of electric double layer. The Ni₂L adsorption in planar and vertical orientations on mercury is addressed as well. The results give a clear explanation of the previously observed self-inhibition of “dark” reduction of Ni₂L on mercury and independent data on the adsorption of these species. The discovered combination of various orientation effects is compared with effects observed for other reactants.