Kinetics and Mechanism of Reduction of Nickel(II) Ethylenediamine Complexes at a Dropping Mercury Electrode

Effect of concentration of ethylenediamine molecules and supporting electrolytes (NaF, NaClO4) on the kinetics of electroreduction of nickel(II) ethylenediamine complexes at dropping and stationary mercury electrodes is studied. The limiting current on the dropping electrode is found to have diffusion–kinetic nature at free ethylenediamine molecule concentrations of ∼0.5 mM to 0.05 M. The slow electrochemical stage is presumably preceded by a slow chemical stage and a reversible chemical stage. In the former, one chelate cycle in the source complex Ni(en)2+3 opens; and in the latter, a monodentate-coordinated ethylenediamine molecule is abstracted. The conclusion is drawn about an inner-sphere mechanism of the electrochemical stage which involves Ni(en)2+2 complexes specifically adsorbed on mercury.