Reduction of silver complexes: towards an ever more elaborated insight in the influence of the type of ligand on the reaction rate

The present work deals with the investigation of the electrochemical reduction of silver thiosulphate (1,2-Ag(S2O3)23−), thiocyanate (1,3-Ag(SCN)32−) and 1,8-dihydroxy-3,6-dithiaoctane (1,2-Ag(DTO)2+) complexes. The influence of the ligand type on the charge transfer rate is explained by the changing positions of the density distributions of electronic energy levels of the three complexes. The basics for this approach are the theories of energy band models (EBMs). An experimental methodology is developed to determine the energy density distributions. A Ti/TiO2 substrate, obtained by galvanostatically anodising Ti, is put forward as an appropriate substrate for this investigation, and its semiconducting properties are determined. On this substrate, charge transfer (CT) controlled currents can be measured in a sufficiently large potential domain for the three systems. A method of pre-plating is optimised such that the overall semiconducting character of the substrate is kept during the monitoring of the (quasi-)stationary current/voltage diagrams. The active surface areas, necessary for the calculation of the current density/potential curves, are calculated. The positions of the energy density distributions, obtained by the derivation of the current density/voltage diagrams, of the three complexes, show that thiosulphate exhibits the smallest density of accepting energy levels in the given potential domain. For potentials above 0.5 V vs. SCE, the DTO complex has the largest density of vacant energy levels, but for lower potentials the situation is reversed.