The accelerating effect of anions for electrode reactions has been known for a long time, but it is much less appreciated that these effects can sometimes be caused by traces of anions. We have demonstrated that the Cu++/Cu+ reaction is strongly catalyzed by trace amounts of chloride ions in the solution. The Cu+/Cu reaction was found to be unaffected by chloride ions. These experimental results were also substantiated by theoretical calculations. We have investigated the electronic coupling for homogeneous electron-transfer reactions that are approximate models for electron transfer in the copper deposition : (i) outer-sphere reaction (water-water bridge), and (ii) inner-sphere reaction (chloride bridge). For Cu++/Cu+ electron transfer we found increased coupling for the chloride bridge, which we attribute to the closer approach found for this complex compared to the water bridge, while for Cu+/Cu electron transfer, coupling was not increased for the chloride bridge reaction.