For adjusting the charging state of a molecular metal complex in the context of a density functional theory description of coherent electron transport through single molecule junctions, we correct for self interaction effects by fixing the charge on a counterion, which in our calculations mimics the effect of the gate in an electrochemical STM setup, with two competing methods, namely the generalized $Δ$ SCF technique and screening with solvation shells. One would expect a transmission peak to be pinned at the Fermi energy for a nominal charge of +1 on the molecule in the junction but we find a more complex situation in this multicomponent system defined by the complex, the leads, the counterion and the solvent. In particular equilibrium charge transfer between the molecule and the leads plays an importanty role, which we investigate in dependence on the total external charge in the context of electronegativity theory.