In a continued effort to gain understanding of the impact of the second coordination sphere on the catalytic activity of the FeFe-H2ase models, we report the results of an electrochemical and computational study of the reactivity of [Fe2(μ-adtH)(CO)6] {1, adtH = (SCH2)2NH} towards protons in the presence of coordinating anions. The voltammetric behavior of 1 in the presence of HCl (pKa ≈ 9.5) in Bu4NPF6/MeCN is very similar to that of adtR derivatives in the presence of HBF4·Et2O (pKa ≈ 3), suggesting an effect of coordinating anion (i.e. Cl– or BF4–) rather than an effect of acid strength on the catalytic pathway. This hypothesis is supported by DFT calculations, which indicate a weak but significant interaction between Cl– and the proton of the amine group the closest to the metal center in the N-protonated and reduced form of 1.