The first quaternary ammonium-group-containing [FeFe]-hydrogenase models [(μ-PDT)Fe (CO) {κ -(Ph P) N(CH ) NMe BzBr}] (2; PDT=propanedithiolate) and [(μ-PDT)Fe (CO) {μ-(Ph P) N(CH ) NMe BzBr}] (4) have been prepared by the quaternization of their precursors [(μ-PDT)Fe (CO) {κ -(Ph P) N(CH ) NMe }] (1) and [(μ-PDT)Fe (CO) {μ-(Ph P) N(CH ) NMe }] (3) with benzyl bromide in high yields. Although new complexes 1-4 have been fully characterized by spectroscopic and X-ray crystallographic studies, the chelated complexes 1 and 2 converted into their bridged isomers 3 and 4 at higher temperatures, thus demonstrating that these bridged isomers are thermodynamically favorable. An electrochemical study on hydrophilic models 2 and 4 in MeCN and MeCN/H O as solvents indicates that the reduction potentials are shifted to less-negative potentials as the water content increases. This outcome implies that both 2 and 4 are more easily reduced in the mixed MeCN/H O solvent than in MeCN. In addition, hydrophilic models 2 and 4 act as electrocatalysts and achieve higher i /i values and turnover numbers (TONs) in MeCN/H O as a solvent than in MeCN for the production of hydrogen from the weak acid HOAc.