Water soluble ferrocenes [Fe(Cp)(CpL)], where Cp− is the η5-cyclopentadienide ligand and the side chain L is (a) the carboxylic acid group –(CH2)xCO2H with x=0–4 (I–V); (b) the complex x=2 with the β-methylene mono-methyl substituted (VI); (c) the amine hydrochloride derivative with L=CH(Me) NH3+ (VII); and (d) the complex with two Cp rings bridged by the amine hydrochloride –CH(NH3+)CH2CH2− (VIII); have been prepared, and are used as one-equivalent reductants for the active-R2 subunit of Escherichia coli ribonucleotide reductase. Formal reduction potentials E1°′ (25°C) of the carboxylates of acids I–VI in 20 mM NaOH, and of the amine hydrochlorides VII and VIII in water were determined by cyclic voltammetry, and are in the range 0.308–0.550 V versus nhe, I=0.100 M (NaCl). Second-order rate constants k12 (25°C) for the reduction of active-R2 were determined by UV–Vis spectrophotometry, and are in the range 0.15–0.50 M−1 s−1 at I=0.100 M. A free-energy plot of logk12 versus E°′ values gives no clearcut unidirectional trend. Since from present information the electron self-exchange rate constant for the [Fe(Cp)2]+/[Fe(Cp)2] couple is favourable (>7×106 M−1 s−1 in methanol at 25°C), it would appear that electron-transfer from the ferrocenes via Trp-48, Asp-237, His-118 to the FeIII2 site on R2 is much slower than expected, and smaller than with the organic radical reductants previously studied. Electron-transfer from some other position on the protein surface to the Tyr is considered as an alternative.