A mechanism is proposed for the electrochemical reduction of [Fe(eta (5)-C6H7)(CO)(3)][PF6] based on cyclic voltammetry and simulation techniques. In [NBu4][X]/CH3CN (X = BF4 or ClO4) but not in [NBu4][BF4]/CH2Cl2, a rapid equilibrium prior to the electron transfer process is identified between [Fe(eta (5)-C6H7)(CO)(3)][PF6] and a species formulated as [Fe(eta (3)-C6H7)(CO)(3)(NCMe)](+). The formation of the species under equilibrium involves solvent coordination and eta (5) to eta (3) ring slippage of the cyclohexadienyl ligand as the response of the system to the high electron count. Electrochemical electron transfer to [Fe(eta (3)-C6H7)(CO)(3)(NCMe)](+) affords a highly reactive 19-clectron intermediate exhibiting chemical reactivity (ECE mechanism) that leads to the formation of dimer-type species. A 'father-son' type mechanism is proposed for the formation of the products of the electrochemical reduction of [Fe(eta (5)-C6H7)(Co)(3)][PF6]. All the species involved in the mechanism were analysed by theoretical means and are proposed on the basis of calculations made with the B3LYP HF/DFT hybrid functional. (C) 2001 Elsevier Science B.V. All rights reserved.