4. Conclusions
By re-examining prudently the electrocatalytic substitution reactions of complexes 1 and 2 by the ligand PPh3, it was found that the two-electron transfer catalysis (TETC) mechanism early reported [25] could not be valid in these cases. Based on recent developments in electrochemical investigations for diiron hexacarbonyl complexes and our own electrocatalytic investigations including digital simulations, alternative mechanisms were proposed. For both complexes, the electrocatalytic species is the monoanion and the electrocatalytic proceeds involving one-electron transfer rather two in spite of the differences in electrochemistry caused by the bridging linkage. The validity of these mechanisms was confirmed by cyclic voltammetric investigations and digital simulations. Our investigations also demonstrated that electrocatalytic substitution could be an more efficient and, relatively, environmentally more benign than chemical substitution in synthesis of diiron pentacarbonyl complexes which are one large category of the most investigated iron carbonyl complexes due to their relevance to the sub-unit of the [FeFe]-hydrogenase