CRYOELECTROCHEMICAL STUDY OF THE CLEAVAGE OF RADICAL-ANIONS OF DIIRON AND DIRUTHENIUM CARBONYL DIMERS

Electrochemical reduction of the diiron complexes [Fe(CO)2(eta-5-Cp)]2 and [Fe(CO)2(eta-5-Cp*)]2 proceeds at room temperature by two electrons, but at low temperature is a one-electron reaction producing dimer radical anions that are stable on the voltammetric time scale. Analysis of the reaction kinetics at intervening temperatures shows that the two-electron reaction occurs through the radical anion, which dissociates to a mononuclear anion and a radical. The second electron is delivered homogeneously to the radical by a second dimer radical anion. The cleavage rate constant for [Fe(CO)2(eta-5-Cp)]2.- is 10(2) times that for [Fe(CO)2(eta-5-Cp*)]2.- (at 0-degrees-C), and the rate for the related dimer [Ru(CO)2(eta-5-Cp)]2.- was too fast to measure and at least 10(4) times larger than that for [Fe(CO)2(eta-5-Cp*)]2.-. The rates of the dimer radical anion cleavage fall in the same order as the rates of isomer interconversion in the parent dimers and raise the possibility that the rate of cleavage may be controlled by an intramolecular cleavage-precursor step.