Ligand exchange of metal carbonyls by chain mechanisms. Electrochemical kinetics of electron transfer catalysis

Abstract

The metal carbonyl derivatives LM(CO) n of manganese, rhenium, molybdenum and tungsten undergo facile ligand substitution by an electrode-mediated process. Phosphine, pyridine and isocyanaide substitutions are shown to be chain reactions by coulometric analysis and by cyclic voltammetry of the metal carbonyl solutions containing the added nucleophiles L . Reversible electrochemical parameters can be obtained for both LM(CO) n and the substitution product L M(CO) n . These allow the digital simulation of the cyclic voltammograms by Feldberg's method, which provides a detailed analysis of the kinetics of the electrocatalytic mechanism for ligand substitution. Generally, the radical cation LM(CO) n + produced initially at the anode undergoes rapid exchange with L to afford the cationic substituted species L M(CO) n +. This step is followed by electron transfer with the reactant LM(CO) n to yield the substitution product L M(CO) n and regenerate the radical cation LM(CO) n +, which completes the chain propagation sequence. Multiple repetition of this cycle is indicated by the high current efficiencies which are obtainable. The second order rate constants for the ligand exchange of the 17-electron radical cations LM(CO) n + with added L are evaluated, and found to be more than 10 6 times larger than that for the neutral, diamagnetic precursor LM(CO) n . The radical cations L M(CO) n + also react readily with phosphine and alkyl isocyanide nucleophiles, leading to a characteristic distortion of the CV waves. Digital simulation of such cyclic voltammograms allows the kinetics of these processes, particularly the redox catalysis in the oxidation of phosphines by LM(CO) n +, to be evaluated quantitatively. The enhanced reactivity of the 17-electron radical cations LM(CO) n + is discussed in relationship to recent reports of the substitution lability in other 17- and 19-electron metal carbonyls.