Kinetics, Thermodynamics, and Mechanism of the Radical Chain Process for Ligand Substitution of Metal Carbonyls

Article abstract of DOI:10.1021/ja00339a012

The radical chain process for the ligand substitution of a variety of carbonylmanganese derivatives (MnL) can be induced chemically or electrochemically with turnover numbers that can exceed 10³.The catalytic cycle is initiated by electron transfer to afford the 17-electron carbonylmanganese cation MnL⁺.The propagation steps in the chain process are (1) the facile ligand exchange of MnL⁺ with the added nucleophile L to afford MnL⁺ followed by (2) the reduction of MnL⁺ by a homogeneous process involving cross electron exchange with MnL and/or heterogeneous electron transfer at the electrode.This carbonylmetal system is sufficiently well-behaved to allow the kinetics and thermodynamics for each step of the catalytic cycle to be examined in quantitative detail by transient and bulk electrochemical techniques.Analysis of the reversible cyclic voltammograms of MnL, both in the presence and in the absence of the nucleophile L, is achieved by Feldberg's digital simulation method.The computer simulation of the experimental cyclic voltammograms provides accurate values of the second-order rate constants for the rapid ligand exchange of MnL+ with a variety of added nucleophiles L.The unusual reactivity patterns for substitution in the paramagnetic MnL are presented in the context of previous studies with other metal carbonyls.