Substitution Catalyzed by Electron Transfer

A general scheme for ligand substitution initiated by electron transfer. 

Overall (MeCp)Mn(CO)2(MeCN) -i PPha. (MeCp)Mn(CO)2(PPh3) -K CH3CN 

Ligand Substitutions by Radical Chains Initiated by Atom Abstractions 

Ligand substitutions of 18-electron complexes can also occur by radical-chain processes initiated by atom abstraction. These radical chains occur through 17-electron intermediates that imdei o facile associative substitutions. Substitutions of metal carbonyl hydrides, halides, and stannyl complexes by this mechanism are all known. These reactions are particularly prevalent in first-row metal hydrides because the M-H bond is weaker than the M-H bond in second- and third-row metal complexes, and hydrogen atom abstraction is one step of the radical chain. However, they have also been proposed to occiu with third-row metal-hydride complexes 

Several observations implied that this substitution process occurred by a chain mechanism (1) no reaction occurred over 60 days at 25 °C when light was excluded and ultrapure reagents were used (2) the rates were not reproducible (3) the reactions occurred with an induction period and unusual product versus time curves (4) the reactions were accelerated by Re(CO)j, which was generated from photolysis of Re2(CO)jp and (5) the reaction was accelerated by small amounts of O, but was inhibited by large amounts of Oj. 

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