The effect of 19-electron formation constants on the electrochemistry and electron transfer induced substitution reactions of cyclopentadienylmetal halide complexes

The reduction of cyclopentadienylmetal halide complexes is generally considered to involve addition of an electron to an orbital that is antibonding with respect to the metal-halide bond. Subsequent metal-halide bond cleavage yields the halide and an organometallic radical. At inert electrodes, this radical is reduced further to an 18-electron anion. This series of reactions constitutes a prototypical ECE mechanism. Chemical reduction can be used to divert the radical into other pathways such as electron transfer chain catalyzed substitution. Attempts to initiate such reductively induced substitution reactions of CpFe(CO)₂I and Cp′Mo(CO)₃I give very different results, suggesting that these very similar complexes are reduced via substantially different mechanisms. Very likely, the molybdenum complex reacts via a DISP mechanism instead of ECE. The difference in electrochemical reduction mechanism as well as the different reactivity toward reductively induced substitution are explained in terms of a difference in the formation constants of 19-electron intermediates.