Cobaltocenes, radical addition

Complexes of the type CpCo(PR3)2 are alkylated at the metal with small alkyl halides to give CpCo(PR3)2R (Scheme 25). Bulky halides produce ring-substituted hydrido cations instead, explained by attack of the electrophile from the exo site followed by ring-to-metal proton transfer. This reaction could be electrophilic addition (see Electrophilic Reaction), 5ei, or more probably radical addition initiated by electron transfer similar to the RX reaction of cobaltocene (Section 7.1). Since the oxidation potential of CpCo(P(alkyl)3)2 is more negative than that of cobaltocene, this latter mechanism is very plausible. 

Otherwise, reaction of 132 with a hydride source such as NaBH4 afforded the neutral allenylethynyl complex 140 by regioselective addition at the Cy atom (Fig. 23) [372]. In contrast, reduction with one equivalent of cobaltocene, and subsequent trapping of the resulting radical with Ph3SnH, yielded the isomeric 5,5-diphenylpenta-l,3-diynyl complex 141, suggesting that the unpaired electron of the pentatetraenylidene radical is predominantly localized on the Ce atom [372]. 

The chemistry of cobaltocene is dominated by its tendency to act as an electron-rich radical that can undergo one-electron oxidation, ring addition,... 

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