Metallocene anions: From electrochemical curiosities to isolable complexes

Since the first reports and structural characterisation of ferrocene in the 1950s, metallocenes [M(Cp)2] (M = metal, Cp = cyclopentadienyl, C5H5) have become a workhorse of organometallic chemistry. The vast majority of metallocenes and derivatized metallocenes [M(CpR)2], where the CpR ring bears alternative substituents or incorporates heteroatoms in the ring itself, contain metal centres in a formal +II oxidation state, or +III for metallocenium cations, [M(CpR)2]+. Here we present an overview of metallocene anions and their derivatives, [M(CpR)2]−, where metal centres formally exhibit +I oxidation states. We focus on d-block examples, from their origins as unstable intermediates in low temperature electrochemical studies to more recent examples of isolated complexes.