One of the most significant new frontiers in chemistry during the past decade has been the stabilization and study of π-organic derivatives of the transition metals. Over 60 elements are now known to form one or more cyclopentadienyl compounds. The aromatic-type reactions displayed by ferrocene and related complexes have also been extensively investigated. Ferrocene will undergo alkylation, acylation, sulphonation, metalation, arylation, formylation, aminomethylation, and other reactions characteristic of a highly reactive aromatic system. π-Cyclopentadienyl derivatives of vanadium, chromium, manganese, ruthenium, and osmium as well as certain π-arene complexes of chromium exhibit ring substitution reactions in varying degrees. Other metallocenes undergo ring addition reactions.The orientation effects of substituents on reactions of substituted ferrocenes have been studied. A variety of unique oxidation, reduction, dehydration, and rearrangement reactions are known in metallocene chemistry. Haloferrocenes and mercuriferrocenes are readily converted to biferrocenyl. Recent rate investigations have established that carbonium ions adjacent to metallocene nuclei possess unusual stabilization as a result of direct metal participation.
