α-metallocenylmethylium ions and isoelectronic fulvene complexes of d6 to d9 metals.: Structural considerations

This review collects the structural features of those metallocene derivatives in which a carbenium ion center is connected adjacent to a cyclopentadienyl ring eta-bound to a transition metal. In the first part we briefly summarize the investigations and observations that led to the structure of the (alpha-ferrocenylmethylium ion. This part is followed by examples of metallocenylmethylium ions and metallocenyldimethylium dications of the iron triad, the isoelectronic ferrocenylboranes, fulvenechromium tricarbonyl complexes, alpha-cymantrenylcarbenium ions, and [cyclopentadienyl-cyclobutadienecobalt] methylium ions. In all cases we notice a bending of the alpha-carbenium ion center toward the metal. The bending angle is largest for fulvenechromium tricarbonyl and the ruthenocenyl- and osmocenylmethylium ions.