Electronic Structure of CpCo(η⁴-C₅H₆): A Catalytic, Organometallic Hydride Donor

In biological and electrochemical processes, electron and proton carriers direct energy transport and storage in chemical bonds. The metallocene CpCo(η⁴-C₅H₆) (CpCo(CpH)) is an organometallic hydride donor that selectively transfers hydrides in a manner analogous to biological hydride transfer agents like NAD(P)H. Hydride transfer from CpCo(CpH) generates cobaltocenium ([Cp₂Co]⁺), which in the presence of mild acids can be electrochemically recycled back to CpCo(CpH) without generating hydrogen. We use Co K-edge X-ray absorption and emission spectroscopy and density functional theory to characterize the electronic structure of CpCo(CpH) and illuminate the origins of these unusual chemical properties. We show that the CpH ligand is a strong π acceptor, resulting in a species with high electron density on H that is poised for hydride delivery. Moreover, H/D isotope exchange experiments and reaction coordinate calculations show that hydride transfer occurs from the CpH site, rather than through the Co center, resulting from unfavorable orbital overlap of a Co-bound hydride intermediate. These electronic properties distinguish CpCo(CpH) from other metal hydride species and give rise to unique electrochemical properties. Using these insights, we compare the behaviors of several metallocene hydride species, to explain how variations in electronic structures lead to different chemical and electrochemical properties.