Size-dependent reactivity of rhodium deuteride cluster anions RhD (n = 0-3) toward dinitrogen: The prominent role of σ donation.

Nitrogen (N) fixation is a challenging task for chemists. Adsorption of N on transition metal (TM) sites has been identified as a prerequisite for activating the very stable N≡N triple bond in both industrial and biological processes. The importance of π back-donation (filled orbitals of TM → π* orbitals of N) between metal sites and N has been well elucidated while the role of another classic orbital interaction, namely σ donation (σ orbitals of N → empty orbitals of TM), remains ambiguous. Herein, the size-dependent reactivity of trinuclear rhodium deuteride cluster anions RhD (n = 0-3) toward N adsorption in the gas phase was investigated experimentally and theoretically. A reverse relationship that higher electron-donating ability of clusters corresponds to lower N adsorption reactivity was experimentally observed, which is uncommon in N activation by gas-phase species. Theoretical analysis revealed that the σ donation rather than the π back-donation plays a predominant role in the adsorption complexes RhDN and the enhanced reactivity upon D addition is ascribed to the lowered energy levels of active orbitals in RhD as n increases. This study provides the first experimental evidence to declare the important role of σ donation and new clues for the design of reactive metal species in nitrogen fixation.