Theoretical, spectroscopic, and mechanistic studies on transition-metal dinitrogen complexes: implications to reactivity and relevance to the nitrogenase problem.

Dinitrogen complexes of transition metals exhibit different binding geometries of N2 (end-on terminal, end-on bridging, side-on bridging, side-on end-on bridging), which are investigated by spectroscopy and DFT calculations, analyzing their electronic structure and reactivity. For comparison, a bis(mu-nitrido) complex, where the N--N bond has been split, has been studied as well. Most of these systems are highly covalent, and have strong metal-nitrogen bonds. In the present review, particular emphasis is put on a consideration of the activation of the coordinated dinitrogen ligand, making it susceptible to protonation, reactions with electrophiles or cleavage. In this context, theoretical, structural, and spectroscopic data giving informations on the amount of charge on the N2 unit are presented. The orbital interactions leading to a charge transfer from the metals to the dinitrogen ligand and the charge distribution within the coordinated N2 group are analyzed. Correlations between the binding mode and the observed reactivity of N2 are discussed.