Nitrides stepping up: Terminal nitrido complexes in nitrogen fixation and nitrogen atom transfer reactions

The fundamental reactivity of 4/5d transition metal complexes with terminal nitrido ligands is reviewed, based on selected, representative examples from our laboratory. Systematic trends in metal–nitride bonding along the transition series are discussed as a basis for harnessing molecular nitrides as intermediates in challenging chemical transformations, specifically nitrogen fixation and C–H functionalization. For early to mid-transition metal nitrido complexes with strong MN triple bonds, strategies towards chemically, electrochemically, and photochemically driven reductive N2 splitting are evaluated as an entry to nitrogen fixation schemes. Subsequent nitride transfer reactions are presented that directly connect N2 activation and splitting with the synthesis of nitrogenous products beyond ammonia, such as nitriles, isocyanates, and N2O. In case of late transition metals with increasing nitride centered radical character, nitride centered activation of strong bonds vs decay via nitride homocoupling is discussed as a basis for efficient nitrogen atom transfer catalysis.