Activation of dinitrogen by group 6 metal complexes

Fixation of atmospheric nitrogen and N-2 reduction to NH3 under mild conditions form an important and, at the same time, challenging area of modern fundamental science. This review considers studies of the reactivity of dinitrogen complexes of group 6 metals (mainly molybdenum and tungsten) with relatively simple phosphine ligands, which made it possible to identify the main stages of N-2 reduction in the coordination sphere of a transition metal and factors affecting the reaction efficiency. The attention is focused on the studies of the protonation of the coordinated N-2 molecule, which is a key step in the catalytic cycle of nitrogen reduction, proceeding as alternating proton and electron transfer steps.