Figure 13.

Visualization of hp and re mechanisms for H₂ release upon N₂ (blue) binding to E₄. The following is shown: the Fe-2,3,6,7 face of resting FeMo-co; the structure of FeMo-co must distort in different stages of catalysis. The Fe that binds N₂ is presumed to be Fe6, as indicated by studies of α-70^Val variants; when bold, red, Fe6 is formally reduced by two equivalents (see text). The bridging hydrides of E₄ (green) are positioned to share an Fe “vertex”, as suggested by re mechanism of H₂ release upon N₂ binding. Alternative binding modes for N₂-derived species can be envisaged.