Biological and industrial nitrogen fixation are the processes by which one of the most inert molecules, N 2 , is transformed into a bioavailable nitrogen source (e.g. NH 3 ) that can be incorporated into all nitrogen-containing biomolecules [1]. As such, nitrogen fixation is essential to sustaining life on this planet, and has attracted intense scrutiny among biological and chemical communities for decades [2][3][4]. The mechanism by which nitrogenase enzymes promote the biological reduction of nitrogen under ambient conditions remains an unsolved and fascinating problem [4-6]. Nature's solution to fixing nitrogen stands in sharp contrast to that which humanity has adopted, i.e. the Haber-Bosch process for ammonia synthesis that is carried out at approximately 200 atm and 500 8C. This Herculean feat of chemical engineering provides, in net, about 50% of the nitrogen atoms that wind up in all human beings on this planet [7]. From a chemist's perspective, however, this remains a brute force and high-energy solution to the nitrogen fixation problem, especially given the knowledge that nature can fix N 2 under ambient conditions.
Chemists began to become more keenly aware of the apparent shortcomings of Haber-Bosch chemistry around 1965, when Allen and Senoff reported their landmark discovery that N 2 could coordinate as a ligand to a transition metal as they established for the complex ([(NH 3 ) 5 Ru(N 2 )] 2+ ) [8,9]. It was already known that metal-rich enzymes in nature could catalyze nitrogen fixation under ambient conditions and therefore it could be reasonably postulated that synthetic metal catalysts might also be able to catalyze nitrogen reduction under suitable conditions. After all, the reduction of plenty of related substrates (e.g. acetylene and ethylene) could already be mediated by synthetic metal catalysts. An exciting surge of research activity followed the Allen and Senoff discovery, and within just a decade profound progress had been made towards the ultimate goal of viable synthetic catalysts for nitrogen fixation under ambient conditions. Literally dozens of dinitrogen complexes were prepared using various transition 81