Within the background of biological nitrogen fixation mediated by the enzyme nitrogenase, transition-metal complexes capable of binding and activating dinitrogen towards protonation and reduction have been synthesized. Of particular interest in this research area has been the transition-metal mediated conversion of dinitrogen to ammonia (stoichiometric and catalytic) and the elucidation of the corresponding mechanistic pathways. Metal dinitrogen complexes can be used for molecular dinitrogen fixation. Most commonly used were Molybdenum, Tungsten and Iron as they form part of nitrogenase enzymes. But late transition metal complexes were also studied for their ability to catalytic reduction of molecular dinitrogen. The different metal complexes and their possible mechanisms from different research papers is discussed in this review.