The structures and reactivity of various catalytic model systems of nitrogenase are reviewed. Firstly, the Schrock cycle is discussed as an example of a nitrogen-fixing system for which the mechanism has been elucidated both experimentally and theoretically. Then a series of mononuclear iron complexes based on tetradentate ligands of the type EP 3 (E = Si, C, B) is discussed. These systems both serve as highly active catalysts for the synthesis of ammonia from N 2 and provide important insights into the role of the ligand situated in the position trans to coordinated N 2 . In addition, new cobalt, iron, ruthenium, and osmium complexes that display catalytic conversion of N 2 into [a] 1337 Felix Tuczek studied chemistry at Johannes-Gutenberg University Mainz and received his PhD in 1989 in the group of Philipp Gütlich for work in Mössbauer emission spectroscopy. After two years as a postdoc at Stanford University in the group of Prof. Ed Solomon he returned to Mainz to work on his habilitation (1997) on copper peroxo and azido complexes with relevance to hemocyanin. In 1999 he received a call to Christian Albrechts University Kiel and since then has been professor of molecular inorganic chemistry in Kiel. His scientific interests include nitrogen fixation, type 3 copper proteins, and corresponding small-molecule model systems, as well as molecular switches. Nadja Stucke studied chemistry at Christian Albrechts University of Kiel and received her master's degree in 2014. Since 2014 she has been a PhD student in the working group of Prof. Dr. F. Tuczek. Her scientific interests are focused on synthetic nitrogen fixation. Benedikt Flöser studied chemistry at Christian Albrechts University of Kiel, received his master's degree in 2014, and proceeded to work as a PhD student in the group of Prof. Dr. F. Tuczek. His research is concerned with quantum chemistry in small molecule activation (N 2 fixation, oxygen activation), and spin crossover systems. Thomas Weyrich studied chemistry at Christian Albrechts University of Kiel, where he obtained his master's degree in 2013 after completion of his thesis dealing with synthetic nitrogen fixation under the supervision of Prof. Dr. Felix Tuczek. He continued his research in this field for his PhD thesis by synthesizing new phosphine ligands for molybdenum-based dinitrogen fixation.

Activating small molecules with transition metal complexes adsorbed on metal surfaces is a novel approach combining aspects of homogeneous and heterogeneous catalysis. In order to study the influence of an Au(111) substrate on the activation of the small-molecule ligand carbon monoxide, a molybdenum tricarbonyl complex containing a PN P pincer ligand was synthesized and investigated in the bulk, in solution, and adsorbed on an Au(111) surface. By means of a platform approach, a perpendicular orientation of the molybdenum complex was achieved and confirmed by IRRAS and NEXAFS. By using vibrational spectroscopy (IR, Raman, IRRAS) coupled to DFT calculations, the influence of the metal substrate on the activation of the CO ligands bound to the molybdenum complex was determined. The electron-withdrawing behavior of gold causes an overall shift of the CO stretching vibrations to higher frequencies, which is partly compensated by dynamic charge transfer from the substrate to the molybdenum center, which increases its (dynamic) polarizability.

Molybdenum(0) dinitrogen complexes, supported by the mixed NHC/phosphine pincer ligand PCP, exhibit an extreme activation of the N2 ligand due to a very π-electron-rich metal center. The low thermal stability of these compounds can be increased using phosphites instead of phosphines as coligands. Through an amalgam reduction of [MoCl3(PCP)] in the presence of trimethyl phosphite and N2 the highly activated and room-temperature stable dinitrogen complex [Mo(N2)(PCP)(P(OMe)3)2] is obtained. As a second product, the first transition metal complex containing the meta-phosphite ligand P(O)(OMe) originates from this reaction.

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Three PN 3 P pincer ligands with a central pyridine ring, amine groups in the ligand backbone and different substituents at the terminal phosphine donors are synthesized and coordinated to molybdenum(III) precursors. Conversion of the resulting Mo III compounds to dinitrogen complexes is investigated and compared to the literature-known complex trans-[Mo(N 2 ) 2 (PMe 2 Ph)(PNP tBu )] supported by a classic PNP pincer ligand. The difference between PN 3 P ligands terminated by di- [a]