Molecular Nitrogen as a Ligand

The discovery and development of the chemistry of dinitrogen complexes are reviewed with particular reference to factors affecting their stabilities, and the implication of those factors in terms of electronic structure and energies is discussed, FIRST I must thank the organizers of this congress for their invitation to give a plenary lecture, and also I must explain my title. As you know, one is usually invited to give a plenary lecture some considerable time before the event, and to provide a title. But the subject changes continually and so I chose the somewhat vague title, 'Some reactions of nitrogen ligands', in the hope that I might be able to talk about the complex compounds containing molecular nitrogen as a ligand, but leaving the way open to talk about nitridocomplexes, or some other aspect of the complex chemistry of nitrogen should the complex chemistry of molecular nitrogen, ie. dinitrogen, not come up to my expectations. However, I am pleased to say that development has been rapid and this lecture concerns only dinitrogen as a ligand. I have adjusted its title accordingly. I shall summariz. briefly the pertinent properties of dinitrogen and of its complexes and discuss the factors which I believe are important in determining the bonding of dinitrogen in a transition metal complex. The discovery of coordination compounds containing the nitrogen molecule as a ligand is one of the most exciting recent developments in the chemistry of nitrogen. Not that such compounds were unexpected; they had been so long in anticipation that most chemists had begun to belive that they could not exist. In 1930 Bortells1 showed that traces of molybdenum were essential for the growth of nitrogen-fixing bacteria in the absence of any source of nitrogen other than dinitrogen, but in the presence of urea or ammonia the bacteria appeared to grow happily in the absence of molybdenum. Thus the molybdenum appeared to be directly involved with the nitrogen molecule in the process of fixation. This knowledge led to attempts to produce dinitrogen complexes by compressing the gas over molybdenum compounds but with no success. Similarly, a development of interest in olefin and acetylene complexes during the 1950s led to attempts to prepare dinitrogen analogues of the complex compounds of acetylene, but again of no avail. Most of these attempts went unpublished. Indeed, all attempts to produce dinitrogen complexes were so negative that chemists became convinced that they would not exist as stable entities,