Compounds containing the dinitrogen molecule, N2, bound to a metal (also called dinitrogen complexes). Outstanding in their ability to form coordination compounds with nitrogen are a number of metals which belong to the group 7 transition metal family. For each metal of this group, several nitrogen complexes have been identified. Nitrogen complexes of these metals occur in low oxidation states, such as Co(I) or Ni(O). The other ligands present in these complexes besides N2 are usually of a type known to stabilize low oxidation states; phosphines appear to be particularly prominent bonding partners in this respect. The illustration shows the structure of a typical N2 complex, elucidated by a crystal structure determination. The NN bond axis in this complex is aimed, within the limits of experimental error, directly toward the position of the metal atom. The CoN2 bond length, 0.18 nanometer, is within the normal range of comparable metal-ligand bonds. See also Coordination chemistry.
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Structure of a coordination compound with N2 (circles represent phenyl groups).
Even in most favorable cases the binding of the dinitrogen molecule to the metal is fairly labile; all the compounds lose their nitrogen on mild heating. Some of the nitrogen complexes are only metastable to loss of dinitrogen even at room temperature; accordingly, they cannot be obtained by direct uptake of gaseous nitrogen. In the synthesis of these metastable complexes, hydrazine or azide compounds serve as a source of nitrogen molecules within the coordination sphere of the metal. Addition of other coordinating agents to the nitrogen complexes usually results in a displacement of N2 from the metal. The cobalt compound in the illustration exchanges its N2 ligand quite reversibly for other ligand molecules, such as NH3 and H2C&dbnd;CH2. Whereas these ligands are easily displaced again by an excess of N2, an irreversible exchange occurs with carbon monoxide. The bulky organic groups on the phosphine ligands are likely to interfere with the approach to the metal of all but the slimmest ligands and thereby help the “thin” dinitrogen molecule to maintain or regain its position on the metal in competition with most other ligands.
