Chemical Engineering Tips – Molecular Nitrogen Complexes

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Transition metal complexes that contain at least one nitrogen molecule in their ligands are known as molecular nitrogen complexes, or alternatively, dinitrogen (N2) complexes. The first molecular nitrogen complex, [Ru(NH3)5N2]X2, was prepared in 1965 by A.D. Allen and others in aqueous solution by reducing ruthenium trichloride with hydrazine hydrate. Soon after, a method for the direct synthesis of molecular nitrogen complexes from nitrogen was developed. For example, replacing a ligand with weak binding by a nitrogen molecule yields molecular nitrogen complexes; such reactions can sometimes take place under mild conditions. Furthermore, in the presence of a strong reducing agent along with a tertiary phosphine or bis-tertiary phosphine, certain transition metal compounds can be treated with nitrogen to produce molecular nitrogen complexes. Almost all transition metal elements in groups IVB–VIII can form molecular nitrogen complexes. Elements that can form stable molecular nitrogen complexes include titanium (Ti), molybdenum (Mo), tungsten (W), manganese (Mn), rhenium (Re), iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru), rhodium (Rh), osmium (Os), iridium (Ir), and platinum (Pt), among others. These elements are mostly post-transition elements; they all occupy low oxidation states in molecular nitrogen complexes and contain a large number of d electrons. N2 and CO are isoelectronic, and their structures are also similar; therefore, the bonding in molecular nitrogen complexes is similar to that in carbonyl complexes. The lone pair of electrons on the nitrogen atom moves into the empty orbitals of the transition metal ion (atom), forming a σ-covalent bond. Meanwhile, the non-bonding d electrons of the transition metal ion (atom) move into the antibonding π* empty orbitals of the nitrogen molecule, forming a backdonor π bond, thereby creating a σ-π double bond structure. However, the nitrogen molecule is a poor σ-electron donor, with a much weaker ability to donate electron pairs to form σ-covalent bonds compared to CO. Furthermore, nitrogen molecules have a weaker ability to accept the d electrons of metal ions (atoms) to form feedback π bonds than CO, so molecular nitrogen complexes are less stable than metal carbonyl complexes. In addition to forming complexes in an end-group manner, nitrogen molecules can also donate bonding π electrons to form side-group complexes. Furthermore, nitrogen molecules can also form dinuclear complexes with metal ions. The nitrogen molecules in the molecular nitrogen complexes are activated to a certain extent. The activation of nitrogen molecules is a prerequisite for their further reduction to ammonia, opening the way for direct nitrogen fixation from the air. Research on molecular nitrogen complexes has advanced the study of chemical simulation of biological nitrogen fixation, such as the search for new catalysts for ammonia synthesis to enable its production under mild conditions, thus holding significant practical importance. Moreover, it is also possible to study the catalytic mechanisms of biological nitrogen fixation and the synthesis of ammonia from nitrogen and hydrogen, thus holding significant theoretical importance as well. Thus, it has attracted widespread attention from chemists and biologists, becoming a very active area of research in modern inorganic chemistry.