Dinitrogen coordination modes

Fig. 2 Dinitrogen coordination modes in transition metai compounds.

Since the first step of all of these reactions is dinitrogen coordination to either the surface of the catalyst or transition metal center of the complex, let us briefly discuss the nature and importance of the M-N2 interaction, and the possible coordination modes ofN2 to transition metal centers. These issues were the subjects of many discussions in the literature [10, 11] and it is commonly agreed that the interaction of the N2 molecule with transition metal centers facilitates the activation of the N=N triple bond the stronger the M-N2 interaction, the easier to break the N=N triple bond. 

Figure I. Possible coordination modes of the dinitrogen molecule in the mono- and dinuclear complexes...

Amongst the model compounds able to coordinate dinitrogen, some can coordinate one dinitrogen and some two dinitrogen molecules. The following coordination modes have been identified for such complexes. 

Examination of the MBs of the n1 (C) coordination mode XII indicates weakening of the C-N2 bond [15, 16], the r 1 (C)-coordinated diazoalkane must thus be kinetically unstable and readily eliminates dinitrogen molecule to give the corresponding carbene complex. Moreover, it is known that the carbon-nitrogen bond of diazoalkane is readily cleaved upon interaction with a Lewis acid such as BF3, etc. The Lewis acidity at the metal coordination centre probably causes similar a-acid-base... 

Several different coordination modes have been found in dinitrogen metal compounds (Fig. 2) and they are strongly influenced by the identity of the metal atom(s), their oxidation state(s), and the ligand environment. This results in different degrees of reduction of the N=N bond, which is often also referred to as activation . For monomeric complexes, the end-on coordination mode A is most frequently encountered. However, in very rare cases side-on coordination B has also been observed, eg, a metastable Tj -bound Nz hgand can be trapped and structurally characterized upon photochemical activation of [Os(NH3)5(r -N2)][PF6]2. The large overlap between the osmium d-orbitals with the 2p orbitals of the bound Nz confers stability to this species. 

Six coordination modes for N2, as depicted in Fig. 23, have been proposed so far. A dinitrogen molecule retains a pair of unshared... 

Dinitrogen might coordinate to metals in at least 4 ways, but only the end-on modes, structures (1) and (2), are well established as common bonding modes by numerous well-defined examples ... 

---