In terms of its bonding to transition metals, N2 is related to
CO and
acetylene as all three species have
triple bonds. A variety of bonding modes have been characterized. Based on whether the N2 molecules are shared by two more metal centers, the complexes can be classified into mononuclear and bridging. Based on the geometric relationship between the N2 molecule and the metal center, the complexes can be classified into end-on or side-on modes. In the end-on bonding modes of transition metal-dinitrogen complexes, the N-N vector can be considered in line with the metal ion center, whereas in the side-on modes, the metal-ligand bond is known to be perpendicular to the N-N vector. In these mononuclear cases, N2 is both as a σ-donor and a π-acceptor. The M-N-N bond angles are close to 180°. In another example, the dinitrogen ligand in
Mo(N2)2(Ph2PCH2CH2PPh2)2 can be reduced to produce ammonia. Because many nitrogenases contain Mo, there has been particular interest in Mo-N2 complexes.
Bridging, end-on N2 also serves as a bridging ligand with "end-on" bonding to two metal centers, as illustrated by {[Ru(NH3)5]2(μ-N2)}4+. These complexes are also called multinuclear dinitrogen complexes. In contrast to their mononuclear counterpart, they can be prepared for both early and late transition metals. The average
bond length of those bridging-end-on dinitrogen complexes is about 1.2 Å. In some cases, the bond length can be as long as 1.4 Å, which is similar to those of N-N single bonds. Hasanayn and co-workers have shown that the
Lewis structures of end-on bridging complexes can be assigned based on π-molecular-orbital occupancy, in analogy with simple tetratomic organic molecules. For example the cores of N2-bridged complexes with 8, 10, or 12 π-electrons can generally be formulated, respectively, as M≡N-N≡M, M=N=N=M, and M-N≡N-M, in analogy with the 8-, 10-, and 12-π-electron organic molecules HC≡C-C≡CH, O=C=C=O, and F-C≡C-F.
Mononuclear, side-on In comparison with their end-on counterpart, the mononuclear side-on dinitrogen complexes are usually higher in energy and the examples of them are rare. Dinitrogen act as a π-donor in these types of complexes. Fomitchev and Coppens has reported the first crystallographic evidence for side-on coordination of N2 to a single metal center in a photoinduced metastable state. When treated with UV light, the transition metal-dinitrogen complex, [Os(NH3)5(N2)]2+ in solid states can be converted into a metastable state of [Os(NH3)5(η2-N2)]2+, where the vibration of dinitrogen has shifted from 2025 to 1831 cm−1. Some other examples are considered to exist in the
transition states of intramolecular linkage isomerizations. Armor and Taube has reported these isomerizations using 15N-labelled dinitrogen as ligands.
Bridging, side-on In a second mode of bridging, bimetallic complexes are known wherein the N-N vector is perpendicular to the M-M vector, which can be considered as side-on fashion. One example is [(η5-C5Me4H)2Zr]2(
μ2,
η2,η2-N2). The dimetallic complex can react with H2 to achieve the artificial
nitrogen fixation by reducing N2. A related ditantalum tetrahydride complex could also reduce N2. ==Reactivity==