Dinitrogen complex



A dinitrogen complex is a coordination compound that contains the dinitrogen ligand, N2. In the area of coordination chemistry, the atomic and diatomic forms are distinguished as nitrogen and dinitrogen occur as ligands.

Metal complexes of N2 have been studied since the 1965's when the first complex, [Ru(NH3)5(N2)]2+, was reported by Allen and Senoff at the University of Toronto. Interest in such complexes arises because N2 comprises the majority of the atmosphere and because many useful compounds contain nitrogen atoms. It is widely accepted that biological nitrogen fixation occurs via the binding of N2 to a metal center in the enzyme nitrogenase, followed by a series of steps that involve electron transfer and protonation.

The hydrogenation of N2 is only weakly exothermic, hence the industrial hydrogenation of nitrogen via the Haber-Bosch Process employs high pressures and high temperatures.

Bonding modes
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.

End-on
As a ligand, N2 usually binds to metals as an "end-on" ligand, as illustrated by Allen and Senoff's complex. Such complexes are usually analgous to related CO derivatives. A good example of this relationship are the complexes IrCl(CO)(PPh3)2 and IrCl(N2)(PPh3)2. Few complexes contain more than one N2 ligand, and no example features three (in contrast metal hexacarbonyls are common). The dinitrogen ligand in W(N2)2(Ph2CH2CH2PPh2)2, the N2 ligand can be reduced to produce ammonia.

Bridging, end-on
N2 also serves as a bridging ligand, as illustrated by {[Ru(NH3)5]2(μ-N2)}4+

Side-on, bridging
In a second mode of bridging, bimetallic complexes are known wherein the N-N vector is perpendicular to the M-M vector. One example is [(η5-C5Me4H)2Zr]2(μ2,η2,η2-N2).