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.^[1] 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.^[2]

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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.^[3] 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.^[4]

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).^[5]

References