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. [cite journal
title = Nitrogenopentammineruthenium(II) complexes
author = A. D. Allen, C. V. Senoff
journal = Journal of the Chemical Society, Chemical Communications
volume =
issue =
pages = 621
year = 1965
url =
doi = 10.1039/C19650000621] 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. [cite journal
title = The continuing story of dinitrogen activation
author = Fryzuk, M. D.: Johnson, S. A
journal = Coordination Chemistry Reviews
volume = 200–202
issue =
pages = 379
year = 2000
url =
doi = 10.1016/S0010-8545(00)00264-2]

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. [cite journal
title = "trans"-Chloro(nitrogen)bis(triphenylphosphine)iridium (I)
author = Collman, J. P.; Hoffman, N. W.; Hosking, J. W.
journal = Inorganic Syntheses
volume = 12
issue =
pages = 8–11
year = 2000
url =
id = ISBN:9780470131718] 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. [Modern Coordination Chemistry: The Legacy of Joseph Chatt” G. J. Leigh (Editor), N. W. Winterton (Editor) Springer Verlag (2002). ISBN 0854044698]

Bridging, end-on

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

ide-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). [cite journal
title = Kinetics and Mechanism of N₂ Hydrogenation in Bis(cyclopentadienyl) Zirconium Complexes and Dinitrogen Functionalization by 1,2-Addition of a Saturated C-H Bond
author = Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J.
journal = Journal of the American Chemical Society
volume = 127
issue =
pages = 14051–14061
year = 2005
url =
doi = 10.1021/ja0538841]

References