Transition metal dinitrogen complex
Metal dinitrogen complexes are coordination compounds that contain the dinitrogen (N2) as a ligand. In the area of coordination chemistry, the atomic and diatomic forms of nitrogen are distinguished, although otherwise "nitrogen" refers to N2.
Metal complexes of N2 have been studied since 1965 when the first complex was reported by Allen and Senoff. This complex, [Ru(NH3)5(N2)]2+ was synthesised from hydrazine hydrate and ruthenium trichloride and consists of a 16e− [Ru(NH3)5]2+ centre attached to one end of N2. Interest in such complexes arises because N2 comprises the majority of the atmosphere and because many useful compounds contain nitrogen atoms. Biological nitrogen fixation probably 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.
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.
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 analogous 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(Ph2PCH2CH2PPh2)2 can be reduced to produce ammonia.
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