Dinitrogen complexes are coordination compounds containing the N2 molecule as a ligand. There are several possible modes of bonding between dinitrogen and metals, including terminal ("end-on") bonding, lateral ("side-on") bonding, and bridge bonding via one or both nitrogen atoms. Only terminal, terminal bridge, and structures with two metal atoms on each nitrogen atom have been conclusively demonstrated. The first dinitrogen complex was discovered in 1965 and contained a stable [(NH3)5RuN2] structure.

1. DINITROGEN COMPLEXES
DEFINITION, NOMENCLATURE, POSSIBLE MODES OF
BONDING.

2. DEFINITION
 Dinitrogen complexes are coordination
compounds which contain the N2 molecule as
ligand.
 The first example of this class of compounds
was discovered by Allen and Senoff in 1965.
 The attempted synthesis of ammine complexes
from RuC13 and hydrazine hydrate did not yield
the expected [Ru(NH3)6]2+ ion but the stable
complex [(NH3)5RuN2]CI2 instead.

3. NOMENCLATURE
 According to the IUPAC rules, the N2 molecule
is named “dinitrogen” when it is a formally
uncharged ligand in coordination compounds.
 Other proposals such as “nitrogenyl” (by
analogy with “carbonyl”), “nitrogeno”, or
“nitrogen” should therefore be avoided.

4. MODES OF BONDING
 In principle, several possibilities are conceivable
for the linkage between dinitrogen and metals.
 These are in particular: terminal (“end-on”)
bonding [(I)], lateral (“side-on”, or “edge-on”)
bonding [(2)], bridge bonding via one of the N
atoms [(3)], terminal bridge bonding via both N
atoms with a metal atom on each nitrogen
[(4),(5)] or two metal atoms on each nitrogen
[(6)].

6.  (1) and (3) correspond to the bonding in
terminal and bridged carbonyl complexes
respectively, (2) is comparable to the bonding in
acetylene complexes.
 In (I), (2),and (4) the triple bond in the N2
molecule is largely retained, but not so in (3),
(5), and (6).
 (5) is therefore to be regarded as a complex of
the doubly deprotonated diimine HN=NH, and
(6) as a fully deprotonated hydrazido complex,
rather than as dinitrogen complexes.

7.  So far, only bonds of types (I),(4),and (6) have been
unequivocally demonstrated in N2 complexes; (2) has been
postulated.
 The structural unit (6) is present in the complex
[((PhLi)3Ni)2N2.2Et2O]2.
 Here the two Ni atoms and the two atoms of the N2
molecule occupy the corners of a distorted tetrahedron:
two such [Ni2N2] units are bridged via two Li atoms.
 A side-on π bond between the N2 molecule and the metal
centres is considered as a possibility; accordingly, ethylene
readily displaces the N2 ligand from the complex.
 The length of the N-N bond was found by X-ray structure
analysis to be 1.35A˚, the highest value yet encountered in
a N2 complex.
 This value approaches that of an N-N single bond (1.47
A˚) and points to a considerable activation of the N2
molecule.

8.  A Pt complex, formed from cis-[PtC12(PPh3)2]
and hydrazine, contains the structural unit (3) in
protonated form.

9.  Molecules related to terminal N2 complexes
include N2O, HN3, CH2N2, C6H5N2X- which
may be considered as an internal diazonium salt.

10. VANDITA RAJ
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