Dinitrogen complexes- organometallic chemistry

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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