The document discusses metal dinitrogen complexes, focusing on their stability, bonding modes, and differences between dinitrogen (N2) and carbon monoxide (CO) ligands. It details the first synthesized dinitrogen complexes and their structural characteristics, highlighting the importance of back-bonding in their formation. Additionally, it emphasizes the potential applications of these complexes in agriculture and nitrogen fixation research.

1. Dr. Geeta Tewari
Department of Chemistry
D. S. B. Campus
Kumaun University, Nainital
Email: geeta_k@rediffmail.com
Metal π Complexes, Part 8,
Metal Dinitrogen Complexes

2. Introduction
 N2 is isoelectronic with NO+ ion and CO ligands.
 Dinitrogen metal complexes are not very stable.
 Lack of polarity of N2 and less tendency to behave as
a π -acceptor.
 [Ru(NH3)5N2]2+ first dinitrogen complex of transition
metal
 [Co(PPh3)3H(N2)] first dinitrogen complex of
transition metal prepared from N2.
 [Mo(PPh2CH2CH2PPh2)2(N2)2] first dinitrogen
complex of molybdenum

3. Preparation
 The first dinitrogen complex (Pentaamine dinitogen ruthenium
(II) cation) [Ru(NH3)5(N2)]2+ was prepared by the reduction of
commercial ruthenium trichloride by hydrazine in aqueous
solution (1965).
RuCl3 + 4N2H4 → [Ru(NH3)5N2]2+ + ...
 Direct reaction of N2
[Co(N2)H(PPh3)3], [RuH2(N2) (PPh3)3] and [FeH2(N2)(PR3)3] (R3
= EtPh2, n-Bu) react direct with nitrogen to form dinitrogen
complex.
[MH2(PR3)3] + N2 → [MHn-2(N2)(PR3)3 + H2

4.  N2 ligand can be easily displaced by water and formation of
dinuclear dinitrogen complex can be formed.
[Ru(NH3)5N2]2+ + [Ru(NH3)5H2O]2+  [Ru(NH3)5N2(NH3)5Ru]4+
+ H2O
 From nitrous oxide
[Ru(NH3)5Cl]Cl2 + Zn/Hg  [Ru(NH3)5H2O]2+
[Ru(NH3)5H2O]2+ + N2O  [Ru(NH3)5NNO]2+
[Ru(NH3)5NNO]2+ + Zn/Hg  [Ru(NH3)5N2]2+
Preparation

5. Mode of Bonding
 Dinitrogen (N2) ligand can be attached to the metal atom via
two modes: terminal and bridged.
 For both terminal and bridged dinitrogen ligands, there are two
structural possibilities: end-on and side-on.

6. https://commons.wikimedia.org/wiki/File:MO_diagramm_von_molekularem_stickstoff.svg

7. HOMO and LUMO in N2
 In nitrogen, the highest filled HOMO is of a very low
energy (-156 eV)
 The vacant LUMO is relatively of high energy (-7
eV)
 Reduction and oxidation of dinitrogen is not easy,
because removal of electrons from HOMO is difficult
and addition of electrons in LUMO is also very
difficult

8. N2 Vs CO ligand
 N2 is isoelectronic with CO.
 CO has a lone pair electron on carbon atom which
can form a σ bond with the metal center (L→M).
 The empty π antibonding MO for back bonding is
also present on carbon (M →L).
 N2 also has a lone pair of electron in sigma BMO, but
the energy of this BMO is lower energy than the
corresponding orbital in CO (may be due to more
electronegativity of N than C).
 Therefore, N2 is weaker σ donor as compared to CO.

9. • Like CO, N2 also has empty π∗ orbitals. These orbitals
are of lower energy as compared to the empty π∗
orbitals of CO. So, the empty π∗ orbitals of N2 are
more approachable than the CO π∗ orbitals.
• In case of N2, the empty π∗ orbitals are equally
distributed over N1 and N2 and therefore, the
overlapping M→N π∗ is smaller than for M→CO π∗
(In CO, the π∗ orbitals are predominantly located on
carbon.
• As a result, the binding of N2 with the metal center is
less efficient than CO.
N2 Vs CO ligand

10.  Always linear.
 π-Back bonding from metal d orbitals into the π*
orbitals of N2 (LUMO).
 Back bonding is highest in Zr, Hf, Ta which are low-
valent metals (electron rich) (stable N2 association).
 Back bonding is least in Fe, Ru, Co, Rh, Ni which are
electronegative.
Bonding in End-on terminal dinitrogen
complexes

11. Bonding in End-on terminal dinitrogen
complexes

12.  π-Back bonding from metal d orbitals into the π*
orbitals of N2 (LUMO) in different manner.
Bonding in side-on terminal dinitrogen
complexes

13. Bonding in side-on terminal dinitrogen
complexes

14. Terminal end-on
 Terminal end-on (one end-on N2 ligand)
 N2 is sigma-donor and pi-accepter.
[Ru(NH3)5N2]2+
[IrCl(N2)(PPh3)2]
 Terminal end-on (more than one end-on N2 ligand)
mer-[Mo(N2)3(PPrn
2Ph)3]
[Mo(N2)2(Ph2PCH2CH2PPh2)2]

15. Terminal side-on
 one side N2 ligand
 High energy compounds
 Less stable
 N2 is pi donor
[Os(NH3)5(η2-N2)]2+ (metastable)

16. Bridging end-on
 Multinuclear dinitrogen complexes
{[Ru(NH3)5]2(μ-N2)}4+
 Iron dinitrogen complexes
Fe-N-N-Fe

17. Both terminal and bridging nitrogen
 [(η1-N2)(η5-C5Me5)2Zr]2(μ2, η2-N2)

18. Bridging side-on
 First f-block dinitrogen complex
Sm
N
N
Sm
C5Me5
C5Me5
C5Me5
C5Me5

19. Bonding and vibrational spectra of
dinitrogen complexes
Donation of electrons from the slightly
antibonding MO of dinitrogen to the metal
and back-bonding from the filled metal d
orbitals to the empty antibonding orbital of
the dinitrogen.
Bond length (109.8 pm)
Bond length (110.7 pm) in complexed
dinitrogen complex.

20. 2330 cm-1
 The N-N stretching frequency (v(N2)) in complexes is 100-400
cm-l lower (2170-2100 cm-1)

21. Uses of dinitrogen complexes
 Dinitrogen complexes may be used in agriculture and
industries and are of great interest, especially as possible
intermediates to study the reactions that may simulate natural
processes of nitrogen fixation.
 The discovery of dinitrogen complexes is helpful in
investigating the possibility of nitrogen fixation via such
complexes.
Mo(N2)2(dpe)2 + 6H+ → 2NH3 + N2 + Mo (VI) products
where dpe = Ph2PCH2CH2PPh2[1,2- bis(diphenyl phosphine
ethane]

22. Thank You