1) 1,3-butadiene acts as a 4e- donor ligand that binds to transition metals. It exists as two conformations and typically binds in a cisoid conformation. 2) Based on the Dewar-Chatt model, 1,3-butadiene can bind as an L2 or LX2 donor type. LX2 binding is more common and results in shortening of the internal C-C bond and lengthening of the terminal C-C bonds. 3) Transition metal-butadiene complexes are typically prepared through reactions of metal carbonyls, metal halides, or metal vapors with 1,3-butadiene. The bonding involves donation

1. Transition metal-1,3-Butadiene complex
DIOLEFIN
ISOLATED DIOLEFIN
CONJUGATED DIOLEFIN
1

2. • The conjugated dienes such as 1,3-butadiene act as
4e- donor ligand
• 1,3-butadiene exists as two conformational isomers
which are in rapid and dynamic equilibrium with each
other
• This ligand binds to a metal in a cisoid conformation
Cis-isomer Trans-isomer
2

3. • The Dewar-Chatt model, when applied to 1,3-butadiene,
predicts that the ligand may bind to metal either as a L2 donor
type, similar to that of an alkene, or as an LX2 donor type,
similar to that of a metallacyclopentene form
L2 binding of 1,3-butadiene is rare, while the LX2 type binding is
more common
C1
C2
C3 C4
C1
C2
C3 C4
3

4. • An implication of the LX2
type binding is:
• shortening of the C2-
C3(1.40Ǻ) distance along
with the lengthening of the
C1-C2(1.46Ǻ) and C3-
C4(1.46Ǻ)
4

5. Methods of preparation
Metal butadiene complexes are usually prepared by the same
methods used for synthesizing metal-alkene complexes
1. Reaction of metal carbonyl with 1,3-butadiene
When a metal carbonyl is treated with 1,3-butadiene, two CO
ligands are replaced by the diene molecule forming transition
metal-butadiene complex
+ Fe(CO)5
High
Pressure
Fe(CO)3
+ 2CO
5

6. • When a mixture of 1,3-butadiene and Fe(CO)5 is irradiated
by UV light, four CO ligands are replaced by two
molecules of 1,3-butadiene forming a bis(η4-butadiene)
complex
Fe(CO)5 + 2CH2=CH-CH=CH2 [Fe(CO)(η4-C4H6)2]
UV
- 4CO
6

7. • Some other examples are:
7

8. 2. Reaction of metal halide with 1,3-butadiene:
Some 1,3-butadiene complexes can be prepared by the
reaction of metal halide with 1,3-butadiene under suitable
conditions
K2[PtCl4] + CH2=CH-CH=CH2 K[PtCl3(η2-C4H6)]
In this complex 1,3-butadiene acts as 2e- donor, dihapto
ligand
- KCl
8

9. . Metal atom vapour phase synthesis:
The condensation of vapour of a metal in the presence of
1,3-butadiene and suitable co-ligand (such as CO,PR3 etc)
produces butadiene complex
Cr(g) + 4CO(g) + CH2=CH-CH=CH2(g) [Cr(CO)4(η4-C4H6)]
9

10. Structure
10

11. [Cr(CO)4(η4-C4H6)]
11

12. Structure and bonding
• To begin with bonding let us consider hybridisation
and molecular orbitals of butadiene molecule
• 1,3-butadiene is a planar molecule with sp2
hybridisation on each carbon atom
• Each carbon atoms form 3 sigma bonds using hybrid
orbitals
12

13. • After C-H and C-C bond formation, each carbon
atom has one singly filled 2p orbital which is
perpendicular to the molecular plane for π-bonding
• Four 2p orbitals take part in linear combination to
form four π-MOs
13

14. The MOs of the 1,3-
butadiene ligand
comprises of two filled
Ψ1(HOMO-1) and Ψ2
(HOMO) orbitals and two
empty Ψ3 (LUMO) and Ψ4
(LUMO+1) orbitals
14

15. • Appropriate combinations of four π MOs of 1,3-
butadiene and metal based orbitals can be used to
construct MOs of transition metal-Butadiene complex
15

16. 16

17. • Ψ1 and Ψ2 π MOs of 1,3-butadiene are involved in
the formation of butadiene → M σ and σ bond
respectively
• Similarly, Ψ3 and Ψ4 π MOs of 1,3-butadiene are
involved in the formation of M → butadiene π and δ
bond respectively (back donation)
17

18. 18

19. • The frontier orbitals of the butadiene, ψ2 (HOMO)
and ψ3 (LUMO) are expected to be the most
important in bonding to the metal.
• In a metal−butadiene interaction the ligand to metal
σ−donation occurs from the filled Ψ2 orbital of the
1,3−butadiene ligand while the metal to ligand
π−back donation occurs on to the empty Ψ3 orbital of
the 1,3−butadiene ligand
19

20. • The molecular orbital diagram shows that both the
depletion of electron density in ψ2 by σ donation to
the metal and population of ψ3 by back donation
from the metal lengthens terminal C-C bond and
shortens internal C-C bond.
• Binding to a metal usually depletes the ligand HOMO
and fills the ligand LUMO.
20

21. • The extent of ligand donation or metal back-donation
depends on the metal, substituents on the diene and
other ligands present
• An electron-rich metal will tend to populate Ψ3; an
electron-poor metal will tend to depopulate Ψ2.
• This is the main reason why binding has such a
profound effect on the chemical character of a ligand.
21

22. • The structure of the bound form of a ligand is similar to
that of the first excited state of the free ligand because to
reach this state we promote an electron from the HOMO
to the LUMO, thus partially depleting the former and
filling the latter.
22

23. • When the butadiene is excited, one of the electron
excited to higher orbital i.e., one of the electron from
ψ2 moves to ψ3
• The internal double bond character, bond length
decreases to 1.39Ǻ and terminal bond is single bond
and bond length increases to 1.45Ǻ
• As a result, lengthening of terminal C-C bonds as
well as shortening of the internal C-C bond can be
observed
23

24. • Similarly in metal-butadiene complexes, depopulation
of ψ2 and population of ψ3 would result in
lengthening of terminal C-C bonds and shortening of
internal C-C bonds
24

25. • The bonding between metal and butadiene may be
represented as follows
• When the donation from ψ2 into metal is important, the
bonding may be represented by (a)
• If donation from ψ1 into metal is important, the bonding
may be represented by (c)
• If ψ3 interactions are important, then represented by (b)
i.e., 1st excited state of butadiene fragment
25

26. For ex: [Fe(CO)3(η4-
C4H6)]. Its structure and
bonding is close to (c) in
which π electron cloud of
butadiene is delocalized on
the entire butadiene moiety
[Fe(CO)3(η4-C4H6)]
26

27. • If the interaction of ψ3 with metal orbital is important,
the structure and bonding may be represented by (b)
which suggests a shorter C2−C3 distance than two
outer C-C distances i.e., C1−C2 and C3−C4
• Ex: in [Cp2Zr(η4-2,3-Me2-C4H4)] C1-C2 and C3−C4
distances are longer than C2−C3 distance
27

28. Therefore, (b) is the main contributor to the
structure and bonding of the compound
In this complex, C1-C2 and C3-C4 distances are
longer than C2-C3 distance
The Zr-C1 and Zr-C4 distances are typical of Zr-
C sigma bonds
28

29. • At the same time Zr is farther from C2 and C3
than C1 and C4
• 2,3-dimethyl butadiene shows extreme LX2
i.e., (b) bonding pattern
• Zr is electron rich, it transfers 2 e-s to
butadiene via π-back-donation and generates
metallacyclopentene
29

30. Properties:
[Fe(CO)3(η4-C4H6)] is a yellow-brown oil which
is soluble in most of the organic solvents
The coordinated butadiene does not undergo
hydrogenation and Diels-Alder reaction
30

31. • Electrophilic addition reaction: the attack by
electrophile on the coordinated butadiene is less
common.
However illustrated by direct protonation and hydrogen
abstraction
31

32. • Sometimes, the carbocation electrophile abstracts
hydride ion from the saturated carbon atom adjacent
to the olefinic carbon atom extending the conjugation
• => lead to a change in the hydrocarbon ligand and its mode of bonding
32

33. • Nucleophilic attack: the coordinated butadiene is
readily attacked by a variety of nucleophiles such as
CH3O-, H-, CN-, R- etc.
33

34. • The addition of nucleophile to butadiene
saturates an olefinic carbon atom and thus
removes it from conjugation
34

35. • The nucleophile may also attack on the metal
atom.
35

36. Displacement reaction: The coordinated 1,3-
butadiene molecule can be displaced by other
neutral ligands such as PPh3
36