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Lecture 33
1) h3-Allyl complexes
•
•
h3-Allyl ligand C3H5- is anionic, donates 4 electrons from its p-system to form one s- and one
p-bond with the metal and forms complexes with many transition metals such as Ni(h3-C3H5)2.
Allyl is capable of p-backbonding but it is not so important as in the case of olefins or CO.
z
LUMO
M
•
•
M
In symmetrical h3-allyl complexes
the M-C1 and M-C3 distances are
identical and slightly longer than
M-C2. In addition, Hmeso and Hsyn
are bent 7-130 towards the metal
while Hanti are bent 300 away.
h3-allyl can easily convert into
h1-allyl:
M
h -allyl
3
12.6 eV
M
x
M
HOMO
y
0.3 eV
H
C1
Hanti
H
C3
Hsyn
C2
-4.7 eV
Hmeso
dyz
pz
dxz
M
M
s-bonding
p-bonding
M
M
h -allyl
1
p-backbonding
2) Coordination can make olefins and allyl electron-poor
•
Alkenes typically considered in organic chemistry as electron-rich compounds can become
electrophilic if coordinated to an electron-poor metal:
R
R
R3N
R3N
Pd
Cl
•
NR3
Cl
R
NHR2
R3N
Pd
NR3
Cl
NHR2
Pd
NR3
Even anionic allyl can behave as an electrophile if it is attached to PdII:
R3P
R3P
Pd
R3P
R3P
-Pd0
Pd0
CHX2
CHX2
3) p-MO’s of cyclic conjugated ligands
For qualitative analysis of bonding between transition metals and p-electron donors such as
organic compounds with C=C bonds it is necessary to know how the ligand p-MO’s look like.
Frost diagrams allow establish the shape, degeneracy and the energy sequence of the ligand pMO’s for the case of cyclic conjugated species.
The diagrams below describe qualitatively the p-MO’s in the n-membered cyclic systems. The
number of the nodal surfaces (shown with dashed lines) of the MO’s increases as you move up
along the diagram.
•
•
•
E
Energy levels of resulting p-MO's are indicated with
d
c
energy level of isolated
p-orbitals
c
b
n=3
a
n=4
b
c
n=5
a
n=6
n=7
Scheme of the p-MO's for benzene molecule:
Scheme of the p-MO's for cyclopentadienyl
a
b
a
b
c
d
4) h5-Cyclopentadienyl complexes
•
p-MOs’ of cyclopentadienyl anion:
•
And bonding in h5-Cp complexes:
z
dz2
y
M
px
M
py
dx2-y2
dxy
M
M
M
x
s-bonding
p-bonding
-backbonding
5) h6-Arene complexes
h6-Bonding in transition metal – arene complexes (5 MO’s only):
z
dz2
M
px
py
dxy
dx2-y2
M
M
M
M
y
x
b
a
s-bonding
c
b
c
p-bonding
-backbonding
h6-, h4- and h2-arene complexes are known:
CO
OC
Fe
2 e-
Cr
Ru0
2+
Ru
1.43
1.41
1.50
D6h
1.31
1.42
CO
1.31
1.45
1.43
1.42
1.38
Fe
OC
CO
CO
6) Coordination can make arenes electron-poor
Me2CCN
OC
OC
Me2CCN
H
Cr
OC
OC
CO
Cr
CO
NuC
H
OC
OC
Nu
CH2
C
H
Cr
OC
OC
CO
Cr
very acidic
CH3
OC
OC
Cr
CO
CO
CH2