Transcript Slide 1

Lecture 34
MO’s of the most important polyatomic ligands
1) Bonding in carbene complexes
•
•
Transition metal carbene complexes are formed by carbenes, CX2 , X = H, Alk, Ar, Hlg,
OR, NR2 etc. These complexes are responsible for catalysis of olefin metathesis (Y.
Chauvin, R. Schrock, R. Grubbs, 2005 Nobel prize for Chemistry).
Frontier orbitals of the singlet CH2 can be readily obtained from those of H2O
(2.0 eV)
CH2
(-10.5 eV)
HOMO
C
C
a1
b2
LUMO
There are two types of carbene bonding to a metal:
M
C
-bonding
M
C
M
C
M
-backbonding
C
2) Nucleophilic carbene complexes
•
Contribution of - and -bonding into the net M-carbene bond determines chemical
reactivity of a carbene complex and depends on the nature of substituents X attached
to the carbene carbon.
•
X = Alk or H (X is a -only donor) corresponds to the case of “nucleophilic” carbene
complexes. For C(Alk)2 -bonding of the carbene carbon to a metal is important so
that the carbene can be considered as a dianionic four-electron ligand attached to a
metal with a double bond, M=CR2.
M
•
Such carbenes behave as nucleophiles:
O CR2
(Me 3CCH2-)3Ta
2.24
•
C
CHCMe 3
(Me3CH2C)3Ta
O +
CR2
CHCMe3
1.94
X = NR2, OR or halogen (X is a -donor) corresponds to the case of “electrophilic”
carbene complexes (“Fisher metal carbenes”). Interestingly, cyclic diaminocarbenes
(Arduengo carbenes) with bulky R (i-Pr, tert-Bu, Ad) are stable in a free form:
R
N
OEt
2.13
(OC)5Cr
C
1.35 (1.41 for a single bond)
1.33 (1.45 for a single bond)
NMe2
C
C
C
N
R
R
N
R
N
N
R
N
R
3) Electrophilic carbene complexes
•
Strong nucleophiles can attack the carbene carbon so proving that the Fisher
carbene complexes are electrophilic in some degree.
NH2R
OMe
(OC)5Cr
C
Me
•
OMe
C NHR
Me
(OC)5Cr
+
-H
NHR
- MeO
(OC)5Cr
C
Me
In metal-free Arduengo carbenes interaction of empty p-orbital of the carbene carbon
with filled p-orbitals of the adjacent atoms raises the energy of the former. It matches
the energy of filled metal orbitals only poorly now and thus is a week acceptor . So,
the metal-to-carbene bonding can be better described with the formula M-CX2 (the
carbene is a neutral two-electron donor).
HN
NH
C
(6.2 eV)
(2.0 eV)
C
(-11.4 eV)
R
N
N
R
HN
NH
C
C
(-9.5 eV)
(-9.2 eV)
HN
NH
C
(-13.0 eV)
+2
N
4) Phosphine complexes. MO diagram for phosphine PH3
•
Phosphine complexes are widely
used in coordination chemistry.
Frontier orbitals of the simplest
phosphine PH3 resemble closely
those of ammonia with the
difference that the LUMO is two
degenerate e-orbitals. What about
phosphorus d-orbitals? Their
energy is too high, +23.1 eV.
3H
P
3a1 (7.5 eV)
2e
(4.6 eV)
a1
e (3px, 3py)
(-10.6 eV)
- s2 + s3
e
2s1 - s2 - s3
(-10.3 eV)
s1 + s2 + s3
a1 (3pz)
2a1
C3v
A1
z
A2
E
(x,y)
1e
x2+y2, z2
z
(-14.4 eV)
a1 (3s)
(-22.7 eV)
y
1a1
(-23.1 eV)
x
5) Bonding in phosphine complexes
•
Phosphine ligands can be not only good -donors but sometime excellent
-acceptors (PF3 form stable complexes ML4 with Pd0 and Pt0 while CO
does not)
y
dyz
dxz
dz2
z
M
M
M
x
P
P
(-10.3 eV)
-bonding
P
(4.6 eV)
-backbonding
Frontier orbitals of some PX3
HOMO, eV
LUMO, eV
PMe3
-8.8
5.5
PH3
-10.3
4.6
PF3
-12.5
4.1
6) Ligand -acceptor properties and M-L orbital overlap
•
LUMO’s of CO (4.6 eV), CH2=CH2 (5.0 eV) and PX3 listed above (4.1 - 5.5 eV)
are close in energy. But -acceptor properties of these ligands are very
different. Why?
•
The energy of interaction of orbitals of the metal and the ligand, Eint, is a
function of two parameters, an overlap integral S and the difference in energy of
overlapping orbitals. For HOMO of a metal and LUMO of a ligand we have:
S2
Eint 
E (ligand) LUMO  E (metal) HOMO
•
Assuming that for one and the same metal and a series of ligands
E(ligand)LUMO - E(metal)HOMO ≈ constant, we get that Eint will be a function of S.
(5.0 eV)
M
(4.6 eV)
M
M
olefin
S increases
CO
(4.1 eV)
(4.6 eV)
M
PF3
PH3
S increases
7) Dihydrogen and alkane -complexes. Agostic interactions
•
Can a species with neither lone pairs (CO, PR3) nor -bonds serve as an electron
donor?
•
Dihydrogen, CH bonds in alkyls groups and alkanes and some other electron-rich
-bonds (Si-H, B-H, etc.) can. A simple way to illustrate the ability of H2 to serve as
a 2e-donor is given below. H3+ is a know species that forms from H2 and H+:
H3+
2 H + H+
e'
3 (1s)
(-4.8 eV)
(-13.5 eV)
a1"
(-33.3 eV)
8) Dihydrogen and alkane -complexes. Agostic interactions
•
While H2 complexes can be isolated, stable alkane complexes are unknown.
Nevertheless, stable complexes with agostic (from Greek “to hold onto oneself”) CH
bond are multiple.
PMe 2
PCy3
CO
H
W
OC
Cl
Me 2P
Cl
0.84 (0.74 in free H 2)
H
PCy3
(7.0 eV)
M
H
2-C,H
M
M
H
H
-backbonding
-bonding
C
C
H
M
H
H
(-14.8 eV)
(6.6 eV)
H
2.10
Cl
H 1.13
OC
(-16.3 eV)
o
84.5
Ti
-bonding
-backbonding
PCy3
+
H
Ir
H
H
H
H
H
PCy3
OC
2
 -H,H
Rh H
H
H
M