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Organometallic Intro
CHEM966 (Tunge)
Coordination number
Electron Counting
CN = (# of monodentate) + 2(# of bidentate) +…
Description of a transition metal
In the example Rh is 4-coordinate. Other cases may be less obvious.
1)
2)
3)
4)
Formal Oxidation State
d Electron Configuration
Total Electron Count at the Metal
Coordination Number
Common Ligands
Anionic Ligands
Cl
R
Consider Wilkinson’s Catalyst:
Ph3P
Ph3P
Rh
H
R2 N
RO
Tr ident ate ( 6 e-)
Bidentate (4 e-)
Monodent ate ( 2 e-)
=
M
Cl
PPh3
O
M
O
M
=
O
Zr
M
M
O
O
M
H
Cl
O
Neutral Ligands
Monodent ate ( 2 e-)
Assignment of Oxidation State
Bidentate (4 e-)
R
a) Remove ligands in closed shell configuration
b) The charge on the metal is the oxidation state [Rh(I)]
Ph 3P
Ph 3P
Rh
Cl
PPh3
Tr ident ate ( 6 e-)
CO PR3
Ph 3P
Ph 3P
Rh
Cl
R
N
N R
R2P
Cr(CO)3
N
n = Valence of neutral metal – charge on the metal
Rh(0) has 9 valence electrons, so Rh(I) has (9-1= 8)
Rh(I) has 8 d-electrons and is said to be d8
Total electron count
The total electron count at the metal is given by:
(metal electrons + ligand electrons). In this case:
the phosphines and chloride are all 2 e- donors so,
Total e- = 8 + 2 + 2 + 2 + 2 = 16 e-
O
PR 2
N
PR2
Variable coordination modes
Ph 3P
Cl
vs.
Pd
d-Electron count (d )
R
R
PPh3
n
PR 2
NR 3
PPh3
Pd
Ph3 P
PPh 3
M
O
O
vs.
S M
S
O M
O
M vs.
O
vs.
Coordination number and electron count dictate geometry.
d6, 6 coordinate metal = octahedral
d8, 3-coordinate = T-shaped
d8, 4-coordinate metal = square planar
d8, 5-coordinate metal = square pyramidal or trigonal bipyramidal
d10, 3-coordinate metal = trigonal planar
d10, 4-coordinate metal = tetrahedral
Coordination mode and electron count affect reactivity
d6, 6 coordinate metal is substitutionally inert
M
Organometallic Intro
CHEM966 (Tunge)
On the 18 e- “rule”:
Practice
Me
O
O Me
O
Rh
Rh
O
O
O
Me O
O
R
PMe3
Zr
N
TMS
Mes
N
Cl
Ph
Cl
H
Me
N
Ru
PCy3
Mes
Ph
H
R
H3C(F3C)2CO
H3C(F 3C)2CO
N
W
CMe3
Where "formal" oxidation state f ails
Mo
Mo(0)
Mo
Mo(VI)
In fact, formal oxidation states often do not correlate with
ionization energies : Green, M. L. H. J. Organometallic
Chemistry 1995,500, 127-148.
Consider WMe6 :the calculated charge on W is +0.4 not +6
Graphs generated by the program mlx (Version 1), Copyright©Cary
Zachmanoglou, 1998 which was graciously provided by Professor Ged Parkin,
Columbia University
The 18 e- rule works well for groups 6-8, but in general you
should regard 18 e- as a maximum (not as you view the octet rule)
= 16 e- compounds
Organometallic Intro
CHEM966 (Tunge)
Where do the electrons go?
Approximate d-orbital MO’s for common coordination geometries.
z
dz2 d x2 -y2
M
d xz
dyz
x
M
d xy
dxz
d xz
dyz
M
x
d z2 dx2- y2
dxy
complex
dz2
H 3C C O
2299
C O
2143
dyz
z
d xy
2060
d x2-y2
Ni(CO) 3PPh3
2068, 1990
dz2
dxy
Ni(CO) 3PtBu 3
2056, 1971
M
dz2
dz2
M
dxy
Cy
N
(O3C)Ni
increasing
backbonding
2049, 1965
N
Cy
Co(CO) 4
2 Fe(CO)
4
1890
1770
Consequence of backbonding: HCo(CO)4 is as acidic as HCl!
M
d yz
νCO cm-1
Ni(CO) 4
d xz d yz
d xz
M C O
donation
into the π*
orbital
dx2- y2
z
x
M C O
C O
d x2 -y2
dxy
d x2-y2
d xz
dyz
Note: Metals have lone pairs that are often not shown
These lone pairs can be involved in π-backbonding
Phosphines and phosphites can be π-acceptors
OR
M P OR
OR
OR
M P OR
OR
OR donation
P OR into the σ*
OR orbital
H H
M
donation
into the π*
orbital
M
H H
Backbonding in metal alkene complexes
1) lengthens the C-C bond
2) makes carbons more tetrahedral
3) formally oxidizes the metal by 2 e-
π-aciditiy
NHC's<PMe 3<PPh3 <P(OMe)3 <P(NR 2) 3<CO
σ-donation
Organometallic Intro
CHEM966 (Tunge)
What type of ligand would be best for the following reaction?
Nu
M
Bond Strengths
Nu
Note: In the d-block bond strengths increase by row. This is due
to better overlap with the larger d-orbitals. 3rd row > 2nd row > 1st
row
M
Other ligand effects
M
The size of a ligand can dramatically influence substitution
reactions at the metal. One convenient measure of ligand size is its
Cone Angle. The cone angle is essentially the area that is blocked
by the ligand. See Tolman, C. A. Chem. Rev. 1977, 313.
M(CO)2
M
M
KD
NiL 4
L
PMe 3
P(O-i-Pr) 3
PPh 3
P(tBu) 3
X
M
KD
<10 -9
2.7 x 10-5
no NiL3
-
PPh2
PPh2
M
(OC)4 Mn-X
o
79
87
o
PPh 2
99o
H
CH 3
Ph
Ac
I
Cl
(CO)4 Mn
Ir(III)-X
55(4)
37(1)
41(3)
31(3)
47(2)
70(2)
~23
60(2)
46(3)
~56
45(3)
45(2)
71(4)
Mn: Skinner, Orgmet 1982, 1166
Ir: Blake et. al. JACS 1979, 74; JACS 1981, 5768
PPh 2
PPh2
PPh2
109 o
(OC)4 Fe
Fe
O
PPh2
85o
PPh 2
PPh 2
PPh2
P
44
53
57
Calibration : BDE(CH3-I) = 58 kcal/mol
Bite Angle: The angle produced when a chelating ligand binds to
a metal center. van Leeuwen, P. W. N. M. Dalton 2003, 1890.
P
Mean BDE
Ti
Zr
Hf
NiL3 + L
Cone Angle
118
130
145
170
46
67
80
Cr
Mo
W
+ CO
M(CO) 3
BDE (kcal/mol)
PPh 2
PPh2
99o
Fe(CO)4 +
(OC)3 Fe
ΔH ~ 24 kcal/mol
(OC)5 Cr
H
R
BDE ~ 50 kcal/mol
Cr(CO)5 +
ΔH ~ 5 kcal/mol
Natural bite angle: Preferred chelation angle of ligand regardless
of metal valence angles. Casey, C. P. Israel J. Chem. 1990, 299.
While BDE’s decrease going down a row in the d-block, Atomic
size does not.
Element
Ti
Zr
Hf
Ni
Pd
Pt
Atomic radius
1.40 Å
1.55 Å
1.55 Å
1.35 Å
1.40 Å
1.35 Å
Ionic radius (IV)
0.56 Å
0.73 Å
0.72 Å
0.62 Å
0.76 Å
0.76 Å
d-orbitals do not shield the nuclear charge well, so elements
decrease in size going to the right.
Lanthanide contraction: electrons in f-orbitals shield the nucleus
more poorly than d-orbitals, so many of the 3rd row d-block
elements are smaller than their 2nd row analogs.