Chem 652 Spring 2013
Introduction to Structure, Ligands, and Bonding
Prof. Donald Watson
"
Assistant Professor"
"
"
Read Hartwig Chapters
1-3
General Anatomy of an Organometallic Complex
CO
Fe
Examples
from last
time:
tBu
OC
PPh2
Cl
PPh3
Ir
OC
CO
CO
OC
Cl
Ta
Ph2P
Ph3P
Ni
O
tBu
Ph3P
OC
O
Ir
Cl
Cl
Cl
HH
H
In general:
•  Organometallic complexes are comprised of one or more
metal atoms supported by one or more ligands.
•  We will focus primarily on soluble transition metal
complexes and intermediates in this class.
•  To understand reactivity, must consider:
•  Type of metal (element)
•  Formal number of electrons around the metal.
•  Type and Number of Ligands
•
Charge of Complex
•
Shape of Complex
CO
CO
Pt
Cl
PPh3
Fe
H
(+, 0, –, etc)
L
L
L
M
L
L
L
“Transition Metals”
M
st
0
1 row
nd
2 row
rd
3 row
III
3
d
Sc
Y
La
early
IV
4
d
Ti
Zr
Hf
V
5
d
V
Nb
Ta
VI
6
d
Cr
Mo
W
VII
7
d
Mn
Tc
Re
dividing line
• Group XII do not meet Cotton and
Wilkinson’s definition of transition metals
“partially filled d-orbitals”
• Assumption: for metals in metal
complexes assume all valence electrons
are in the d-orbitals
e.g. Rh s2d7 actually d9; Sc 3s2d1 actually d3
VIII
8
d
Fe
Ru
Os
IX
9
d
Co
Rh
Ir
X
10
d
Ni
Pd
Pt
late
XI
d s
Cu
Ag
Au
10 1
XII
10 2
d s
Zn
Cd
Hg
Sweeping Stereotypes
Early metals
high oxidation states dominate
Ti(0) → Ti4+ easy!
Late metals
low oxidation states dominate
hard ligands tend to be favored
less polarizable (Cl–; O2–)
:NH3 harder than :PR3
soft ligands more favored
more electropositive
less electropositive
M–L bonds have more ionic
character important for M–C bonds
M-L bonds have more covalent
character for M–C bonds
ex. Zr(CH2Ph)4 less typical [V(CO)6]–
Important Periodic Trends to Keep in Mind
•
•
•
•
size decreases →
size increases ↓ (1st to 2nd)
sze stays approx. the same 2nd to 3rd
redox states:
(1) ability to oxidize increases ↓
(2) covalency increases ↓
•  3d often too small for good M–C overlap, 4d and 5d much better.
Result? 2nd row elements are often important catalysts!
Orbitals: 3s
http://winter.group.shef.ac.uk/orbitron/AOs/3d/index.html
Orbitals: 3p
http://winter.group.shef.ac.uk/orbitron/AOs/3d/index.html
Orbitals: 3d
z
dx2-y2
dz2
y
on axis
(Oh)
x
dxy
L
L
L
M
dxz
dyz
L
L
off axis
(Oh)
L
http://winter.group.shef.ac.uk/orbitron/AOs/3d/index.html
Orbitals: 4d
http://winter.group.shef.ac.uk/orbitron/AOs/3d/index.html
18 Electron “Rule”
Many, but not all, stable, isolatable TM complexes are 18e– complexes.
Why?
Simple Answer:
TM Valance Orbitals: 1 x nS orbital, 3 x nP orbitals, 5 x (n-1)D orbitals
= 9 total valance orbitals
= 18e– for close shell valance
Real Answer:
More complex… as we are dealing with complexes, not metal atoms…
more to come on this.
NOTE: 18e– “rule” is more of a suggestion. Many of the most interesting,
i.e. reactive, complexes do not follow this “rule”.
Determining Electron Count of TM Complex
Two methods for counting electrons around metal center (ionic and
covalent).
Electron counting is a formalism.
Covalent Method: All ligands are disassociated as neutral ligands.
L-type ligands = neutral, 2 e- donor
X-type ligands = neutral radical, 1 e- donor
X
n
X
L
M
n
L
L
M
X
X
L
Example of Covalent Method Electron Counting
H
O
C
H
OC
OC
Fe
H
CO
O C
Fe0
CO
C
O
4 X CO
2 X H·
Fe(0) (d8)
4 X 2e2 X 1e-
8e2e8e-
________________________________________
18e-
H
C O
Ionic Method of Electron Counting
Covalent Method: All ligands are disassociated with their electrons ligands.
L-type ligands = neutral, 2 e- donor
X-type ligands = anionic 2 e- donor
n
X
X
L
M
n+y
L
L
M
X
X
mnemonic:
X
L
M
X
X
L
L
M+ L
X–
L
where y = number
of x-type ligands
Example of Ionic Method Electron Counting
H–
O
C
H
OC
OC
Fe
H
CO
O C
Fe2+
CO
C
O
4 X CO
2 X H–
Fe(2+) (d6)
4 X 2e2 X 2e-
C O
H–
8e4e6e-
________________________________________
18e-
Note: Both Ionic and Neutral Methods give the same electron count,
but differ in how they treat the metal center.
Charged Complexes
2– 2K+
Cl
Cl
Pd
Cl
Covalent:
4 X Cl·
4 X 1ePd(2-) (d10+2)
4e12e-
________________________________________
16e-
Cl
Ionic:
4 X ClPd(2+) (d8)
4 X 2e-
8e8e-
________________________________________
16e-
Oxidation State
•  Formalism to account for “oxidation state” of metal center.
•  This is only a formalism, in reality electrons (and thus charge) are
shared over both the ligand and metal.
•  None the less, oxidation state guides understanding of metal
reactivity.
•  Using ionic method of electron counting arrives directly at metal
oxidation state.
•  Example:
H
OC
OC
Fe
H
CO
CO
Oxidation State = Fe(II)
Ionic:
4 X CO
2 X H–
Fe(2+) (d6)
4 X 2e2 X 2e-
8e4e6e-
________________________________________
18e-
Formalism in Action
•  Recall this is oxidation state is formalism!
•  Analysis describes H2Fe(CO)4 as Fe(II), suggesting hydridic H’s (H–).
•  Note: this is consistent with electronegativity (H = 2.1; Fe = 1.8).
•  However:
H2Fe(CO)4 + H2O
HFe(CO)4– + H3O+
acidic, not hydridic!
pka ~ 5
•  Lesson: Need to think about the whole complex, not just metal salts.
δ+
δ-
Fe
H
Examples of X-type Ligands
Ligand
Disconnection
Complex
ligand charge
ligand e-
hydride
M H
M+ +
H
-1
2
halide
M Cl
M+ +
Cl
-1
2
alkyl
M CR3
M+ +
CR3
-1
2
aryl (σ bound)
M
M+ +
-1
2
-1
2
M+ +
acetylide
M
silane
M SiR3
M+ +
SiR3
-1
2
amido
M NR2
M+ +
NR2
-1
2
alkoxide
M OR
M+ +
OR
-1
2
R
R
Simple Bonding Picture
metal-hydride
σ*
E
dx2-y2
1s
σ
M H
Simple Bonding Picture
metal-halide
σ*
E
dx2-y2
3p
σ
M Cl
Simple Bonding Picture
M CR3
M
M
R
σ*
E
dx2-y2
sp, sp2, or sp3 hybrid
σ
M R
Examples of X-type Ligands (More Complex)
Ligand
Disconnection
Complex
O
acetate (η1)
acetate (η3)
allyl (η1)
allyl (η3)
cyclopentadienyl (Cp)
M O
M
M
M
ligand e-
O
Me
O
M
ligand charge
Me
M+ +
-1
2
-1
4
M+ +
-1
2
M+ +
-1
4
-1
6
M+ +
O
Me
O
O
Me
O
M+ +
•  𝜼 (eta) denotes “hapticity” = how many atoms of ligand are bond to metal from
single donor site.
Examples of 2e– Donor “L”-type Ligands
ligand charge
ligand e-
PPh3
0
2
M+
NEt3
0
2
M+
N CR
0
2
0
2
0
2
0
2
0
2
0
2
Ligand
Complex
Disconnection
phopshine
M PPh3
M+
amine
M NEt3
nitrile
M N CR
H
sigma bond (H–H)
sigma bond (C–H)
H
M
M+
H
H
H
H
M
M+
CR3
alkene
CR3
M+
M
carbonyl
M C
ether
M OR2
O
M+
C
M+
OR2
O
Bonding Picture
Simple σ donors.
σ*
E
dx2-y2
sp3
σ
M PR3
Bonding Picture
dihydrogen
H
H
σ*
E
dx2-y2
H
=
H
σ (H-H)
σ
H
H
H
M
H
H
H
Bonding Picture
C-H σ bond
σ*
E
dx2-y2
H
C
σ (C-H)
σ
H
M
CR3
Bonding Picture
Alkenes - Bonding
σ*
E
dx2-y2
(or dz2)
π (C-C)
σ
M
Bonding Picture
Alkenes – “Back-Bonding”
π*
E
dxz
π∗ (C-C)
(or dxy)
π
M
•  “Back bonding” occurs if
there is a filled (or partially)
d-orbital with correct
symmetry to overlap empty
ligand orbital.
•  Filled d-orbital depends on
metal and electron-count.
Bonding Picture
Back bonding:
σ*
π*
dxz
dx2-y2
E
π∗ (C-C)
π
π (C-C)
σ
R
R
M
R
R
π complex
•  Increases metal-ligand
bond order.
•  Decreases C=C
character (populating
π*).
•  Increases sp3 character
of alkene carbons.
•  Think of as resonance
with metallocyclopropane.
Zeiss Salt
R
R
M
Cl
Cl
R
R
metallocyclopropane
Cl
HH
Pt
H
H
yellow crystals
C O
C O
Bonding Picture
Carbon Monoxide
2π
2 π*
σ*
E
dx2-y2
sp
(or dz2)
σ
M
CO
C O
Bonding Picture
C O
Carbon Monoxide
2π
2 π*
π*
E
π*
E
dxz
dxy
π∗ (C-O)
π∗ (C-O)
π
M
CO
π
M
CO
C O
C O
Bonding Picture
Carbon Monoxide
2π
2 π*
σ*
π*
π∗ (C-C)
dx2-y2
E
sp
(or dz2)
π
σ
M
CO
“L”-type Ligands Donating More than 2e–
Ligand
Disconnection
Complex
diene
M
ligand charge
ligand e-
M+
0
4
arene (η6)
M
M+
0
6
arene (η2)
M
M+
0
2
0
4
Ph
Ph
Ph
P
bisphosphine (κ2)
Ph
P
M+
M
P
Ph
Ph
Ph
Ph
R
R
R
R
N
diamine (κ2)
2 X 2e– L-type ligands
P
N
M+
M
P
R
R
4
2 X 2e– L-type ligands
N
R
0
R
•  𝜅 (kappa) denotes “denticity” = how many binding sites of a polydentate ligand
are bound .
Singlet Carbenes and NHC-Ligands
Ligand
Disconnection
Complex
R
N
N-heterocyclic carbenes
Fisher Carbene
ligand charge
R
N
M
M+
N
R
0
2
0
2
N
R
O R
O R
M
M+
R
R
Sigma Bond
Pi-Backbond
R
M
ligand e-
N
C
N
R
M
C
N
N
Bridging Ligands
Ligand
Disconnection
Complex
Cl
M
µ-bridged
ligand charge
Cl
M
M
Cl
1X neutral, 2 e1X anionic, 2 e-
M
Cl
σ*
σ*
E
ligand e-
dx2-y2
E
dx2-y2
p (Cl)
3p
σ
σ
M Cl
•  µ (mu) denotes bridging ligands.
M
Cl M'
Bridging Ligands
Bridging CO
O
O
M
C
M
M
M
1 e- neutral donor
to each metal
O
C
O
C
σ donation
π backbonding
Bridging Ligands
Bridging Hydride
M
H
H
H
M
hydride has only 2 e–!
M
M
M
M
treat as one X type and one L type
Metal-Metal Bonds
Complex
Disconnection
"ligand" charge
"ligand" e-
M
M
M
M
0
1
M
M
M
M
0
2
etc....
Bonding Picture
δ*
E
δ
M M
Shape depends on orbitals involved… can be “delta” bonding.
XL Ligands
π donor ligands
M Cl
M NR2
halide
amido
M OR
alkoxide
π*
σ*
E
E
dx2-y2
dxz
pCl
3p
assuming
empty d
σ
π
M Cl
M
Cl
Bonding Picture
σ*
σ*
π*
π*
E
dx2-y2
E
dxz
dx2-y2
dxz
3p
3p
π
π
σ
empty d orbital
stabilizing
σ
filled d orbital (p → d repulsion)
destabilizing
X2 Ligands
Ligand
Complex
Disconnection
2+ CR2
ligand charge
ligand e-
-2
4
triplet carbenes
(alkylidines)
M CR2
M2+
oxo
M O
M2+
+ O
-2
4
imido (bent)
M N
M2+
2+ NR
-2
4
2-
R
Singlet Carbenes and NHC-Ligands
Ligand
Disconnection
Complex
R
N
N-heterocyclic carbenes
Fisher Carbene
ligand charge
R
N
M
M+
N
R
0
2
0
2
N
R
O R
O R
M
M+
R
R
Sigma Bond
Pi-Backbond
R
M
ligand e-
N
C
N
R
M
C
N
N
Understanding Carbenes
Sigma Bond
Singlet
Carbene
M
R
N
C N
R
Pi-Backbond
M
C
Triplet
Carbene
(alkylidene)
M
C
R
R
empty p-orbital
stablized by donor
atom or group
pC
nN
Sigma-donation
H
N
Pi-donation
M
C
R
R
treat as filled p-orbital
Bonding Picture
σ*
π*
dx2-y2
E
E
dxz
pCl
sp2
C
σ
R
R
C
assuming
empty d
π
M CR2
M
CR2
R
R
Bonding Picture
alkylidenes, bent imidos, oxos
σ*
π*
E
dx2-y2
dxz
3p
sp2
π
σ
“Triplet” Carbene
σ*
π*
E
dx2-y2
dxz
3p
π
σ
Covalent method: neutral, 2e– donor
C
R
R
X3 Ligands
Ligand
Complex
carbyne
M CR
Disconnection
M2+
3+ CR
ligand charge
ligand e-
-3
6
-3
6
3nitrido
M N
M2+
+
N
Bonding Picture
Very Similar to CO
σ*
π*
E
dx2-y2
dxz
dxy
pz, py
sp
π
σ
X2L
Ligand
Complex
M
imido (linear)
N R
Disconnection
M2+
+
2NR
-2
σ*
π*
E
dx2-y2
dxz
dxy
pz, py
sp
π
σ
ligand charge
ligand e6
Consider W(NH3)6
H3N
H3N
NH3
NH3
W NH3
NH3
Symmetry Adapted Linear Combinations
(SALCs)
eg
2 nodes
t1u
1 node
a1g
no node
d6, 18 electron
6X
H
N
H
H
Full MO Diagram
σ*s (a1g)
σ*p (t1u)
t1u
a1g
σ*d (eg)
eg
nd (t2g)
t2g
eg
t1u
σp (t1u)
σd (eg)
σs (a1g)
a1g
Adding Electrons
σ*s (a1g)
σ*p (t1u)
t1u
a1g
σ*d (eg)
eg
nd (t2g)
t2g
eg
t1u
σp (t1u)
σd (eg)
σs (a1g)
a1g
Adding Electrons
σ*s (a1g)
σ*p (t1u)
t1u
a1g
σ*d (eg)
eg
Δ0
nd (t2g)
t2g
eg
t1u
σp (t1u)
σd (eg)
a1g
σs (a1g)
•  18 e– required to fill bonding and non-bonding orbitals.
Now Consider W(CO)6
CO
OC
OC
CO
W CO
6X
C O
+6X
CO
+6X
d6, 18 electron
C O
σ-Bonding Picture Unchanged
σ*s (a1g)
σ*p (t1u)
t1u
a1g
σ*d (eg)
t2g
eg
t2g
eg
t1u
σp (t1u)
σd (eg)
σs (a1g)
a1g
π-contributions
σ*s (a1g)
σ*p (t1u)
t1u
πd* (t2g)
a1g
σ*d (eg)
t2g
eg
t2g
eg
πd (t2g)
t1u
σp (t1u)
σd (eg)
σs (a1g)
a1g
π-contributions
σ*s (a1g)
σ*p (t1u)
t1u
πd* (t2g)
a1g
σ*d (eg)
t2g
eg
Δ0
t2g
eg
πd (t2g)
t1u
σp (t1u)
σd (eg)
a1g
σs (a1g)
•  18 e– required to fill bonding orbitals.
•  Δ0 increased with back bonding.
Real Origin of the 18 Electron Rule
σ*s (a1g)
σ*s (a1g)
σ*p (t1u)
σ*p (t1u)
πd* (t2g)
σ*d (eg)
σ*d (eg)
• complexes with strong σdonating and π-accepting
ligands more strongly
adhere to this rule
Δ0
Δ0
nd (t2g)
πd (t2g)
σp (t1u)
σp (t1u)
σd (eg)
σd (eg)
σs (a1g)
σs (a1g)
W(NH3)6
• in octahedrons: 18 e– fills
bonding and non-bonding
orbitals
W(CO)6
Δ Crystal Field Splitting Term
σ*s (a1g)
σ*s (a1g)
σ*s (a1g)
σ*p (t1u)
σ*p (t1u)
σ*p (t1u)
πd* (t2g)
σ*d (eg)
σ*d (eg)
σ*d (eg)
Δ0
Δ0
Δ0
πd (t2g)
nd (t2g)
πd (t2g)
σp (t1u)
σp (t1u)
σp (t1u)
σd (eg)
σd (eg)
σd (eg)
σs (a1g)
σs (a1g)
σs (a1g)
W(NH3)6
W(CO)6
[Cr(OH2)6]2+
•  Δo is dependent on:
(1) ligand
(2) T.M. (row) increases ↓
•  consequence of (2):
high spin v. low spin
para- v. diamagnetic
Crystal Field Strength
I– < Br– < Cl– < OH– < H2O < NH3 << CN– < PR3 < CO
σ*s (a1g)
σ*p (t1u)
t1u
πd* (t2g)
a1g
Largest Δo with:
•  strong σ-donors (raises eg)
•  strong π-acceptors (lowers t2g)
σ*d (eg)
t2g
eg
t2g
eg
πd (t2g)
t1u
σp (t1u)
σd (eg)
σs (a1g)
a1g
Relative Energies Of Common Geometries
z
y
x
z2
x2-y2
eg
x2-y2
a1g
z2
E
xy
xz
yz
t2g
eg
xz
octahedral
(Oh)
xy
xz
z2
yz
x2-y2
t2
e
yz
xy
square
planar
a1
z2
b1g
xy
x2-y2
e"
xz
yz
b2g
tetrahedral
(Td)
e'
trigonal
bipyramidal
Understanding Trends
z
y
x
eg
x2-y2
x2-y2
z2
a1g
z2
E
b1g
eg
t2g
xz
xz
yz
xy
yz
xy
b2g
•  Removal of
ligands on zaxis lowers dz2
MO (recall this
is anti-bonding).
•  How do you
explain xy, xz
and zy? Does it
depend on the
ligands?
•  Explains why d8
16e-, sq. planar
are so
common… high
lying x2-y2.
σ*dz2 (eg)
σ*dz2 (eg)