Download Week 1 - School of Chemical Sciences

Course Overview
Instructor:
Professor M.-Christina White: [email protected]
Mallinckrodt 314: office hrs. by appointment
Teaching Fellows:
Qinghao Chen: [email protected]
Matthew Kanan: [email protected]
Mark Taylor: [email protected]
Course Meeting:
Lectures :Tuesday and Thursday, 8:30-10 AM
Sections: Alternate Wednesdays
Begin September 25
Pfizer Lecture Hall
Mallinckrodt Rm. 318
Section 1: 1-2:30 PM
Section 2: 2:30-4 PM
Section 3: 4-5:30 PM
Course Objective:
Introduction to transition metal-mediated organic chemistry. Organometallic mechanisms will be discussed in
the context of homogeneous catalytic systems currently being used in organic synthesis (e.g. cross coupling,
olefin metathesis, asymmetric hydrogenation, etc.). Emphasis will be placed on developing an understanding of
the properties of transition metal complexes and their interactions with organic substrates that promote chemical
transformations.
Course Requirements:
Exams: 20 pts (each)
In class exams (three) will be given every 7-8 lectures. Although these exams will focus primarily on recent
lecture topics, they will be cumulative.
Exam I: October 10
Exam II: November 12
Exam III: December 12
Literature Discussions & Summaries: 20 pts
Three papers from the recent literature will be distributed in class on alternating weeks and will be posted on the
web. A one-page summary of one paper is due in section (JACS communication format recommended). All
papers will be discussed in section and a familiarity with each is expected and may be tested for on exams.
Literature summaries should clearly and succinctly convey the principal objective, results, and conclusions of
the paper. A detailed, step-wise mechanism of the transition metal mediated reaction must be p roposed
(preferably through figures) that describes the chemistry going on at the metal (d-electron count, complex
electron count, oxidation state, ligand association/dissociation, etc) and at the organic substrate. Summaries
submitted that exceed the 1 page limit will not be graded- no exceptions. No late summaries will be graded.
Final Project: 20 pts
Starting with a well-characterized transition metal complex from the inorganic literature, propose its
development into a viable catalytic system for application towards a synthetically useful process. NIH
postdoctoral fellowship style recommended. Length may not exceed 4 pages (including all figures and
references). Papers submitted that exceed the 4 page limit will not be graded- no exceptions. No late papers will
be graded. Due January 15th, 2003.
References
The majority of material in this course is drawn from the primary literature. References
are provided on the appropriate slides.
The following texts have been used as general reference guides in the preparation of these
lectures:
· C rabtree, R.H. T he Organometallic Chemistry of the Transition Metals; 3rd Edition;
Wiley: New York; 2001. (Available at the Harvard Coop).
· Huheey, J.E.; Keiter, E.A.; Keiter, R.L. Inorganic Chemistry: Principles of Structure
and Reactivity; 4th Edition; HarperCollins: New York; 1993.
· Co tton, F.A.; Wilkinson, G.; Murillo, C.A.; Bochmann, M. A dvanced Inorganic
Chemistry; 6th Edition; Wiley: New York; 1999.
· Collman, J.P.; Hegedus, L.S.; Norton, J.R.; Finke, R.G. Principles and Applications of
Organotransition Metal Chemistry; University Science: Mill Valley, CA; 1987.
· Hegedu s, L.S. Transition Metals in the Synthesis of Complex Organic Molecules;
University Science: Mill Valley, CA; 1994.
· Spessard, G.O.; Miessler, G.L. Organometallic Chemistry. Prentice Hall: Upper Saddle
River, NJ; 1996.
· Fleming, I. F rontier Orbitals and Organ ic Chemical Reactions. W iley: New York;
1976.
· Corey, E.J.; Cheng, X.-M. The Logic of Chemical Synthesis. Wiley: New York; 1989.
· Nicolaou, K.C.; Sorensen, E.J. Classics in Total Synthesis. VCH: Weinheim, Germany;
1996.
Non-Standard Journal Abbreviations
ACIEE
HCA
JACS
JOC
JOMC
OL
OM
TL
Angewandte Chemie International Edition (English)
Helvetica Chimica Acta
Journal of the American Chemical Society
Journal of Organic Chemistry
Journal of Organometallic Chemistry
Organic Letters
Organometallics
Tetrahedron Letters
M.C. White, Chem 153
Structure & Bonding -1-
Week of September 17, 2002
Organotransition Metal Chemistry
Organotransition Metal Chemistry (MCW definition): Transition metal mediated reactions that solve (or have potential to solve)
challenging problems in the synthesis of organic molecules.
Coordination Chemistry:
The chemistry of transition metal complexes that have
noncarbon ligands (Werner complexes). Classification
applies to the catalyst and all reaction intermediates.
R'(O)C
Organometallic Chemistry:
The chemistry of transition metal complexes that have
M-C bonds (organometallic complexes). Classification
applies to the catalyst and/or reaction intermediates.
+
OR
PPh 3
RO
O
RO
TiIV
O
R'
O
O
C(O)R'
O
O
TiIV
R'
OH
R
OTf
B(OH)2
t-BuOOH, 4Å MS
CH2Cl 2, -20oC
+
O
TiIV
O
O
TiIV CO2R
O
O
C(O)R'
O
t-Bu
proposed intermediate
R'
CO2Me
NCCH3
O
O
N
R
OR
RO
PPh3
Suzuki cross-coupling catalyst
Trost enyne cycloisomerization catalyst
R
R'(O)C
Ph3P
Ph3P
NCCH3
H3CCN
H3CCN
Sharpless titanium-tartrate
epoxidation catalyst
R
Pd
(PF6-)
Ru
OR
Ru
(PF6-)
N
Ph3P
proposed intermediate
CO2Me
Pd
Ph3P
proposed intermediate
R
R
O
OH
CO2Me
70-90% yield
94->98% ee
N
Sharpless JACS 1987 (109) 5765.
Trost JACS 2002 (124) 5025.
de Lera Synthesis 1995 285.
M.C. White, Chem 153
Structure & Bonding -2-
Week of September 17, 2002
Complexity Generating Reactions
Wender's [5+2] Cycloadditions
O
6
OC
Cl
CO
Rh
1
OC
3
1
Rh
Cl
CO
0.5 mol%
6
3
C4H 4Cl2, 80oC, 3.5h
OH
OH
90%
10
O
H
10
12
12
Wender OL 2001 (3) 2105.
Tandem Heck
O
Ph3 P
I
H
O
OAc
Pd
PPh3
AcO
10mol%
Ag2CO3, THF, reflux
TBSO
O
H
O
82%
OTBS
Overman JOC 1993 (58) 5304.
M.C. White, Chem 153
Structure & Bonding -3-
Week of September 17, 2002
Reactive Site Selectivity in Multifunctional
Molecules
No protecting groups used! The majority of the mass recovered after reaction termination was unreacted starting material.
OMe
OMe
O
O
OMe
N
O
O
H
O
H
O
H
OH
OH
MeO
PPh3
Cl
Cl
N
Ph
Ru
O
O
H
PPh3
O
10 mol%
CH2Cl2 , rt, 22h
49%
E:Z ; 1:1
O
OH
O
H
MeO
OMe
O
H
OH
HO
HO
HO
FK 506
MeO
H
OH
O
O
O
N
OH
O
O
H
OMe
O
OMe
Schreiber JACS 1997 (119) 5106.
M.C. White, Chem 153
Structure & Bonding -4-
Week of September 17, 2002
Asymmetric Catalysis
Nobel Prizein Chemistry 2001:
William S. Knowles, Ryoji Noyori,
K. Barry Sharpless
The Monsanto Process
Wilkinson : Investigations into the reactivity of (PPh3)RhCl uncovered its
high activity as a homogeneous hydrogenation catalyst. This was the 1st
MeO
homogeneous catalyst that compared in rates with heterogeneous
counterparts (e.g. PtO2).
AcO
Ph3P
CO2H
NHAc
PPh3
+
Rh
Ph3P
Cl
H2 (1 atm)
OMe
P
Wilkinson J.Chem. Soc. (A) 1966 1711.
BF4-
Rh
P
OMe
W. Knowles: Replacement of achiral PPh3 ligands with non-racemic
phosphines ((-)-methylpropylphenylphosphine, 69%ee) demonstrated that a
chiral transition metal complex could transfer chirality to a non-chiral
substrate during hydrogenation.
*
Pr(Ph)(Me)P
CO2H
*
P(Me)(Ph)Pr
Rh
*
Pr(Ph)(Me)P
H2
cat.
MeO
CO2H
CO2H
Cl
H2 (1 atm)
Knowles Chem. Commun. 1968, 1445.
H
NHAc
AcO
15 % ee
95% ee, 100 % yield
Electronically tuning the metal center and using a C2 symmetric, bidentate
chiral phosphine ligand led to highly enantioselective hydrogenations of
enamides (very good substrates for asymmetric hydrogenations). The
Monsanto Process (1974) that resulted is the 1st commercialized asymmetric
synthesis using a chiral transition metal complex. Asymmetric
hydrogenation is the key step in the industrial synthesis of L-DOPA (a rare
amino acid used to treat Parkinson's disease).
H3O+
CO2H
MeO
Royal Swedish Academy
of Sciences:www.kva.se
H
AcO
L-DOPA
NH2
M.C. White, Chem 153
Structure & Bonding -5-
1
Week of September 17, 2002
18
The Transition Metals
H
Transition metals (d-block metals):
elements that can have a partially filled d
valence shell. Typically group 4-10 metals.*
Li
Na
3
4
4s23d2
Sc
K
Y
Rb
Cs
Ti
4s23d3
V
6
4s13d5
Cr6
3d 4
5s24d2
3d 5
5s14d4
Zr
Nb
6s25d2
6s25d3
6s25d4
Hf
Ta
W
4d 4
La
5
5d 4
4d 5
5d 5
3d
5s14d5
7
4s23d5
Mn7
3d
8
4s23d6
9
10
Co9
3d
Ni
3d 10
3d
5s24d5
5s14d7
5s14d8
Mo
Tc
Ru
Rh
Pd
4d 6
4d 7
4d 8
4d 9
4d 10
6s25d5
6s25d6
6s25d7
6s15d9
Os
Ir
Pt
5d 6
Re
5d 7
5d 8
B
Ne
11
12
Al
Ar
Cu
Zn
Ga
Kr
Ag
Cd
In
Xe
Au
Hg
Tl
Rn
* d electrons in group
3 are readily removed
via ionization, those in
group 11 are stable and
generally form part of
the
core
electron
configuration.
5s04d10
5d 9
EARLY
He
4s23d8
4s23d7
Fe8
13
5d 10
LATE
valence (d) electron count:
Fe
4s2 3d6
for free
(gas phase)
transition metals: (n+1)s is
below (n)d in energy (recall:
n = principal quantum #).
for complexed transition
metals: the (n)d levels are
below the (n+1)s and thus get
filled first. note that group # =
d electron count
CO
OC Fe
N
Fe II
Cl
CO
CO
CO
N
N
3d6
Cl
3d8
for oxidized metals, subtract the oxidation
state from the group #.
M.C. White, Chem 153
Structure & Bonding -6-
Week of September 17, 2002
Transition Metal Valence Orbitals
(n+1)p orbitals
z
(n+1)s orbital
y
x
pz
px
s
py
· 9 Valence Orbitals: upper limit of 9 bonds may be formed. In most cases a maximum
of 6 σ bonds are formed and the remaining d orbitals are non-bonding. It's these
non-bonding d orbitals that give TM complexes many of their unique properties.
· 18 electron rule: upper limit of 18 e- can be accomodated w/out using antibonding
molecular orbitals (MO's).
(n)d orbitals
dz2
dx2-y 2
dxy
· dz2 and dx2-y2 orbital lobes located on the axes
· dxy, dxz, and dyz lobes located between the axes
dxz
dyz
· orbitals oriented 90o with respect to each other
creating unique ligand overlap possibilities
M.C. White, Chem 153
Structure & Bonding -7-
Week of September 17, 2002
Electron Counting
Step 1: Determine the oxidation state of the metal.
To do this, balance the ligand charges with an equal
opposite charge on the metal. This is the metal's formal
oxidation state.
Step 2: Determine the d electron count. Recall: subtract
the metal's oxidation state from its group #.
9
Co
H
3d9
OC
Ph2P
Rh
O P
O
CO
Rh
4d
O
Ir
5d9
To determine ligand charges, create an ionic model by
assigning each M-L electron pair to the more
electronegative atom (L).
This should result in
stable ligand species or ones known as reaction
intermediates in solution.
H
-1
OC
RhI
P
Ph2
O
O P
O
RhI = d8
9
CO
neutral (0)
Step 3: Determine the electron count of the complex
by adding the # of electrons donated by each ligand to the
metal's d electron count.
ligands: 10emetal: 8 ecomplex: 18 eH
2e-
RhI
CO
OC
P
Ph2
O
P
O
O
2e-
M.C. White, Chem 153
η1 ligands
Structure & Bonding -8Formal
charge
# of edonated
H (hydride)
-1
2
CH3 (alkyl)
-1
2
CO
0
2
X (halides)
-1
2
(monodentate):
-1
µ-X (bridging)
X
M
4
(2/metal)
M
-1
OR (terminal
2
-1
µ-OR (bridging)
M
OR2 (ether)
0
2
O 2 (superoxide)
-1
2
O (terminal oxo)
-2
4
µ-O (bridging)
-2
O
to the metal
η2-alkyl
peroxo
terminal oxo
t-Bu
O
O
V
O
O
OR
O
O
V
O
OR
O
η1-alkyl peroxo
Proposed intermediates in VO(acac)2 catalyzed directed epoxidation of
allylic alcohols.
Sharpless Aldrichimica Acta 1979 (12), 63.
µ): the ligand bridges 2 or more metals
Bridging ligands (µ
linear µ-oxo
4
(2/metal)
M
PR2 (phosphide)
-1
2
PR3 (phosphine)
0
2
NR2 (amide)
-1
2
NR3 (amine)
0
2
imines
0
2
nitriles
0
2
+1
2
NO (nitrosyl )
η x): The number of atoms (x) in the ligand binding
Hapticity (η
4
(2/metal)
R
O
M
η1-Ligands
t-Bu
alkoxide)
M
Week of September 17, 2002
linear
N
N
N
Cl
Fe
N
N
O
Cl
Fe
N
N
N
Nishida Chem. Lett. 1995 885.
M.C. White, Chem 153
Structure & Bonding -9-
Week of September 17, 2002
Electron Counting
PPh 3
Cl
Cl
RhI
Rh
Ph3P
PPh3
Ph3P
Wilkinson's catalyst
(Ph3P)3RhCl
PPh 3
Cl
Me
N
Me
N
ligands: 8emetal: d8, 8ecomplex: 16 e-
Brookhart polymerization
catalyst precursor
Brookhart JACS 1995 (117) 6414.
PPh3
O
Ru
Pd0
Pd
N
H2
P
Ar 2 Cl
Me
PdII
PPh3
H2
N
N
Me
Pd
ligands: 8emetal: d8, 8ecomplex: 16 e-
O
Ar 2
P
N
PPh 3
Ph3P
PPh3
Ph3P
Ph3P
Ph3P
Palladium "tetrakis" triphenylphosphine
cross coupling catalyst
Noyori hydrogenation
catalyst
PPh 3
ligands: 8emetal: d10, 10ecomplex: 18 e-
O
Cl
Ar 2
P
Ru
N
N
H2
N
O
N
II
Cl
Noyori JACS 1998 (120) 13529.
N
N
N
H2
P
Ar 2
Fe
OTf
OTf
ligands: 12emetal: d6, 6ecomplex: 18 e-
Olefin dihydroxylation
catalyst
Que JACS 2001 (123) 6722.
N
FeII
N
N
OTf
OTf
ligands: 12emetal: d6, 6ecomplex: 18 e-
M.C. White, Chem 153
Structure & Bonding -10-
Week of September 17, 2002
Unsaturated Ligands
η1-coordination
M
Formal
charge
# of edonated
-1
2
ηx-coordination
# of edonated
0
6
0
2
0
2
-1
6
-1
4
-1
4
M
η6-arene
η 1-aryl
-1
2
M
1
η -alkenyl
M
η2-alkene
R
R
Formal
charge
M
-1
H
2
η1-alkynyl
M
η2-alkyne
H
M
η 1-Cp (cyclopentadienyl)
-1
2
η5-Cp
M
-1
2
η 1-allyl
M
M
(cyclopentadienyl)
M
=
M
η3-allyl
O
O
O
η1-acetate
M
M
-1
2
O
η2-acetate
M.C. White, Chem 153
Structure & Bonding -11-
Week of September 17, 2002
Electron Counting II
Cp*
H
H
P(Cy)3
P(Cy)3
O
Ir
O
P(Cy)3
O
H
CF3
Ir
III
CF3
Rh
O
H
H
H
Me3P
Crabtree JACS 1987 (109) 8025.
Bergman:
direct observation
of C-H-> C-M
ligands: 12emetal: d6, 6ecomplex: 18 e-
Bergman OM 1984 (3) 508.
Zr
Cl
Cl
ZrIV
Cl
Cl
S
Ru
Cl
Cl
CH 3
Ru-Ru bond = 2 enote: metal oxidation
state doesn't change
Brintzinger catalyst
Brintzinger JOMC 1985 (228) 63.
ligands: 16emetal: d0, 0ecomplex: 16 e-
S
Ru
Ru
S
ligands: 12emetal: d6, 6ecomplex: 18 e-
CH 3
CH 3
Cl
H
Me3 P
P(Cy)3
Crabtree's dehydrogenation
catalyst
H
Rh III
Hidai catalyst for
propargylic substition
Hidai JACS 2002 (124) 7900
III
RuIII
S
Cl
CH 3
Ru 1
ligands: 12 emetal: d5, 5eRu 2: 1 ecomplex: 18 e-
Ru 2
ligands: 12 emetal: d5, 5eRu 1: 1 ecomplex: 18 e-
M.C. White, Chem 153
Structure & Bonding -12-
Week of September 17, 2002
Weakly Coordinating Counterions
Common weakly coordinating counterions used in organotransition metal catalysis to generate
cationic catalysts:
Weakly coordinating anions generally
TfO-< ClO4- < BF4- < PF6- < SbF6- < BAr'4 (B[3,5-C6 H3(CF3 )2]4 )
More weakly coordinating
Synthesis
Metathesis: Ag (I) halide abstraction. Most general approach for the introduction of
weakly coordinating counterions.
have: 1. low charge, 2. high degree of
charge
delocalization
(i.e.
no
individual
atom
has
a high
concentration of charge), 3. steric bulk.
The least coordinating anion:
hexahalocarboranes (CB11H6X6-)
2+
N
N
Me
N
Fe
N
Me
+
Cl 2 equiv. Ag SbF6
CH3CN
Cl
Me
N
Fe
N
Me
NCCH3
(SbF6-)2
NCCH3
N
N
note: neutral solvent
Jacobsen JACS 2001 (123) 7194.
replaces L- in rxn.
Protonolysis
Ar
Ar
Me
N
Ni
N
Ar
Me
H +(OEt2)2 BAr'4Et2O
N
Me
(BAr'4-)
Ni
N
+
OEt2
Ar
Brookhart JACS 1999 (121) 10634.
Strem: Silver hexabromocarborane
(Ag +CB11H6Br6-) 1g = $594
Strauss Chem. Rev. 1993 (93) 927.
Reed Acc. Chem. Res. 1998 (31) 133.
M.C. White, Chem 153
Structure & Bonding -13-
Week of September 17, 2002
Electron Counting III
COD = 1,5-cyclooctadiene
weakly coordinating anion
does not contribute to the
electron count for complex
+
P(Cy) 3
Ir
(PF6-)
Crabtree's catalysts
for hydrogenations
+
+
IrI
P(Cy) 3
PF6
(PF6-)
Ru
H3CCN
H3CCN
N
N
+
RuII
NCCH 3
ligands: 12 emetal: d6, 6ecomplex: 18 e-
ligands: 8 emetal: d8, 8ecomplex: 16 e-
review: Crabtree Acct. Chem. Res. 1979 (12) 331.
PF6
NCCH3
CH3CN
CH3CN
1st synthesis:Mann OM 1982 (1) 485.
catalytic enyne cycloisomerizations:Trost JACS 2002 (124) 5025.
3+
BPh3
BPh3
N
N
Rh+
RhI
Me
N
N
O
Me
NBD = norbornadiene
N
"Zwitterionic complex"
used in hydroformylations
ligands: 10 emetal: d8, 8ecomplex: 18 e-
1st synthesis: Schrock and OsbornInorg. Chem. 1970 (9) 2339.
hydroformylation: Alper Chem. Comm. 1993, 233.
N
O
Fe
Me
Fe
O
(SbF6-)3
N
N
Me
epoxidation catalyst
Question:
Jacobsen JACS 2001 (123) 7194.
Fe 1
ligands: x emetal: dx, 5ecomplex: x e-
Fe 2
ligands: x emetal: dx, xecomplex: x e-
M.C. White, Chem 153
Structure & Bonding -14-
Week of September 17, 2002
Common Geometries for TM Complexes
Coordination number (CN):The
number of ligands (L) bonded to
the same metal (M).
CN = 4 ,ML 4:
L
Sterics. to a 1st approximation,
geometry of TM complexes
determined by steric factors
(VSEPR -valence shell electron
pair repulsion). The M-L bonds
are arranged to have the
maximum possible seperation
around the M.
109.5o
M
L
L
tetrahedral
L
L
180
L
90o
L
Lax
CN = 3, ML 3
L
L
trigonal planar
L
L
L 180o, trans
L
square planar
CN = 6, ML 6:
180o, trans
120o
L
M
trigonal bipyramidal
linear
M
90o, cis
Leq
L
L
90o
T-shaped
Leq
M
120o
M
L
Leq
o
M
L
CN = 5, ML 5:
Lax
CN = 2, ML2:
Electronics: d electron count combined
with the complex electron count must be
considered when predicting geometries for
TM complexes with non-bonding d
electrons. Often this leads to sterically less
favorable geometries for electronic reasons
(e.g. CN = 4, d8 , 16 e- strongly prefers
square planar geometry) .
Lapical
90o, cis
L
M
L
L
L
octahedral
L
L
M
~90 o
~90-100o
Lba sal
Lba sal
square pyramidal
M.C. White, Chem 153
Structure & Bonding -15-
Week of September 17, 2002
MO Description of σ bonding in ML6
L
Metal Valence
Orbitals
L M
L
L
L
L
Linear Combinations of
Ligand σ Donor Orbitals
t1u
pz
px
18 e- Rule:
The octahedral geometry is strongly favored
by d6 metals (e.g. Fe (II), Ru (II), Rh(III)). A
stable electronic configuration is achieved at
18 e-, where all bonding (mostly L character)
and non-bonding orbitals (mostly M d
character) are filled.
py
a1g
σ*
LUMO
s
z
L
y
∆
eg
L
L
L
L
L
x
dz2
eg
t2g
dx2-y2
t2g
n
HOMO
2 nodes
t2g
eg
dxy
dxz
dyz
Mulliken symbols: in an octahedral
enviroment, the degenerate d orbitals split into
orbitals of t2g and eg symmetries. Orbitals with
different symbols have different symmetries
and cannot interact.
1 node
t1u
σ
a1g
0 node
Albright Tetrahedron 1982 (38) 1339.
M.C. White/ Q. Chen, Chem 153
Structure & Bonding -16-
Week of September 17, 2002
Octahedral
CO
H
RuII
94.37o
Ph3P
101.35o
H
PPh3
91.21o
PPh3
metal: d6 , 6 ecomplex: 18 e-
Bond angles (o)
Bond Lengths (Å)
C1-Ru-P2: 91.21
P3-Ru-P2: 102.78
P1-Ru-P2: 101.35
H2-Ru-P2: 94.37
Ru-H1: 1.590
Ru-H2: 1.651
Ru-C1: 1.893
Ru-P1: 2.324
Ru-P2: 2.311
Ru-P3: 2.401
H1-Ru-P2: 176.77
P1-Ru-P3: 147.86
H2-Ru-C1: 173.13
Ru(H)2(PPh3)3(CO)
ligands: 12 e-
M.C. White, Chem 153
Structure & Bonding -17-
Week of September 17, 2002
MO Description of σ bonding in ML4 square planar
L M
L
Metal ValenceOrbitals
L
L
Linear Combinations of
Ligand σ Donor Orbitals
a2u
16 e - Rule:
The square planar geometry is favored by d8
pz
a2u
eu
eu
px
py
a1g
a2u
metals (e.g. Ni (II), Pd (II), Pt(II), Ir (I), Rh(I)).
A stable electronic configuration is achieved at
LUMO
σ*
16 e-, where all bonding and non- bonding
orbitals are filled. Spin-paired compounds
display diamagnetic behavoir (i.e. weakly
repelled by magnetic fields) and may be
readily characterized by NMR.
a1g
s
a1g
When combining orbitals, the resulting
MO's must be symmetrically dispersed
between bonding and antibonding.
y Thus, combining 3 orbitals (i.e. a1g's)
requires one of the orbitals to be nonx bonding.
n
HOMO
a1g
b1g
eg
b2g
dz2
L
L
eg
b2g
b1g
n
dx2-y2
b1g
eg
dxz
dyz
b2g
dxy
In a square planar ligand field the
degenerate d orbitals split into
orbitals of a1g, b 1g, eg, and b2g
symmetries. The degenerate p
orbitals split into orbitals of eu and
a2u symmetries.
eu
σ
a1g
L
L
M.C. White/ Q. Chen, Chem 153
Structure & Bonding -18-
Week of September 17, 2002
Square planar
91.42 o
OC
92.07o
Rh
PPh3
87.53 o
I
Ph3P
Cl
89.12
o
Bond angles (o)
ligands: 8 emetal: d8 , 8 ecomplex: 16 e-
Bond lengths (Å)
cis
P1-Rh-C1: 92.07
C1-Rh-P2: 91.42
P2-Rh-Cl1: 87.53
P1-Rh-Cl1: 89.12
trans
P1-Rh-P2: 176.09
C1-Rh-Cl1: 175.45
Rh(CO)(Cl)(PPh3)2
Rh-P1: 2.327
Rh-P2: 2.333
Rh-C1: 1.820
Rh-Cl1: 2.395
M.C. White/ Q. Chen, Chem 153
Structure & Bonding -19-
Week of September 17, 2002
Distorted square planar
84.45 o
Cl
85.28o
Ph3P
Rh
I
97.73
o
PPh3
96.45 o
ligands: 8 emetal: d8 , 8 e-
PPh3
complex: 16 e-
Bond Angles (o)
cis
P1-Rh-Cl1: 85.28
Cl1-Rh-P3: 84.45
P3-Rh-P2: 96.45
P1-Rh-P2: 97.73
trans
Cl1-Rh-P2: 166.68
P1-Rh-P3: 159.03
Wilkinson’s catalyst
(Ph3P)3RhCl
Bond Lengths (Å)
Rh-P1: 2.305
Rh-P2: 2.224
Rh-P3: 2.339
Rh-Cl1: 2.405
Steric bulk of PPh3 ligands results
in significant bond angle distortion
from ideal square planar.
M.C. White, Chem 153
Structure & Bonding -20-
Week of September 17, 2002
MO Description of σ bonding in ML4 tetrahedral
L
L
L
Metal Valence
Orbitals
Linear Combinations of
Ligand σ Donor Orbitals
M
L
The tetrahedral geometry is electronically
LUMO
t2
σ*
t2
pz
px
favored by d 4 or d10 metal complexes where
the non-bonding orbitals are either 1/2 or
entirely filled, respectively.
py
a1
a1
n
HOMO
s
y
L
L
L
e
x
dz
2
2 2
dx -y
e
e
t2
n
L
t2
t2
dxz
dyz
dxy
σ
a1
M.C. White/ Q. Chen, Chem 153
Structure & Bonding -21-
Week of September 17, 2002
Tetrahedral
PPh3
108.8o
ligands: 8 ePd0
Ph3P
Ph3P
PPh3
metal: d10, 10 ecomplex: 18 e-
Bond angles (o)
P1-Pd-P2a: 108.79
P2-Pd-P2a: 110.14
Bond lengths (Å)
Pd-P1: 2.427
Pd-P2: 2.458
Palladium “tetrakis”
Pd(PPh3)4
M.C. White, Chem 153
Structure & Bonding -22-
Week of September 17, 2002
MO Description of σ bonding in ML4 tetrahedral
L
L
L
Metal Valence
Orbitals
L
LUMO
t2
σ*
t2
pz
px
Linear Combinations of
Ligand σ Donor Orbitals
M
py
a1
a1
n
d8 metal complexes may adopt a tetrahedral
geometry for steric reasons (i.e. L very large or
M very small). These complexes have
diradical character and are unstable (generally
in equilibrium with square planar geometry).
These compounds exhibit paramagnetic
behavoir (i.e. unpaired electrons are attracted to
magnetic fields) making NMR's difficult to
interpret.
HOMO
s
L
y
L
L
e
x
dz2
2
2
dx -y
e
e
t2
n
L
t2
t2
dxz
dyz
dxy
σ
a1
M.C. White, Chem 153
Structure & Bonding -23-
Week of September 17, 2002
Ligand sterics
Ligands
Cone angle*
θ (ο )
Ligands
R
87
3o amines
NH3
94
PF3
104
NMe3,
132
P(OMe)3
107
quinuclidine,
PMe3
118
NMe2Et
PCl3
124
NMeEt2
145
125
NEt3
150
NPr3
160
NPh3
166
NEt2Ph
170
NBz3
210
N(i-Pr)3
220
phosphines
PH3
Ph2P
PPh 2
PPhMe2
127
R
Cone angle*
θ (ο)
R
P
average of Ni-P bond
2.28 Å lengths obtained from
crystal data
θ
PEt3
132
PPh2Me
136
PPh2Et
140
PPh2Pr
140
PPh3
145
others
PPh2Cy
153
H
75
PPhCy2
161
Me
90
PCy3
170
CO
95
P(t-Bu)3
182
Cp
136
P(o-tol) 3
194
P(mesityl)3
212
M
Tolman Chem. Rev. 1977, 77, 313.
R
R
R
N
θ
M
average of Pd-N bond
2.2 Å lengths obtained from
crystal data
Trogler JACS 1991, 113, 2520.
∗θ values measured using strain-free CPK model of
M(L). For ligands with many internal degrees of
freedom, the values do not account for distortions in
geometry due to contacts with other atoms in the
complex. Very valuable as a relative scale.
M.C. White, Chem 153
Structure & Bonding -24-
Week of September 17, 2002
Effect of ligand sterics on structure
cis-trans isomerization
L
Pt
L
Cl
Cl
K
L
Pt
L
Cl
Cl
trans
cis
Most common cis/trans isomerization in MX2L 2 complexes
where M= Pd, Pt. The trans/cis ratio is favored by bulkier L
(large θ).
square planar/tetrahedral isomerization
Ligand Cone angle
[tetrahedral]
ο
K
=
θ( )
[sq. planar]
109.5 o L
Cl
L
Ni
K
Cl
L
o
90
Cl
L
Ni
Cl
PPhMe2
136
1.78
PEtPh2
140
2.03
PPrPh2
140
2.33
square planar:
electronically favored
PPh 3
145
>>>
for C.N.=4, d8
PPh 2Cy
153
2.45
PPhCy2
161
0.14
PCy3
170
0.00
Ni(II) smaller cation --> ligands always trans.
Increasing the size of L (or X) may lead to a
tetrahedral distortion to relieve steric strain. If the θ of
L becomes too large, severe steric repulsion of L with
L will favor going back to square planar.
Tolman Chem. Rev. 1977 (77) 313.
Pignolet Inorg. Chem. 1973 (12) 156.
tetrahedral:
sterically favored
M.C. White/ Q. Chen Chem 153
Structure & Bonding -25-
Week of September 17, 2002
Effect of ligand sterics on coordination number
Generalizations about CN:
Low CN favored by:
1. Low oxidation state (e- rich) metals.
2. Large, bulky ligands.
High CN favored by:
1. High oxidation state (e- poor) metals.
2. Small ligands.
Ph(But)2P
Pd0
P(t Bu)2Ph
176.51o
Pd(PtBu2Ph)2
Bond length (Å)
ligands: 4 emetal: d10 , 10 ecomplex: 14 e-
Although Pd(PtBu2Ph)2 is
coordinatively unsaturated
electronically, the steric bulk
of PtBu2Ph ligands prevents
additional ligands from
coordinating to the metal.
Pd-P1: 2.251
Bond angle (o)
P1-Pd-P1a: 176.51
Otsuka JACS, 1976 (98)5850.
M.C. White, Chem 153
Structure & Bonding -26-
σ-bonding
Week of September 17, 2002
MO Description of σ−bonding
in an octahedral complex
z
z
ligand σ-bonding
orbitals
Metal d
orbitals
L
M
y
x
L
M
y
LUMO
x
Best Overlap
σ*
Worst overlap
best shape complementarity
eg
Pure σ-donors
t2g
HOMO
R
M
C
M
H
Alkyl
Hydride
R
R
σ
R
N
M
3o Amines
R
R
Conclusion:
The energy of the LUMO is directly affected
by M-L σ bond strength. Weak bonds will
have low-lying LUMO's making the metal
more electrophilic.
M.C. White, Chem 153
Structure & Bonding -27-
Week of September 17th, 2002
Periodic table trends:electronegativity
13
14
1
2
H
15
16
17
increasing electronegativity
2.2
TRANSITION METALS (TM)
Li
Be
B
C
N
O
F
1.0
1.6
2.0
2.5
3.0
3.4
4.0
Mg
0.9
1.3
K
Ca
3
4
5
6
7
8
9
10
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
1.8
1.9
1.9
0.8
1.0
1.3
1.5
1.6
1.6
1.6
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
0.8
1.0
1.2
1.3
1.6
2.1
1.9
2.2
2.3
2.2
Cs
Ba
La*
Hf
Ta
W
Re
Os
Ir
0.8
0.9
1.1
1.5
2.3
1.9
2.2
2.2
The electronegativity of the
elements increases substantially
as in progressing from left to
right (EM to LM) across the
periodic table.
H
H
H
H
H
N
increasing electronegativity
Si
P
S
Cl
1.9
2.2
2.6
3.1
Cu
Zn
Ga
Ge
As
Se
Br
1.9
1.7
1.8
2.0
2.2
2.5
2.9
Ag
Cd
In
Sn
Sb
Te
I
1.9
1.7
1.6
1.8
2.0
2.1
2.6
Au
Hg
Tl
Pb
Bi
Po
At
2.5
2.0
1.6
1.9
2.0
2.0
2.2
Whereas the electronegativity of main group elements
increases in progressing up a column, that of the TM
increases in progressing down.
Pauling The Nature of the Chemical Bond, 3rd Ed.;1960
H
N
N
H
H
3+
N
N
H
H
H
Al
1.6
H
H
H
12
H
H
H
Co
H
H
Pt
2.3
11
increasing electronegativity
Na
LATE (LM)
N
H
Electrostatic Model
H
H
increasing electronegativity
EARLY (EM)
The most accurate description of
σ-bonding in TM complexes lies
somewhere in between the 2 extremes and
depends in large part on the relative
electronegativities of the metal and ligands
H
N
H
H
N
N
H
H
N
H
3-
Co
H
H
N
H
H
H
H
N
H
Covalent Model
H
M.C. White, Chem 153
Structure & Bonding -28-
Week of September 17th, 2002
Electronegativity II
1
13
2
H
14
15
16
17
increasing electronegativity
2.2
TRANSITION METALS (TM)
Li
Be
B
C
N
O
F
1.0
1.6
2.0
2.5
3.0
3.4
4.0
Mg
1.3
3
4
5
LATE (LM)
6
7
8
9
10
12
11
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
0.8
1.0
1.3
1.5
1.6
1.6
1.6
1.8
1.9
1.9
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
0.8
1.0
1.2
1.3
1.6
2.1
1.9
2.2
2.3
2.2
Cs
Ba
La*
Hf
Ta
W
Re
Os
Ir
0.8
0.9
1.1
1.5
2.3
1.9
2.2
2.2
increasing electronegativity
K
Pt
2.3
Al
1.6
Si
P
S
Cl
1.9
2.2
2.6
3.1
Cu
Zn
Ga
Ge
As
Se
Br
1.9
1.7
1.8
2.0
2.2
2.5
2.9
Ag
Cd
In
Sn
Sb
Te
I
1.9
1.7
1.6
1.8
2.0
2.1
2.6
Au
Hg
Tl
Pb
Bi
Po
At
2.5
2.0
1.6
1.9
2.0
2.0
2.2
increasing electronegativity
Na
0.9
EARLY (EM)
increasing electronegativity
HML n
+ MLn
H
O
IV
Zr
Cl
H
+
IV
Zr
EtO
R
Cl
R
Schwartz's reagent
adds H-Zr across alkenes and alkynes
(hydrozirconation). incompatible with
most carbonyls b/c of hydridic properties.
H
O
OEt
H
Labinger ACIEE 1976 (15) 333.
Ionic bonding is greater when orbitals of unequal
electronegativities interact. M-L σ-bonding in
electropositive metals (e.g. early metals) has
significant ionic character.
H· + ·MLn
HML n
(easier to break heterolytically)
OC
RhI
H
O
(easier to break homolytically)
OR
EtO2C
OR
O
EtO2C
O
intermediate in catalytic
hydroformylation
of alkenes
Leighton JACS 2001 (123) 11514.
Covalent bonding is greater when orbitals of similar
electronegativities interact. Therefore, M-L σ-bonding
in electronegative metals (e.g. late metals) is primarily
covalent in nature.
M.C. White, Chem 153
Structure & Bonding -29-
Week of September 17th, 2002
σ-bonding
H
H
H
H
H
N
EB ∝
bonding
energy
H
H
N
N
H
H
3+
Co
H
H
N
N
H
H
H
H
EI
+ EC
ionic
covalent
bonding bonding
H
H
H
N
H
H
N
N
H
H
N
H
3-
Bond strength in polarized M-L bonds results
from a gain in covalent and ionic bonding energy.
The degree to which each type of bonding
influences bond strength is highly dependent on
the relative electronegativities of the metal and
ligands.
H
N
H
Co
H
H
N
H
H
H
H N
H
H
Covalent Model
Electrostatic Model
Electrostatic Model: Ionic Bonding
Covalent Model
LUMO
M
ML σ∗
M+
EI ∝
−(QMQ L)
Q = charge density
EI
− (QMQ L)∝ − (εM-εL)
L
L-
EI ∝ (εM-εL)
increasing energy
increasing ionization potential ( ε)
H
H
H
EC
LUMO
M
HOMO
EI
L
EC
ML σ
∝ orbital overlap
(EM-EL )
EM ∝ 1 EL∝ 1
εM
εL
EC
∝ orbital overlap
(εM-εL)
HOMO
Ionic bonding is greater when elements of high and opposite charge
interact. Differences in charge are paralleled in differences in
electronegativities. Large differences in electronegativity favor strong
ionic bonding. M-L σ-bonding in early metals has significant ionic
character.
Fleming Frontier Orbitals and Organic Chemical Reactions, 1976.
Pauling The Nature of the Chemical Bond, 3rd. Ed.; 1960.
Covalent bonding is greater when orbitals of similar energies
interact. The energy of atomic orbitals is inversely proportional to
the element's electronegativity (i.e. the orbital energy of an
electronegative element is lower than that of a electropositive
element). Small differences in electronegativity favor strong
covalent bonding. M-L σ-bonding in late metals has a high degree
of covalent bonding.
M.C. White, Chem 153
Structure & Bonding -30-
Week of September 17th, 2002
Periodic table trends II: hard/soft
HARD
nucleophile
SOFT
nucleophile
1
H
13
14
15
16
17
increasing electronegativity/
decreasing orbital energy
2
2.2
HARD
electrophile
Na
Mg
0.9
1.3
K
Ca
Sc
Ti
0.8
1.0
1.3
1.5
Y
Zr
Nb
Rb
3
4
1.0
1.2
1.3
Cs
Ba
La*
Hf
0.8
0.9
1.1
0.8
EARLY
(EM)
5
SOFT
electrophile
6
7
8
V
Cr
Mn
Fe
Co
Ni
1.6
1.6
1.6
1.8
1.9
1.9
Mo
Tc
Ru
Rh
Pd
2.2
9
2.3
1.6
2.1
1.9
Ta
W
Re
Os
Ir
1.5
2.3
1.9
2.2
2.2
increasing electronegativity/
decreasing orbital energy
Hard nucleophiles (ligand): have a low energy HOMO
with high charge density (negative charge).
Hard electrophiles (metal) : have a high energy LUMO
with high charge density (positive charge).
Hard (L) - Hard (M) interaction: is predominantly ionic
in character. It is favorable because of strong Coulombic
attraction.
10
2.2
Pt
2.3
11
12
B
C
N
O
F
2.0
2.5
3.0
3.4
4.0
Al
1.6
Si
P
S
Cl
1.9
2.2
2.6
3.1
Cu
Zn
Ga
Ge
As
Se
Br
1.9
1.7
1.8
2.0
2.2
2.5
2.9
Ag
Cd
In
Sn
Sb
Te
I
1.9
1.7
1.6
1.8
2.0
2.1
2.6
Au
Hg
Tl
Pb
Bi
Po
At
2.5
2.0
1.6
1.9
2.0
2.0
2.2
increasing electronegativity
Be
1.6
increasing electronegativity
Li
1.0
LATE
(LM)
Soft nucleophiles (ligand): have a high energy HOMO
with low charge density.
Soft electrophiles (metal) : have a low energy LUMO
with low charge density.
Soft (L) - Soft (M) interaction: is predominantly covalent in
character. It is favorable because of small ∆E between the
HOMO of the ligand and the LUMO of the metal (EM-EL ).
Fleming Frontier Orbitals and Organic Chemical Reactions, 1976.
M.C. White, Chem 153
Structure & Bonding -31-
Week of September 17th, 2002
Periodic table trends II: hard/soft
Hard/Soft: in part accounts for the extraordinary
functional group tolerance of late transition metal
complexes towards organic functionality.
1
increasing electronegativity/
decreasing orbital energy
2
Li
1.0
Be
1.6
Na
0.9
Mg
1.3
3
4
5
6
7
8
9
10
K
0.8
Ca
1.0
Sc
1.3
Ti
1.5
V
1.6
Cr
1.6
Mn
1.6
Fe
1.8
Co
1.9
Ni
1.9
0.8
1.0
Y
1.2
Zr
1.3
Nb
1.6
Mo
2.1
Tc
1.9
Ru
2.2
Rh
2.3
Pd
2.2
Cs
0.8
Ba
0.9
La*
Hf
Ta
1.5
W
2.3
Re
1.9
Os
2.2
Ir
2.2
Pt
2.3
HARD
electrophile
EARLY
(EM)
increasing electronegativity/
decreasing orbital energy
increasing electronegativity
1.1
SOFT
electrophile
B
2.0
C
2.5
N
3.0
O
3.4
F
4.0
11
12
Al
1.6
Si
1.9
P
2.2
S
2.6
Cl
3.1
Cu
1.9
Zn
1.7
Ga
1.8
Ge
2.0
As
2.2
Se
2.5
Br
2.9
Ag
1.9
Cd
1.7
In
1.6
Sn
1.8
Sb
2.0
Te
2.1
I
2.6
Au
2.5
Hg
2.0
Tl
1.6
Pb
1.9
Bi
2.0
Po
2.0
At
2.2
LATE
(LM)
Nicolaou JACS 1993 (115) 4419.
Nicolaou's Rapamycin Synthesis: Note* last step!!!
O
OCH3 O
SnBu3
+
Bu3Sn
I
O
OH
O
Cl
PdII
N
H
O
OH
O
OMe
NCCH3
Cl
O
NCCH3
OMe
OH
DMF, THF
25oC, 24h
N
H
O
(i-Pr)2NEt
H
OH
O
OCH3 O
20 mol%
O
I
increasing electronegativity
H
2.2
Rb
HARD
nucleophile
SOFT
nucleophile
O
O
OH
28%
O
OMe
H
OMe
OH
M.C. White, Chem 153
Structure & Bonding -32-
Week of September 17th, 2002
M-C Bond Strengths
Me3P
X
120
X
Bergman's C-H activation
complex
110
Ir-X bond dissociation enthalpies for (η5-Me5C5)(PMe3)Ir(X)2
X
DIr-X
(kcal/mol)
D(H-X) kcal/mol
IrIII
M-C Bond Strength Trends: the trends in M-C σ
bond strengths generally parallel those found in H-Cσ
bond strengths.
Ph
Me
Vy
100
H
Neopentyl
Pentyl
Cy
Ph
82
90
H
74
Vy
71
Pentyl
58
Me
56
Cy
52
Neopentyl
48
40
50
60
70
D(Ir-X) kcal/mol
80
90
As in C-H σ bonding, there is a general trend towards weaker M-C
with increased substitution. Large deviations occur when the alkyl
group is very bulky or when it is methyl. Bulky ligands like
neopentyl are thought to destabilize the M-C bond because of steric
hinderance, making it much weaker than the correlation would
predict. There is a strong thermodynamic preference to form the
sterically less hindered M-C bond.
sp C-M > sp 2 C-M > sp3 C-M
1o C -M > 2o C-M > >> 3o C-M
As in C-H σ bonding, an increase in % s character of the carbon
strengthens the M-C σ bond because of better orbital overlap. The
correlation between C-H and M-C (C = aryl, vinyl) BDE's is not
perfect with M-C bonds being stronger than predicted because of
π-bonding with the metal.
Bergman Polyhedron 1988 (7) 1429.
M.C. White, Chem 153
Structure & Bonding -33-
Six valence metal orbitals that participate in σ-bonding in
an octahedral complex along the x,y, and z axes.
Week of September 17th, 2002
σ and π bonding in ML6
z
y
σ-bonding
x
dz2
z
dx2-y2
s
L
M
y
x
pz
px
py
Three valence metal orbitals that may participate in π-bonding in
an octahedral complex with ligands that have orbitals of matching
symmetry (i.e. p, d, π, π*).
π-bonding
z
M
y
x
dxy
dxz
dyz
L
M.C. White, Chem 153
Structure & Bonding -34-
Week of September 17th, 2002
σ and π donors
z
MO Description for M-L π-donor
system in an octahedral complex
z
σ−complex
M
M
y
y
x
Best overlap
x
Worst overlap
1o , 2o Amines
eg*
σ*
Alkoxides
R
N
M
LUMO
LUMO
∆
σ-bonding: Lsp2 -> Mdσ
π-donation: Lp -> Mdπ
Halides
π∗
O
M
HOMO
R
R
σ-bonding: Lsp2 -> Mdσ
π-donation: Lp -> Mdπ
t2g
HOMO
t2g
other π-donors
O-
M
ligand π-bonding
orbitals
π
O
Cl
acac (acetylacetonate)
or I-, Br -, F-
N
Cp
N
O- -O
σ-bonding: Lsp2 -> Mdσ
π-donation: Lp -> Mdπ
salen
benzene
Conclusion:
The energy of the HOMO is directly affected by M-L π
bonding. Ligand to metal π donation increases the energy
of the HOMO making the metal more basic. π-donor
ligands stabilize electron poor, high oxidation state metals.
Very prevalent for early TM complexes (low d electron
count) and less so for late TM (high d electron count).
M.C. White, Chem 153
Structure & Bonding -35-
Week of September 17th, 2002
Oxidation state formalism
Electroneutrality principle (Pauling): "stable complexes are those with structures such that each atom has only
a small electric charge." Stable M-L bond formation generally reduces the positive charge on the metal as well
as the negative charge and/or e- density on the ligand. The result is that the actual charge on the metal is not
accurately reflected in its formal oxidation state.
Pauling The Nature of the Chemical Bond, 3rd Ed.;1960, pg. 172.
R'(O)C
RO
O
N
N
III
Mn
t-Bu
O
O
O
t-Bu
Cl
t-Bu
V
O
RO
IV
O
R'
t-Bu
Jacobsen epoxidation catalyst
Mn (salen)
ligands: 10emetal: d4 ,4ecomplex: 14 e-
O
O
VO(acac)2 "vanadium acac"
epoxidation catalyst
ligands: 12 emetal: d1 ,1ecomplex: 13 e-
OR
O
TiIV
O
O
O
TiIV
O
C(O)R'
self-assembling
dimer based on
OR crystal structure.
R'
Sharpless titanium-tartrate
epoxidation catalyst
ligands: 12 emetal: d0 ,0ecomplex: 12 e-
Sharpless JACS 1987 (109) 1279.
The "18 electron rule" often fails for early transition metals. Formal oxidation state is not an accurate
description of electron density at the metal. Low oxidation state, early TM complexes are stabilized
via π-donation (i.e. a shifting of electron density from π-donor ligands to the metal). This in part
accounts for the extreme oxophilicity of early TM.
M.C. White, Chem 153
Structure & Bonding -36-
Week of September 17th, 2002
σ and π acceptors
MO Description for M-L π -acceptor
system in an octahedral complex
C
C
M
M
O
C
ligand π-bonding
orbitals
σ−complex
π∗
σ-bonding: L n -> Mdσ
π-backbonding: Md π -> Lπ*
σ-bonding: L π -> Md σ
π-backbonding: Md π -> Lπ*
LUMO
t2g*
LUMO
LUMO
H
eg
M
N
N
N
bpy
H
R'
σ-bonding: L σ-> Md σ
π-backbonding: Md π -> Lσ*
M
P
Orpen Chem. Comm. 1985, 1310.
Braga Inorg. Chem. 1985, 2702.
R
N
σ*
*
N
∆
phen
R'
N
R
CH3 CN, NO, N2 , CN-
∆
t2g
HOMO
Rationalization of M -> P backbonding is
controversial. The classic picture envokes
a Mdπ -> P 3d interaction. Quantum
mechanical calculations indicate that P-X
σ* orbitals play a major role.
Hybridization of phosphorus 3d and P-R
σ* resulting in π-acceptor orbitals has
been envoked.
π
HOMO
Conclusion:
Metal to ligand π donation (π backbonding) lowers the
energy of the HOMO making the metal less basic.
π-backbonding stabilizes electron rich, low oxidation state
metals. Very prevalent in late TM complexes.
M.C. White, Chem 153
Structure & Bonding -37-
Week of September 17th, 2002
π-backbonding
Phosphorus
Ligand (L)
P(t-Bu)3
CO v, cm-1
2056
R
O
P
Ni
C
R
PCy3
P(i-Pr)3
2059
P(NMe2 )3
2062
PMe3
2064
PPhMe2
2065
PBz3
2066
PPh 2Me
2067
PPh 3
2069
PPh 2(OEt)
2072
P(p-C6 H4Cl)3
2073
PPh(OEt)2
2074
P(OEt)3
2077
PH3
2083
PCl3
2097
PF3
2111
CO stretching frequencies measured for
Ni(CO)3L where L is PR3 ligands of
different σ-donor abilities. Free CO vibrates
at 2143 cm-1 .
R
The increase in electron density at the nickel from phosphine σ-donation is
dispersed through the M-L π system via π-backbonding. Much of the electron
density is passed onto the CO π* and is reflected in decreased v(CO) IR
frequencies which corresponds to weaker CO bonds.
v=
1
2πc
1/2
f
(Mx My)/(Mx+M y)
Recall: Band position in IR is governed by :
1. force constant of the bond (f) and
2. individual masses of the atoms (Mx and My).
Stronger bonds have larger force constants than
weaker bonds.
Tolman Chem. Rev. 1977 (77) 313.
M.C. White, Chem 153
Structure & Bonding -38-
Week of September 17th, 2002
π-acids: effect on the metal
NC
NC
O
MeO
MeO
LDA
Cr
OC
OC
Cr
CO
CO
OC
OC
π-acid
Cr(0), d6, 18e-
(±)-Acorenone B
Semmelhack JACS 1980 (102) 5926
CO's render the electron rich Cr metal electrophilic via strong π-backbonding. Complexation of
benzene with the electrophilic Cr(CO)3 fragment withdraws electon density from the aromatic ring
activating it towards nucleophilic attack.
acidic
H
H
π-acid
OC
C O
O
H
N
Ni
II
N
24oC
OC
H
Ni0
CoI
CO
Norton JACS 1987 (109) 3945.
+
other π-acids
F
N
N
no reaction without π acid
Yamamoto JACS 1971 (93)3350.
Acrolein is thought to act as a π-acid, withdrawing electron density from the
Ni(II) complex via π-backbonding and promoting elimination of the diethyl
fragment to reduce the metal.
pka < 1 H2O
CO
NC
CN
NC
CN
F
F
F
N
F3C
NO2
F
M.C. White, Chem 153
Structure & Bonding -39-
Week of September 17th, 2002
olefin-metal complexes
C
M
C
Dewar-ChattDuncanson Model
The balance of electron flow can be shifted predominantly in
one direction dependent on the electronic properties of the
metal. If the metal is electron withdrawing, M-L σ-bonding
predominates and withdraws electron density from the
π-bond of the olefin. This results in the induction of a δ+
charge on the olefin that activates it towards nucleophilic
attack.
Olefin-metal bonding is thought to
occur via a 2-way donor-acceptor
mechanism that involves σ-donation
from the bonding π-electrons of the
olefin to empty σ orbitals of the metal
and π-backbonding from the metal to
the empty π* orbitals of the olefin. Both
interactions are important in forming a
stable M-olefin complex
If the metal is electron donating (i.e. low oxidation state metals like Pd(0),
Ni(0), Pt(0)) π-backbonding predominates and the metal alkene complex
begins to approach a metallocyclopropane structure. In complexes involving
electropositive metals in low oxidation states, the metallocyclopropane
carbons are rendered nucleophilic as evidenced by their reaction with
electrophiles (i.e. aldehydes). Cp2Ti metallocyclopropane is a stable complex,
crystal obtained by Bercaw.
R
Cl
δ+
Pd
L
OH2
Cl
II
Pd
Cl
OH2
L
note: convention is to not
change formal oxidation state
of the metallocyclopropane.
II
H R
Cl R
Cp
σ donation>>
π-backbonding
II
H
Cp
R'CHO
Cp
δH
intermediates in Wacker oxidation (commercial production
of acetaldehyde)
R
δII
Ti
Ti
Cp
R
Cp
Cp
O
R'
H
π-backbonding >>
σ donation
TiIV
Bercaw JACS 1983 (105) 1136.
Takaya OM 1991 (10) 2731.
Powerful take-home message: the appropriate metal complex can invert the chemical behavior of an alkene.
M.C. White/M.W. Kanan Chem 153
Structure & Bonding -40-
Week of September 17th, 2002
Metallocyclopropanes
*
Cp
TiII
*
Cp
Ph3P
H
H
Pt0
H
H
Bercaw JACS 1983 (105) 1136
Ph3P
Cheng Canadian J. Chem. 1972 (50) 912.
M.C. White, Chem 153
Structure & Bonding -41-
Week of September 17th, 2002
Spectrochemical series
strong π acceptor L
strong π donor ligand
π-backbonding lowers the
energy of the HOMO and
thus increases the energy
difference ∆ between the σ*
and π metal orbitals.
Ligand to metal π donation
increases the energy of the
HOMO, making ∆ smaller.
recall: σ* orbitals.
LUMO
eg
LUMO
eg
LUMO
LUMO
eg
eg
∆
∆
t2g
HOMO
The energy difference
between the metal π and σ*
orbitals is often referred to
as the crystalfield splitting
and labeled ∆.
∆
∆
t2g
t2g
HOMO
recall: non-bonding
orbitals capable of π
bonding
HOMO
t2g
strong σ donor L
HOMO
Strong σ bonding orbitals are
low in energy and have
antibonding σ* orbitals that
are proportionally high in
energy .
Spectrochemical series: The colors of TM complexes often arrise from the absorption of visible light that corresponds to the
energy gap ∆. Electronic spectra (UV-vis) can often be used to measure∆ directly.
I - < Br - < Cl - < N3 -, F- < OH - < O2 - < H2O < NCS - < py, NH3 < en < bpy, phen < NO2 - < CH3 -, C 6H5- < CN- < CO,Hπ-donor
low ∆
"low field ligand"
π-acceptor/strong σ-donor
high ∆
"high field ligand"
M.C. White, Chem 153
Structure & Bonding -42-
Week of September 17th, 2002
High spin/low spin
high-spin/low-spin
If ∆ is low enough, electrons may rearrange to give a "high spin" configuration to reduce electron- electron repulsion that
happens when they are paired up in the same orbital. In 1st row metals complexes, low-field ligands (strongπ-donors) favor
high spin configurations whereas high field ligands (π-acceptors/ strong σ donors) favor low spin. The majority of 2nd and
3rd row metal complexes are low-spin irrespective of their ligands.
Primarily for 1st row metal complexes:
LUMO
LUMO
eg
eg
low-spin
high-spin
∆
∆
t2g
HOMO
t2g
HOMO
strong π donor L
strong σ donor L/
π-acceptor L
For a given geometry and ligand set , first row metals tend to have lower∆ than second or third row metals. Low oxidation
state (low-valent) complexes also tend to have lower∆ than high oxidation state (high-valent) complexes.
Mn2+ < V2+ < Co2+ < Fe2+ < Ni2+ < Fe3+ < Co3+ < Mn4+ < Rh3+ < Ir3+ < Pt4+
1st row/low-valent
low ∆
2nd,3rd row/high-valent
high ∆