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 havenoncarbon ligands (Werner complexes). Classification applies to the catalyst and all reaction intermediates.
Organometallic Chemistry:The chemistry of transition metal complexes that have M-C bonds (organometallic complexes). Classification applies to the catalyst and/or reaction intermediates.
RuH3CCN NCCH3
H3CCN
(PF6-)
R
R
RuNCCH3
(PF6-)
R
Trost enyne cycloisomerization catalyst
Trost JACS 2002 (124) 5025.
+
+
proposed intermediate
PPh3
PdPh3P PPh3
Ph3P
Suzuki cross-coupling catalyst
B(OH)2 N
OTf
CO2Me
Ph3PPd
Ph3P
N CO2Me
proposed intermediate
N
CO2Me
de Lera Synthesis 1995 285.
OTiIV
RORO
OTiIV
O O
O
R'(O)C
R'OR
OR'
OR
OH t-BuOOH, 4Å MS
OTiIV
ROOR
OTiIV
O O
R'(O)C
CO2RO
OR'
O
O
t-Bu
R
R
OHRO
Sharpless JACS 1987 (109) 5765.
Sharpless titanium-tartrateepoxidation catalyst
CH2Cl2, -20oC
70-90% yield94->98% ee
proposed intermediate
C(O)R'
C(O)R'
M.C. White, Chem 153 Structure & Bonding -2- Week of September 17, 2002
Cross-Couplings
Tandem Heck
IO
OH
TBSO O
O
H
OTBS
AcOPd
Ph3P OAc
PPh310mol%
Ag2CO3, THF, reflux
82%
Overman JOC 1993 (58) 5304.
Nobel Prize in Chemistry 2010: Richard Heck, Ei-ichi Negishi, Akira Suzuki
Nicolaou's Rapamycin Synthesis: Note* last step!!!
O
I
OCH3
OHO
NOH
OO
HOMe
OHO
OH
OMe
O
I+
SnBu3
Bu3Sn
ClPdII
Cl NCCH3
NCCH3
20 mol%
(i-Pr)2NEtDMF, THF25oC, 24h
28%
O
OCH3
OHO
NOH
OO
HOMe
OHO
OH
OMe
O
Nicolaou JACS 1993 (115) 4419.
M.C. White, Chem 153 Structure & Bonding -3- Week of September 17, 2002
Reactive Site Selectivity in Multifunctional Molecules
HO
MeO
O OH
N
O
H
O
OO
OH
OMe
OMe
OH
HO
MeO
O OH
N
O
H
O
OO
OH
OMe
OMe
O
HO
MeO
O OH
N
O
H
O
OO
OH
OMe
OMe
O
H
H H
FK 506
Ru
PPh3
PPh3
Cl
Cl Ph
H
10 mol%CH2Cl2, rt, 22h
49%E:Z ; 1:1
Schreiber JACS 1997 (119) 5106.
No protecting groups used! The majority of the mass recovered after reaction termination was unreacted starting material.
Nobel Prize in Chemistry 2005: Yves Chauvin, Robert Grubbs, Richard Schrock
M.C. White, Chem 153 Structure & Bonding -4- Week of September 17, 2002
Asymmetric Catalysis Nobel Prize in Chemistry 2001: William S. Knowles, Ryoji Noyori, K. Barry Sharpless
Wilkinson : Investigations into the reactivity of (PPh3)RhCl uncovered its high activity as a homogeneous hydrogenation catalyst. This was the 1st
homogeneous catalyst that compared in rates with heterogeneouscounterparts (e.g. PtO2).
RhPh3P
Ph3P
Cl
PPh3
H2 (1 atm)
Wilkinson J.Chem. Soc. (A) 1966 1711.
MeO
AcO
CO2H
NHAc
RhP
P OMe
OMe
+
BF4- H2
MeO
AcO
CO2H
NHAcH
95% ee, 100 % yield
MeO
AcO
CO2H
NH2H
H3O+
L-DOPA
cat.
The Monsanto Process
W. Knowles: Replacement of achiral PPh3 ligands with non-racemicphosphines ((-)-methylpropylphenylphosphine, 69%ee) demonstrated that a chiral transition metal complex could transfer chirality to a non-chiralsubstrate during hydrogenation.
CO2H RhPr(Ph)(Me)P
Pr(Ph)(Me)P
Cl
P(Me)(Ph)PrCO2H
Electronically tuning the metal center and using a C2 symmetric, bidentatechiral phosphine ligand led to highly enantioselective hydrogenations ofenamides (very good substrates for asymmetric hydrogenations). TheMonsanto Process (1974) that resulted is the 1st commercialized asymmetric synthesis using a chiral transition metal complex. Asymmetrichydrogenation is the key step in the industrial synthesis of L-DOPA (a rareamino acid used to treat Parkinson's disease).
H2 (1 atm)
**
*
15 % eeKnowles Chem. Commun. 1968, 1445.
Royal Swedish Academy of Sciences:www.kva.se
The Transition Metals
* d electrons in group3 are readily removedvia ionization, those ingroup 11 are stable and generally form part ofthe core electronconfiguration.
valence (d) electron count:
for complexed transitionmetals: the (n)d levels arebelow the (n+1)s and thus getfilled first. note that group # =d electron count
OC FeCOCO
CO
CO
3d8
K Sc Ti V Cr Mn Fe Co Ni Cu Zn
Rb Y Zr Nb Mo Tc Ru Rh Pd Ag Cd
Cs Hf Ta W Re Os Ir Pt Au Hg
Na
B
Al
Ga
In
Tl
Li Ne
Ar
Kr
Xe
Rn
H He
3 4 5 6 7 8 9 10 11 12
13
1 18
4s23d2 4s23d3
3d4 3d5
5s24d2
4d4
5s14d4
4d5
4s13d5
3d6
5s14d5
4d6
6s25d2
5d4
6s25d3
5d5
6s25d4
5d6
4s23d5 4s23d6
3d7 3d8
4s23d7
3d9
4s23d8
3d10
5s24d5
4d7
5s14d7
4d8
5s14d8
4d9
5s04d10
4d10
6s25d5
5d7
6s25d6
5d8
6s25d7
5d9
6s15d9
5d10
Transition metals (d-block metals):elements that can have a partially filled dvalence shell. Typically group 4-10 metals.*
EARLY LATE
La
M.C. White, Chem 153 Structure & Bonding -5- Week of September 17, 2002
N
NNFeII
Cl Cl
3d6
for oxidized metals, subtract the oxidation state from the group #.
Fe 4s23d6
for free (gas phase)transition metals: (n+1)s isbelow (n)d in energy (recall: n = principal quantum #).
Electron Counting
Ph2PCORh
P
HOC
OO
O
To determine ligand charges, create an ionic model by assigning each M-L electron pair to the moreelectronegative atom (L). This should result instable ligand species or ones known as reactionintermediates in solution.
COOC
P
POO
O
RhI
H
Ph2
-1
neutral (0)
Step 1: Determine the oxidation state of the metal.To do this, balance the ligand charges with an equalopposite charge on the metal. This is the metal's formal oxidation state.
Co
Rh
Ir
9
3d9
4d9
5d9
RhI = d8
Step 3: Determine the electron count of the complexby adding the # of electrons donated by each ligand to the metal's d electron count.
COOC
P
POO
O
RhI
H
Ph2
2e-
2e-
ligands: 10e-metal: 8 e-complex: 18 e-
M.C. White, Chem 153 Structure & Bonding -6- Week of September 17, 2002
Step 2: Determine the d electron count. Recall: subtract the metal's oxidation state from its group #.
η1-Ligands Hapticity (ηx): The number of atoms (x) in the ligand binding
to the metal
V
OO OR
O
O
t-Bu
VO
OOR
Ot-Bu
O
η1-alkyl peroxo
terminal oxo η2-alkyl peroxo
Proposed intermediates in VO(acac)2 catalyzed directed epoxidation of allylic alcohols.
Sharpless Aldrichimica Acta 1979 (12), 63.
Bridging ligands (µ): the ligand bridges 2 or more metals
FeN
O FeN
NCl
Cl
NN
N
N
N
Nishida Chem. Lett. 1995 885.
linear µ-oxo
M.C. White, Chem 153 Structure & Bonding -7- Week of September 17, 2002
MX
M
M
RO
M
MO
M
η1 ligands (monodentate):
H (hydride)CH3 (alkyl)COX (halides)µ-X (bridging)
OR (terminal alkoxide)µ-OR (bridging)
OR2 (ether)O2 (superoxide)O (terminal oxo)µ-O (bridging)
PR2 (phosphide)PR3 (phosphine)NR2 (amide)NR3 (amine)iminesnitrilesNO (nitrosyl )
Formal charge
# of e-donated
-1-1 0-1-1
-1
-1
0-1-2-2
-1 0-1 0 0 0+1
22224
2
4
2244
2222222
(2/metal)
(2/metal)
(2/metal)
linear
Electron Counting M.C. White, Chem 153 Structure & Bonding -8- Week of September 17, 2002
Ph3PRh
Cl PPh3
PPh3 Ph3PRhI
Cl PPh3
PPh3
PRu
PH2N
NH2
Cl
Cl
Ar2
Ar2
O
O
PRuII
P N
N
Cl
Cl
O
OAr2
Ar2
H2
H2
N
NPd
Me
Me N
NPdII
Me
Me
NFe
N NOTf
N
OTf
NFeII
N NOTf
N
OTf
PPh3
PPh3Pd
Ph3PPh3P
PPh3
PPh3Pd0
Ph3PPh3P
Wilkinson's catalyst(Ph3P)3RhCl
ligands: 8e-metal: d8, 8e-complex: 16 e-
ligands: 12e-metal: d6, 6e-complex: 18 e-
Noyori hydrogenationcatalyst
Brookhart polymerizationcatalyst precursor
ligands: 8e-metal: d8, 8e-complex: 16 e-
Olefin dihydroxylation catalyst
ligands: 12e-metal: d6, 6e-complex: 18 e-
ligands: 8e-metal: d10, 10e-complex: 18 e-
Palladium "tetrakis" triphenylphosphinecross coupling catalyst
Noyori JACS 1998 (120) 13529. Que JACS 2001 (123) 6722.
Brookhart JACS 1995 (117) 6414.
M
M
R M
H
M
O
O M
η1-coordinationFormal charge
# of e-donated
-1
-1
-1
-1
-1
-1
2
2
2
2
2
2
η1-aryl
η1-alkenyl
η1-alkynyl
η1-Cp (cyclopentadienyl)
η1-acetate
M
η1-allyl
M
M
R HM
M
O
OM
0
0
0
-1
-1
-1
6
2
2
6
4
4
η6-arene
η2-alkene
η2-alkyne
η5-Cp (cyclopentadienyl)
η2-acetate
ηx-coordinationFormal charge
# of e-donated
M
η3-allyl
= M M
Unsaturated Ligands M.C. White, Chem 153 Structure & Bonding -9- Week of September 17, 2002
Electron Counting II M.C. White, Chem 153 Structure & Bonding -10- Week of September 17, 2002
HIr
H O
O
P(Cy)3
P(Cy)3
CF3H
IrIIIH O
O
P(Cy)3
P(Cy)3
CF3
ligands: 12e-metal: d6, 6e-complex: 18 e-
Crabtree's dehydrogenationcatalyst
RhH
HMe3P
Cp*
RhIIIH
HMe3P
ligands: 12e-metal: d6, 6e-complex: 18 e-
Bergman: direct observation
of C-H-> C-M
ZrClCl
Brintzinger catalyst
ZrIVCl
Cl
ligands: 16e-metal: d0, 0e-complex: 16 e-
RuS
RuS
CH3
CH3
Cl
ClRuIII
SRuIII
S
CH3
CH3
Cl
Cl
Ru-Ru bond = 2 e-note: metal oxidation state doesn't change
Hidai catalyst forpropargylic substition
ligands: 12 e-metal: d5, 5e-Ru 2: 1 e-complex: 18 e-
Ru 1ligands: 12 e-metal: d5, 5e-Ru 1: 1 e-complex: 18 e-
Ru 2
Hidai JACS 2002 (124) 7900Brintzinger JOMC 1985 (228) 63.
Crabtree JACS 1987 (109) 8025. Bergman OM 1984 (3) 508.
Weakly Coordinating Counterions
The least coordinating anion: hexahalocarboranes (CB11H6X6
-)
Strem: Silver hexabromocarborane (Ag+CB11H6Br6
-) 1g = $653
Strauss Chem. Rev. 1993 (93) 927. Reed Acc. Chem. Res. 1998 (31) 133.
Fe ClCl
N
N
Me
Me
N
N
CH3CNFe NCCH3
NCCH3
N
N
Me
Me
N
N
(SbF6-)2
2 equiv. Ag+SbF6-
2+
Jacobsen JACS 2001 (123) 7194.
Common weakly coordinating counterions used in organotransition metal catalysis to generate cationic catalysts: Weakly coordinating anions generally
have: 1. low charge, 2. high degree ofcharge delocalization (i.e. noindividual atom has a highconcentration of charge), 3. steric bulk.
SynthesisMetathesis: Ag (I) halide abstraction. Most general approach for the introduction of weakly coordinating counterions.
Protonolysis
M.C. White, Chem 153 Structure & Bonding -11- Week of September 17, 2002
note: neutral solventreplaces L- in rxn.
TfO-< ClO4- < BF4
- < PF6- < SbF6
- < BAr'4 (B[3,5-C6H3(CF3)2]4)
More weakly coordinating
N
NAr
Ar
NiMe
MeEt2O
N
NAr
Ar
NiMe
OEt2
H+(OEt2)2 BAr'4-
+
(BAr'4-)
Brookhart JACS 1999 (121) 10634.
Electron Counting III M.C. White, Chem 153 Structure & Bonding -12- Week of September 17, 2002
Ir
N
P(Cy)3(PF6
-) IrI
N
P(Cy)3 PF6
BPh3
Rh+
BPh3
RhI
RuH3CCN NCCH3
H3CCN
(PF6-) RuII
CH3CN NCCH3CH3CN
PF6
Fe ON
N
Me
Me
N
N
(SbF6-)3Fe
N
N
N
NO O
Me
Me
+
Crabtree's catalystsfor hydrogenations
+
weakly coordinating anion does not contribute to theelectron count for complex
ligands: 8 e-metal: d8, 8e-complex: 16 e-
review: Crabtree Acct. Chem. Res. 1979 (12) 331.
COD = 1,5-cyclooctadiene
NBD = norbornadiene
"Zwitterionic complex"used in hydroformylations
1st synthesis: Schrock and Osborn Inorg. Chem. 1970 (9) 2339.hydroformylation: Alper Chem. Comm. 1993, 233.
ligands: 10 e-metal: d8, 8e-complex: 18 e-
ligands: 12 e-metal: d6, 6e-complex: 18 e-
1st synthesis: Mann OM 1982 (1) 485.catalytic enyne cycloisomerizations: Trost JACS 2002 (124) 5025.
+ +
Jacobsen JACS 2001 (123) 7194.ligands: x e-metal: dx, 5e-complex: x e-
Fe 1ligands: x e-metal: dx, xe-complex: x e-
Fe 2
3+
epoxidation catalyst
Question:
Common Geometries for TM Complexes M.C. White, Chem 153 Structure & Bonding -13- Week of September 17, 2002
CN = 6, ML6:
LM
L L
L
L
L
90o, cis180o, trans
octahedral
CN = 5, ML5:
L
L
ML
L
109.5o
CN = 4 ,ML4:
tetrahedral
Leq M
Leq
Leq
Lax
Lax120o
90o
trigonal bipyramidal
CN = 2, ML2:
M LL
180o
CN = 3, ML3
L ML
L
linear
trigonal planar
120o
Coordination number (CN):The number of ligands (L) bonded to the same metal (M).
Sterics. to a 1st approximation, geometry of TM complexesdetermined by steric factors(VSEPR -valence shell electronpair repulsion). The M-L bondsare arranged to have themaximum possible seperationaround the M.
LM
L L
L
90o, cis
square planar
L
ML Lba sal
Lba sal
Lapical
square pyramidal
180o, trans
~90o
~90-100o
Electronics: d electron count combined with the complex electron count must beconsidered when predicting geometries forTM complexes with non-bonding delectrons. Often this leads to sterically lessfavorable geometries for electronic reasons (e.g. CN = 4, d8, 16 e- strongly preferssquare planar geometry) .
M LL
L90o
T-shaped
M.C. White, Chem 153 Structure & Bonding -14- Week of September 17, 2002
Ligand sterics
P
RR
R
M
θ 2.28 Åaverage of Ni-P bondlengths obtained from crystal data
Tolman Chem. Rev. 1977, 77, 313.
N
RR
R
M
θ 2.2 Åaverage of Pd-N bond lengths obtained fromcrystal data
Trogler JACS 1991, 113, 2520.
∗θ values measured using strain-free CPK model ofM(L). For ligands with many internal degrees offreedom, the values do not account for distortions in geometry due to contacts with other atoms in thecomplex. Very valuable as a relative scale.
PH3
PF3
P(OMe)3
PMe3
PCl3
Ph2P PPh2
PPhMe2
PEt3PPh2MePPh2EtPPh2PrPPh3
PPh2CyPPhCy2
PCy3
P(t-Bu)3
P(o-tol)3
P(mesityl)3
Ligands Cone angle*θ (ο)
87104107118124
125
127132136140140145153161170182194212
Ligands
NH3
NMe3,quinuclidine, NMe2EtNMeEt2NEt3
NPr3
NPh3
NEt2PhNBz3
N(i-Pr)3
Cone angle*θ (ο)
94132
145150160166170210220
others
HMeCOCp
759095136
phosphines 3o amines
M.C. White/ Q. Chen Chem 153 Structure & Bonding -15- Week of September 17, 2002
Effect of ligand sterics on coordination number
Pd(PtBu2Ph)2
Bond length (Å) Pd-P1: 2.251 Bond angle (o) P1-Pd-P1a: 176.51
Otsuka JACS, 1976 (98)5850.
Although Pd(PtBu2Ph)2 is coordinatively unsaturated electronically, the steric bulk of PtBu2Ph ligands prevents additional ligands from coordinating to the metal.
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.
Pd0 P(tBu)2PhPh(But)2P
176.51o
ligands: 4 e-metal: d10, 10 e-complex: 14 e-
Transition Metal Valence Orbitals
� 18 electron rule: upper limit of 18 e- can be accomodated w/out using antibonding molecular orbitals (MO's).
dz2 dx2-y2 dxy dxz dyz
(n)d orbitals
� dz2 and dx2-y2 orbital lobes located on the axes� dxy, dxz, and dyz lobes located between the axes
� orbitals oriented 90o with respect to each other creating unique ligand overlap possibilities
pz px py
(n+1)p orbitalsz
y
x
(n+1)s orbital
s
M.C. White, Chem 153 Structure & Bonding -16- Week of September 17, 2002
� 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 thesenon-bonding d orbitals that give TM complexes many of their unique properties.
M.C. White, Chem 153 Structure & Bonding -17- Week of September 17, 2002
M H
Pure σ-donors
Hydride Alkyl
M C
R
RR
3o Amines
M N
R
RR
σ-bonding
M
z
y
x
L
Best Overlap
M
z
y
x Worst overlap
L
best shape complementarity
t2g
eg
LUMOσ*
Metal d orbitals
ligand σ-bondingorbitals
MO Description of σ−bonding in an octahedral complex
Conclusion:The energy of the LUMO is directly affected by M-L σ bond strength. Weak bonds willhave low-lying LUMO's making the metalmore electrophilic.
σ
HOMO
M.C. White, Chem 153 Structure & Bonding -18- Week of September 17th, 2002
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd
Cs Ba Hf Ta W Re Os Ir Pt Au Hg
Na
Be
Mg
B C
Al
Ga
In
Tl
Si
Ge
Sn
Pb
Li N O
P
As
Sb
Bi
S
Se
Te
Po
F
Cl
Br
I
At
H
La*
1
2
3 4 5 6 7 8 9 10 11 12
13 14
EARLY (EM) LATE (LM)
15 16 17
2.2
1.0
0.9
0.8
0.8
0.8
1.6
1.3
1.0
1.0
0.9
1.3
1.2
1.1
1.5
1.3
1.6
1.6
1.5
1.6
2.1
2.3
1.6
1.9
1.9
1.8
2.2
2.2
1.9
2.3
2.2
1.9
2.2
2.3
1.9
1.9
2.5
1.7
1.7
2.0
2.0
1.6
1.8
1.6
1.6
2.5
1.9
2.0
1.8
1.9
3.0
2.2
2.2
2.0
2.0
3.4
2.6
2.5
2.1
2.0
4.0
3.1
2.9
2.6
2.2
increasing electronegativityincreasing electronegativity
increasing electronegativity
increasing electronegativity
TRANSITION METALS (TM)
Periodic table trends:electronegativity
The electronegativity of theelements increases substantially as in progressing from left toright (EM to LM) across theperiodic table.
Whereas the electronegativity of main group elementsincreases in progressing up a column, that of the TMincreases in progressing down.
Co
HN H
H
H
N HH
H NHH
HN
HH
H
NHH HN
HH
Electrostatic Model
3+Co
HN HH
H
N HHH N
HH
HN
HH
H
NHH HN
HH
Covalent Model
3-The most accurate description ofσ-bonding in TM complexes liessomewhere in between the 2 extremes anddepends in large part on the relativeelectronegativities of the metal and ligands
Pauling The Nature of the Chemical Bond, 3rd Ed.;1960
M.C. White, Chem 153 Structure & Bonding -19- Week of September 17th, 2002
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd
Cs Ba Hf Ta W Re Os Ir Pt Au Hg
Na
Be
Mg
B C
Al
Ga
In
Tl
Si
Ge
Sn
Pb
Li N O
P
As
Sb
Bi
S
Se
Te
Po
F
Cl
Br
I
At
H
La*
1
2
3 4 5 6 7 8 9 10 11 12
13 14
EARLY (EM) LATE (LM)
15 16 17
2.2
1.0
0.9
0.8
0.8
0.8
1.6
1.3
1.0
1.0
0.9
1.3
1.2
1.1
1.5
1.3
1.6
1.6
1.5
1.6
2.1
2.3
1.6
1.9
1.9
1.8
2.2
2.2
1.9
2.3
2.2
1.9
2.2
2.3
1.9
1.9
2.5
1.7
1.7
2.0
2.0
1.6
1.8
1.6
1.6
2.5
1.9
2.0
1.8
1.9
3.0
2.2
2.2
2.0
2.0
3.4
2.6
2.5
2.1
2.0
4.0
3.1
2.9
2.6
2.2
increasing electronegativityincreasing electronegativity
increasing electronegativity
increasing electronegativity
TRANSITION METALS (TM)
Electronegativity II
Ionic bonding is greater when orbitals of unequal electronegativities interact. M-L σ-bonding inelectropositive metals (e.g. early metals) hassignificant ionic character.
Covalent bonding is greater when orbitals of similar electronegativities interact. Therefore, M-L σ-bonding in electronegative metals (e.g. late metals) is primarilycovalent in nature.
ZrIV
H
Cl + ZrIV
O
Cl
R HOEt
Schwartz's reagent
Labinger ACIEE 1976 (15) 333.
HMLn H + MLn
adds H-Zr across alkenes and alkynes(hydrozirconation). incompatible withmost carbonyls b/c of hydridic properties.
O
REtO
(easier to break heterolytically)
intermediate in catalytichydroformylation
of alkenes
HMLn H�+ �MLn
RhI
O
HO
OREtO2C
OREtO2C
OC
HO
Leighton JACS 2001 (123) 11514.
(easier to break homolytically)
M.C. White, Chem 153 Structure & Bonding -20- Week of September 17th, 2002
σ-bonding EB EI + EC
bondingenergy
ionicbonding
covalentbonding
Co
HN H
H
H
N HHH N
HH
HN
HH
H
NHH HN
HH
Electrostatic Model
3+Co
HN HH
H
N HHH N
HH
HN
HH
H
NHH HN
HH
Covalent Model
3-
Bond strength in polarized M-L bonds resultsfrom a gain in covalent and ionic bonding energy. The degree to which each type of bondinginfluences bond strength is highly dependent onthe relative electronegativities of the metal andligands.
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Ionic bonding is greater when elements of high and opposite chargeinteract. Differences in charge are paralleled in differences inelectronegativities. Large differences in electronegativity favor strong ionic bonding. M-L σ-bonding in early metals has significant ioniccharacter.
M
L
M+
L-incr
easi
ng io
niza
tion
pote
ntia
l (ε)
EI
EI (εM-εL)
EI −(QMQL)Q = charge density
− (QMQL) − (εM-εL)
Fleming Frontier Orbitals and Organic Chemical Reactions, 1976.Pauling The Nature of the Chemical Bond, 3rd. Ed.; 1960.
Electrostatic Model: Ionic Bonding
HOMO
LUMO
Covalent bonding is greater when orbitals of similar energiesinteract. The energy of atomic orbitals is inversely proportional tothe element's electronegativity (i.e. the orbital energy of anelectronegative element is lower than that of a electropositiveelement). Small differences in electronegativity favor strongcovalent bonding. M-L σ-bonding in late metals has a high degree of covalent bonding.
M
LEI
ECML σ
ML σ∗
ECorbital overlap
incr
easi
ng e
nerg
y
(εM-εL)
ECorbital overlap
(EM-EL)
EM1εM
EL1εL
Covalent Model
HOMO
LUMO
M.C. White, Chem 153 Structure & Bonding -21- Week of September 17th, 2002
Periodic table trends II: hard/soft
La*
1
2
3 4 5 6 7 8 9 10 11 12
13 14
EARLY (EM)
LATE (LM)
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn
Rb Y Zr Nb Mo Tc Ru Rh Pd Ag Cd
Cs Ba Hf Ta W Re Os Ir Pt Au Hg
Na
Be
Mg
B C
Al
Ga
In
Tl
Si
Ge
Sn
Pb
Li
15 16
N O
P
As
Sb
Bi
S
Se
Te
Po
17
F
Cl
Br
I
At
H2.2
1.0
0.9
0.8
0.8
0.8
1.6
1.3
1.0
1.0
0.9
1.3
1.2
1.1
1.5
1.3
1.6
1.6
1.5
1.6
2.1
2.3
1.6
1.9
1.9
1.8
2.2
2.2
1.9
2.3
2.2
1.9
2.2
2.3
1.9
1.9
2.5
1.7
1.7
2.0
2.0
1.6
1.8
1.6
1.6
2.5
1.9
2.0
1.8
1.9
3.0
2.2
2.2
2.0
2.0
3.4
2.6
2.5
2.1
2.0
4.0
3.1
2.9
2.6
2.2
increasing electronegativity/decreasing orbital energy
increasing electronegativity
increasing electronegativity/decreasing orbital energy
increasing electronegativity
HARDelectrophile
SOFTelectrophile
SOFTnucleophile
HARDnucleophile
Hard nucleophiles (ligand): have a low energy HOMOwith high charge density (negative charge).Hard electrophiles (metal) : have a high energy LUMOwith high charge density (positive charge).Hard (L) - Hard (M) interaction: is predominantly ionic in character. It is favorable because of strong Coulombicattraction.
Soft nucleophiles (ligand): have a high energy HOMO with low charge density.Soft electrophiles (metal) : have a low energy LUMOwith low charge density.Soft (L) - Soft (M) interaction: is predominantly covalent in character. It is favorable because of small ΔE 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 -22- Week of September 17th, 2002
Ir-X bond dissociation enthalpies for (η5-Me5C5)(PMe3)Ir(X)2
X
Ph
Vy
H
Pentyl
Me
Cy
Neopentyl
DIr-X (kcal/mol)
82
74
71
58
56
52
48
IrIIIX
XMe3P
Bergman's C-H activationcomplex
M-C Bond Strengths M-C Bond Strength Trends: the trends in M-C σ bond strengths generally parallel those found in H-C σ bond strengths.
90
100
110
120
40 50 60 70 80 90
Cy
Neopentyl
Me
Pentyl
Vy H
Ph
D(Ir-X) kcal/mol
D(H
-X) k
cal/m
ol
sp C-M > sp2 C-M > sp3 C-M 1o C -M > 2o C-M > >> 3o C-M
Bergman Polyhedron 1988 (7) 1429.
As in C-H σ bonding, there is a general trend towards weaker M-Cwith increased substitution. Large deviations occur when the alkylgroup is very bulky or when it is methyl. Bulky ligands likeneopentyl are thought to destabilize the M-C bond because of steric hinderance, making it much weaker than the correlation wouldpredict. There is a strong thermodynamic preference to form thesterically less hindered M-C bond.
As in C-H σ bonding, an increase in % s character of the carbonstrengthens the M-C σ bond because of better orbital overlap. The correlation between C-H and M-C (C = aryl, vinyl) BDE's is notperfect with M-C bonds being stronger than predicted because ofπ-bonding with the metal.
M.C. White, Chem 153 Structure & Bonding -23- Week of September 17th, 2002
M L
z
y
x
σ-bonding
M
z
y
x
L
π-bonding
σ and π bonding in ML6
dxy dxz dyz
Six valence metal orbitals that participate in σ-bonding inan octahedral complex along the x,y, and z axes.
dz2 dx2-y2
pz px py
z
y
x
s
Three valence metal orbitals that may participate in π-bonding inan octahedral complex with ligands that have orbitals of matching symmetry (i.e. p, d, π, π*).
M.C. White, Chem 153 Structure & Bonding -24- Week of September 17th, 2002
σ and π donors
M
z
y
x Best overlap
M
z
y
x Worst overlap
t2g
HOMO
Δ
LUMO
t2g
eg*
LUMOσ*
π
π∗
HOMO
σ−complex ligand π-bondingorbitals
MO Description for M-L π-donor system in an octahedral complex
M
σ-bonding: Lsp2 -> Mdσπ-donation: Lp -> Mdπ
Cl
or I-, Br-, F-
Halides
M OR
Alkoxides
M NR
R
1o, 2o Amines
σ-bonding: Lsp2 -> Mdσπ-donation: Lp -> Mdπ
σ-bonding: Lsp2 -> Mdσπ-donation: Lp -> Mdπ
OO-
N N
O- -O
acac (acetylacetonate)
other π-donors
salen
Cp
benzene
Conclusion:The energy of the HOMO is directly affected by M-L πbonding. Ligand to metal π donation increases the energyof the HOMO making the metal more basic. π-donorligands stabilize electron poor, high oxidation state metals. Very prevalent for early TM complexes (low d electroncount) and less so for late TM (high d electron count).
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 wellas the negative charge and/or e- density on the ligand. The result is that the actual charge on the metal is notaccurately reflected in its formal oxidation state.
Pauling The Nature of the Chemical Bond, 3rd Ed.;1960, pg. 172.
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 stabilizedvia π-donation (i.e. a shifting of electron density from π-donor ligands to the metal). This in partaccounts for the extreme oxophilicity of early TM.
M.C. White, Chem 153 Structure & Bonding -25- Week of September 17th, 2002
N N
O OMnIII
Clt-Bu
t-Bu
t-Bu
t-Bu
Jacobsen epoxidation catalystMn (salen)
ligands: 10e-metal: d4 ,4e-complex: 14 e-
VIV
O
OO
OO
VO(acac)2 "vanadium acac"epoxidation catalyst
ligands: 12 e-metal: d1 ,1e-complex: 13 e-
Sharpless titanium-tartrateepoxidation catalyst
self-assemblingdimer based oncrystal structure.
ligands: 12 e-metal: d0 ,0e-complex: 12 e-
OTiIV
RORO
OTiIV
O O
O
R'(O)C
R'OR
OR'
OR
C(O)R'
M.C. White, Chem 153 Structure & Bonding -26- Week of September 17th, 2002
σ and π acceptors
Conclusion:Metal to ligand π donation (π backbonding) lowers theenergy of the HOMO making the metal less basic.π-backbonding stabilizes electron rich, low oxidation state metals. Very prevalent in late TM complexes.
HOMO
Δ
LUMO
t2g
eg*
t2g*
LUMOσ*
π
HOMO
π∗
Δ
ligand π-bonding orbitals
MO Description for M-L π -acceptor system in an octahedral complex
σ−complex
LUMO
M C OC
CM
σ-bonding: Ln -> Mdσπ-backbonding: Md π -> Lπ*
H
H
M
σ-bonding: L π -> Mdσπ-backbonding: Md π -> Lπ*
σ-bonding: L σ-> Mdσπ-backbonding: Md π -> Lσ*
P
Rationalization of M -> P backbonding iscontroversial. The classic picture envokes a Mdπ -> P 3d interaction. Quantummechanical calculations indicate that P-Xσ* orbitals play a major role.Hybridization of phosphorus 3d and P-Rσ* resulting in π-acceptor orbitals hasbeen envoked.
M
Orpen Chem. Comm. 1985 , 1310.Braga Inorg. Chem. 1985 , 2702.
N N NN
CH3CN, NO, N2, CN-
R'
R N N R
R'
bpy phen
M.C. White, Chem 153 Structure & Bonding -27- Week of September 17th, 2002
π-backbonding
P(t-Bu)3PCy3P(i-Pr)3P(NMe2)3PMe3PPhMe2PBz3PPh2MePPh3PPh2(OEt)P(p-C6H4Cl)3PPh(OEt)2P(OEt)3PH3PCl3PF3
PhosphorusLigand (L) CO v, cm-1
2056
20592062206420652066206720692072207320742077208320972111
Tolman Chem. Rev. 1977 (77) 313.
CO stretching frequencies measured forNi(CO)3L where L is PR3 ligands ofdifferent σ-donor abilities. Free CO vibrates at 2143 cm-1.
The increase in electron density at the nickel from phosphine σ-donation isdispersed 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) IRfrequencies which corresponds to weaker CO bonds.
P
R
R
RNiCO
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.
v = 12πc
f(MxMy)/(Mx+My)
1/2
π-acids: effect on the metal M.C. White, Chem 153 Structure & Bonding -28- Week of September 17th, 2002
N
NNiII
C
N
N
Ni0 +24oC
no reaction without π acid
OH
H OH
π-acid
Yamamoto JACS 1971 (93)3350.
CO's render the electron rich Cr metal electrophilic via strong π-backbonding. Complexation ofbenzene with the electrophilic Cr(CO)3 fragment withdraws electon density from the aromatic ring activating it towards nucleophilic attack.
CNNC
CNNC
F3C
N
FF
FF
F
NO2
other π-acids
Acrolein is thought to act as a π-acid, withdrawing electron density from theNi(II) complex via π-backbonding and promoting elimination of the diethylfragment to reduce the metal.
OC
CoIOC CO
CO
H acidic
pka < 1 H2O
Norton JACS 1987 (109) 3945.
CrCO
CrCO
Cr(0), d6, 18e-
OCOC
OCOC
LDA
MeOMeO
NCO
(±)-Acorenone B
π-acid
Semmelhack JACS 1980 (102) 5926
NC
M.C. White, Chem 153 Structure & Bonding -29- Week of September 17th, 2002
C
CM
Dewar-Chatt-Duncanson Model
Olefin-metal bonding is thought tooccur via a 2-way donor-acceptormechanism that involves σ-donationfrom the bonding π-electrons of theolefin to empty σ orbitals of the metal and π-backbonding from the metal tothe empty π* orbitals of the olefin. Both interactions are important in forming astable M-olefin complex
olefin-metal complexes
The balance of electron flow can be shifted predominantly in one direction dependent on the electronic properties of themetal. If the metal is electron withdrawing, M-L σ-bondingpredominates and withdraws electron density from theπ-bond of the olefin. This results in the induction of a δ+charge on the olefin that activates it towards nucleophilicattack.
LPdII
Cl
Cl
R
OH2
σ donation>>π-backbonding
OH2
RLPdII
Cl
Cl
δ+
intermediates in Wacker oxidation (commercial production of acetaldehyde) 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.
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 complexbegins to approach a metallocyclopropane structure. In complexes involvingelectropositive metals in low oxidation states, the metallocyclopropanecarbons are rendered nucleophilic as evidenced by their reaction withelectrophiles (i.e. aldehydes). Cp2Ti metallocyclopropane is a stable complex, crystal obtained by Bercaw.
CpTiII
Cp
RH
H H
δ-
δ-
R
CpTiII
Cp
H
R'CHO
CpTiIV
Cp
O
R
R'
π-backbonding >>σ donation
note: convention is to not change formal oxidation state of the metallocyclopropane.
M.C. White/M.W. Kanan Chem 153 Structure & Bonding -30- Week of September 17th, 2002
*Cp
TiII
*CpH
H
H
H
Bercaw JACS 1983 (105) 1136
Ph3P
Pt0
Ph3P
Cheng Canadian J. Chem. 1972 (50) 912.
Metallocyclopropanes