Elements of organometallic chemistry

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Elements of organometallic chemistry
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Elements of organometallic chemistry
Complexes containing M-C bonds Complexes with
p-acceptor ligands Chemistry of lower oxidation
states very important Soft-soft interactions
very common Diamagnetic complexes
dominant Catalytic applications
18-electron rule (diamagnetic complexes) Most
stable complexes contain 18 or 16 electrons in
their valence shells Most comon reactions take
place through 16 or 18 electron intermediates
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A simple classification of the most important
ligands
X
LX
L
L2
L2X
L3
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Counting electrons
The end result will be the same
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(No Transcript)
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Why 18 electrons?
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Organometallic complexes
18-e most stable
16-e stable (preferred for Rh(I), Ir(I), Pt(II),
Pd(II))
lt16-e OK but usually very reactive
gt 18-e possible but rare
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Organometallic Chemistry Fundamental Reactions
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Fundamental reaction of organo-transition metal
complexes
Reaction D(FOS) D(CN) D(NVE)
Association-Dissociation of Lewis acids 0 1 0
Association-Dissociation of Lewis bases 0 1 1
Oxidative addition-Reductive elimination 2 2 2
Insertion-deinsertion 0 0 0
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Association-Dissociation of Lewis acids
D(FOS) 0 D(CN) 1 D(NVE) 0
Lewis acids are electron acceptors, e.g. BF3,
AlX3, ZnX2
This shows that a metal complex may act as a
Lewis base The resulting bonds are weak and
these complexes are called adducts
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Association-Dissociation of Lewis bases
D(FOS) 0 D(CN) 1 D(NVE) 2
A Lewis base is a neutral, 2e ligand L (CO,
PR3, H2O, NH3, C2H4,) in this case the metal is
the Lewis acid
For 18-e complexes, dissociative mechanisms
only For lt18-e complexes dissociative and
associative mechanisms are possible
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Oxidative addition-reductive elimination
D(FOS) 2 D(CN) 2 D(NVE) 2
Very important in activation of hydrogen
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Insertion-deinsertion
D(FOS) 0 D(CN) 0 D(NVE) 0
Very important in catalytic C-C bond forming
reactions (polymerization, hydroformylation)
Also known as migratory insertion for mechanistic
reasons
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Metal Carbonyl Complexes
CO as a ligand strong s donor, strong
p-acceptor strong trans effect small steric effect
CO is an inert molecule that becomes activated by
complexation to metals
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C-like MOs
Larger homo lobe on C
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Mo(CO)6
18 electrons
6CO ligands x 2s e each
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Mo(CO)6 s-only bonding
The bonding orbitals will not be further modified
6 s ligands x 2e each
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p-bonding may be introduced as a perturbation of
the t2g/eg set Case 1 CO empty p-orbitals on
the ligands M?L p-bonding (p-back bonding)
t2g (p)
t2g
eg
eg
t2g
t2g (p)
Mo(CO)6 s-only
Mo(CO)6 s p
(empty p-orbitals)
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Metal carbonyls may be mononuclear or polynuclear
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Synthesis of metal carbonyls
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Characterization of metal carbonyls
IR spectroscopy
(C-O bond stretching modes)
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Effect of charge
Effect of other ligands
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The number of active bands as determined by group
theory
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13C NMR spectroscopy
13C is a S 1/2 nucleus of natural abundance
1.108
For metal carbonyl complexes d 170-290 ppm
(diagnostic signals) Very long T1 (use
relaxation agents like Cr(acac)3 and/or enriched
samples)
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Typical reactions of metal carbonyls
Ligand substitution
Always dissociative for 18-e complexes, may be
associative for lt18-e complexes
Migratory insertion
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Metal complexes of phosphines
PR3 as a ligand Generally strong s donors, may be
p-acceptor strong trans effect Electronic and
steric properties may be controlled Huge number
of phosphines available
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Tolmans electronic and steric parameters of
phosphines
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Typical reactions of metal-phosphine complexes
Ligand substitution
Very important in catalysis Mechanism depends on
electron count
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Metal hydride and metal-dihydrogen complexes
Terminal hydride (X ligand)
Bridging hydride (m-H ligand, 2e-3c)
Coordinated dihydrogen (h2-H2 ligand)
Hydride ligand is a strong s donor and the
smallest ligand available
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Synthesis of metal hydride complexes
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Characterization of metal hydride complexes
1H NMR spectroscopy
High field chemical shifts (d 0 to -25 ppm usual,
up to -70 ppm possible) Coupling to metal nuclei
(101Rh, 183W, 195Pt) J(M-H) 35-1370 Hz Coupling
between inequivalent hydrides J(H-H) 1-10
Hz Coupling to 31P of phosphines J(H-P) 10-40
Hz cis 90-150 Hz trans
IR spectroscopy
n(M-H) 1500-2000 cm-1 (terminal) 800-1600 cm-1
bridging n(M-H)/n(M-D) v2 Weak bands, not very
reliable
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Some typical reactions of metal hydride complexes
Transfer of H-
Transfer of H
A strong acid !!
Insertion
A key step in catalytic hydrogenation and related
reactions
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Bridging metal hydrides
Anti-bonding
Non-bonding
2-e ligand
4-e ligand
bonding
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Metal dihydrogen complexes
Characterized by NMR (T1 measurements)
Very polarized d, d-
If back-donation is strong, then the H-H bond is
broken (oxidative addition)
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NMR characterization of organometallic complexes
If X CO
1H NMR
1 n(CO) band
2 n(CO) bands
1 n(CO) band
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Metal-olefin complexes
2 extreme structures
sp3
sp2
p-bonded only
metallacyclopropane
Zeises salt
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Effects of coordination on the CC bond
Compound C-C (Å) M-C (Å)
C2H4 1.337(2)
C2(CN)4 1.34(2)
C2F4 1.31(2)
KPtCl3(C2H4) 1.354(2) 2.139(10)
Pt(PPh3)2(C2H4) 1.43(1) 2.11(1)
Pt(PPh3)2(C2(CN)4) 1.49(5) 2.11(3)
Pt(PPh3)2(C2Cl4) 1.62(3) 2.04(3)
Fe(CO)4(C2H4) 1.46(6)
CpRh(PMe3)(C2H4) 1.408(16) 2.093(10)
CC bond is weakened (activated) by coordination
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Characterization of metal-olefin complexes
IR n(CC) 1500 cm-1 (w)
NMR 1H and 13C, d lt free ligand
X-rays CC and M-C bond lengths
indicate strength of bond
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Synthesis of metal-olefin complexes
PtCl42- C2H4 ? PtCl4(C2H4)- Cl-
RhCl3.3H2O C2H4 EtOH ? (C2H4)2Rh(m-Cl)22
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Reactions of metal-olefin complexes
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Metal cyclopentadienyl complexes
Metallocenes (sandwich compounds)
Bent metallocenes
2- or 3-legged piano stools
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Cp is a very useful stabilizing
ligand Introducing substituents allows modulation
of electronic and steric effects
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Metal alkyl, carbene and carbyne complexes
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Main group metal-alkyls known since old
times (Et2Zn, Frankland 1857 R-Mg-X, Grignard,
1903))
Transition-metal alkyls mainly from the 1960s
onward
Ti(CH3)6
W(CH3)6
PtH(C?CH)L2
Cp(CO)2Fe(CH2CH3)6
Cr(H2O)5(CH2CH3)62
Why were they so elusive?
Kinetically unstable (although thermodynamically
stable)
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Reactions of transition-metal alkyls