Download Metal-olefin complexes

Elements of organometallic chemistry
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
A simple classification of the most important ligands
X
LX
L
L2
L2X
L3
Counting electrons
Method A
Method B
Determine formal oxidation state of metal
Deduce number of d electrons
Ignore formal oxidation state of metal
Count number of d electrons for M(0)
Add d electrons + ligand electrons (A)
Add d electrons + ligand electrons (B)
The end result will be the same
Why 18 electrons?
antibonding
Organometallic complexes
18-e most stable
16-e stable (preferred for Rh(I), Ir(I), Pt(II), Pd(II))
<16-e OK but usually very reactive
> 18-e possible but rare
Organometallic Chemistry
Fundamental Reactions
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
0
0
0
Insertion-deinsertion
Association-Dissociation of Lewis acids
D(FOS) = 0; D(CN) = ± 1; D(NVE) = 0
Lewis acids are electron acceptors, e.g. BF3, AlX3, ZnX2
H
H
+ BF3
W:
H
BF3
W
H
This shows that a metal complex may act as a Lewis base
The resulting bonds are weak and these complexes are called adducts
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
HCo(CO) 4
HCo(CO) 3 + CO
For 18-e complexes, dissociative mechanisms only
For <18-e complexes dissociative and associative mechanisms are possible
Oxidative addition-reductive elimination
D(FOS) = ±2; D(CN) = ± 2; D(NVE) = ±2
Mn+ + X-Y
M(n+2)+
X
Y
H
Ph3P
Cl
I
Ir
CO
PPh3
Vaska’s compound
+ H2
Ph3P
IrIII
Cl
H
PPh3
CO
Very important in activation of hydrogen
Insertion-deinsertion
D(FOS) = 0; D(CN) = 0; D(NVE) = 0
M-X + L
(CO)5Mn-CH3 + CO
M-L-X
O
(CO)5Mn-C-CH 3
Very important in catalytic C-C bond forming reactions
(polymerization, hydroformylation)
Also known as migratory insertion for mechanistic reasons
Metal Carbonyl Complexes
M-CO
CO as a ligand
strong s donor, strong π-acceptor
strong trans effect
small steric effect
CO is an inert molecule that becomes activated by complexation to metals
“C-like MO’s”
Frontier orbitals
Larger homo lobe on C
Mo(CO)6
anti bonding
“metal character”
non bonding
“18 electrons”
6CO ligands x 2s e each
12 s bonding e
“ligand character”
Mo(CO)6 s-only bonding
anti bonding
“metal character”
non bonding
6 s ligands x 2e each
The bonding orbitals will not be further modified
12 s bonding e
“ligand character”
π-bonding may be introduced
as a perturbation of the t2g/eg set:
Case 1: CO
empty π-orbitals on the ligands
ML π-bonding (π-back bonding)
t2g (π*)
t2g
eg
eg
Do
D’o
D’o > Do
t2g
Energy gain
t2g (π)
Mo(CO)6
s-only
Mo(CO)6
s+π
(empty π-orbitals)
Metal carbonyls may be mononuclear or polynuclear
Synthesis of
metal carbonyls
Characterization of metal carbonyls
IR spectroscopy
M-C-O
(C-O bond stretching modes)
Effect of charge
Effect of other ligands
The number of active bands
as determined by group theory
13C
13C
NMR spectroscopy
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)
Typical reactions of metal carbonyls
Ligand substitution:
Cr(CO)6 + CH3CN
Cr(CO)5(CH3CN) + CO
Always dissociative for 18-e complexes, may be associative for <18-e complexes
Migratory insertion:
CH3
OC
Mn
OC
CO
CO
CO
H3C CO
H3C
C
CO
O
Mn
OC
CO
CO
CO
C
CO
O
Mn
OC
CO
CO
Metal complexes of phosphines
M-PR 3
PR3 as a ligand
Generally strong s donors, may be π-acceptor
strong trans effect
Electronic and steric properties may be controlled
Huge number of phosphines available
Tolman’s electronic and steric parameters of phosphines
Typical reactions of metal-phosphine complexes
Ligand substitution:
HCo(CO) 4 + PBu3
HRh(CO)(PPh 3)3 + C2H4
HCo(CO) 3(PBu3) + CO
HRh(CO)(PPh 3)2(C2H4) + PPh 3
Very important in catalysis
Mechanism depends on electron count
Metal hydride and metal-dihydrogen complexes
M
H
Terminal hydride (X ligand)
H
M
H
Bridging hydride (m-H ligand, 2e-3c)
H
Coordinated dihydrogen (h2-H2 ligand)
M
H
Hydride ligand is a strong s donor and the smallest ligand available
Synthesis of metal hydride complexes
IrCl(CO)(PPh 3)2 + H2
RuCl 2(PPh 3) 3 + H 2
Ir(H) 2Cl(CO)(PPh 3)2
Et3N
RuHCl(PPh 3) 3 + Et 3N.HCl
Co2(CO) 8 + H 2
2 HCo(CO)4
[Fe(CO) 4] 2- + H+
[HFe(CO) 4]-
Cp2ZrCl2 + NaBH 4
Cp2ZrHCl
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) = √2
Weak bands, not very reliable
Some typical reactions of metal hydride complexes
Transfer of HCp2Zr(H)2 + 2CH2O
Cp2Zr(OCH3)2
Transfer of H+
HCo(CO)4
H+ + [Co(CO)4]-
A strong acid !!
Insertion
IrH(CO)(PPh3)3 + (C2H4)
Ir(CH2CH3)(CO)(PPh3)3
A key step in catalytic hydrogenation and related reactions
Bridging metal hydrides
H
M
H
H
M
M
M
M
M
H
Anti-bonding
M
H
Cl:
:
H
M
M
M
2-e ligand
M
M
Non-bonding
M
M
4-e ligand
H
bonding
M
M
Metal dihydrogen complexes
M
Characterized by NMR (T1 measurements)
H
H
H
M
H
OC
OC
PiPr3
CO H
W
H
Very polarized
d+, d-
PiPr3
If back-donation is strong, then the H-H bond is broken (oxidative addition)
NMR characterization of organometallic complexes
1H
NMR
If X = CO
1 n(CO) band
2 n(CO) bands
1 n(CO) band
Metal-olefin complexes
2 extreme structures
sp3
metallacyclopropane
sp2
π-bonded only
Zeise’s salt
Effects of coordination on the C=C bond
Compound
C-C (Å)
M-C (Å)
C2H4
1.337(2)
C2(CN)4
1.34(2)
C2F4
1.31(2)
K[PtCl3(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)
C=C bond is weakened (activated) by coordination
Characterization of metal-olefin complexes
IR
n(C=C) ~ 1500 cm-1 (w)
NMR
1H
and 13C, d < free ligand
X-rays
C=C and M-C bond lengths
indicate strength of bond
Synthesis of metal-olefin complexes
[PtCl4]2- + C2H4  [PtCl4(C2H4)]- + Cl-
RhCl3.3H2O + C2H4 + EtOH  [(C2H4)2Rh(m-Cl)2]2
Reactions of metal-olefin complexes
Metal cyclopentadienyl complexes
M
Metallocenes
(“sandwich compounds”)
M
Bent metallocenes
“2- or 3-legged
piano stools”
M
M
L
L
L
L
L
Cp is a very useful stabilizing ligand
Introducing substituents allows modulation of electronic and steric effects
Metal alkyl, carbene and carbyne complexes
Main group metal-alkyls known since old times
(Et2Zn, Frankland 1857; R-Mg-X, Grignard, 1903))
Transition-metal alkyls mainly from the 1960’s onward
W(CH3)6
Ti(CH3)6
Cp(CO)2Fe(CH2CH3)6
PtH(CCH)L2
[Cr(H2O)5(CH2CH3)6]2+
Why were they so elusive?
Kinetically unstable (although thermodynamically stable)
Reactions of transition-metal alkyls
R
LnM
LnM + R-X
X
LnM R
+ H+
LnM+ + R-H