Download Organometallic compounds of the d-block elements

CH307 Organometallic Compounds of d-block elements
Organometallic compounds contain at least
one metal-carbon bond.
Ligands
Organometallic complexes have a wide range of ligand
types from simple M-R to the -cyclopentadienyl rings
of ferrocene.
Hapticity of a ligand
The hapticity of a ligand indicates the atoms that are
directly bonded to the metal.
The Greek letter  (eta) is used,
(5-C5H5) M indicates that all five carbons are bonded to M.
P. McArdle 2009
5-C5H5 ligands are usually
drawn as in (23,1b)
 - bonded or 1-alkyls of the M-R type are well known.
Examples are WMe6, TiMe4 and MeMn(CO)5. Which
Contain simple (2c-2e) M-R bonds.
The compound (5C5H5)(1C6H5)Fe(CO)2
also contains one (2c-2e) bond.
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The Carbonyl Ligand, CO
CO is an unlikely ligand as it does not alter the pH of H2O
(i.e. will not complex a H+)
It also has a very small dipole moment and the strongest
known chemical bond (1100 kJmol-1).
Mopac calculation of CO HOMO and LUMO orbitals
HOMO
LUMO
P. McArdle 2009
The accepted bonding scheme has two steps:
2. M to CO back –donation
1. -donation from a filled
C based orbital to an empty from a filled metal d-orbital
metal orbital.
to an empty CO (*) orbital.
Step 2 enhances step 1 and thus the scheme is often said
to be synergic.
P. McArdle 2009
In this scheme CO is acting as a -acceptor or -acid ligand
and step 2 enchances step 1 (a synergic effect).
CO ligands may be terminal, doubly bridging (2) or triply
bridging (3).
MC-O IR spectra are very useful as the bands are very
intense and narrow.
Free CO is at 2143cm-1 and neutral M(CO)x
are at ~ 2000cm-1 ~ 100cm-1 lower than free CO.
This indicates a reduction in CO bond order on complexation
and shows the importance of step 2 in the bonding scheme.
P. McArdle 2009
C-O bond lengths from electron diffraction and X-ray
diffraction studies show that
free CO is 1.13Å MC-O is 1.17 and 2M2C-O is 1.20Å.
IR spectra show this even more clearly
CO
2143
2000
1800cm-1
Within an isoelectronic series (same no. of valence electrons)
The following is generally true
a +ve charge increases MC-O ~ 100cm-1 and
a –ve charge decreases MC-O ~ 100cm-1.
P. McArdle 2009
Hydride Ligands
Many molecular hydrides are known which involve
transition metal organometallic systems.
M-H systems vary from hydridic (basic) types which
react with H+ to M-H systems which are strong acids.
1H
NMR is the best way to characterise M-H systems.
The chemical shift range is from  -8 to –30ppm.
This is for the most part outside the normal chemical shift
range of 0 to 10 ppm and it is easy to detect M-H by NMR.
The M-H stretching vibration is observed close to 2000cm-1
However it is often weak and hard to see if CO is present.
P. McArdle 2009
Phosphine ligands
Substituted PH3, PR3, derivatives are good ligands
PF3 ~ CO due to extensive back bonding enhanced by F.
PR3 in general are weaker -acceptors than CO.
In terms of the bonding scheme used for CO PR3
uses a P d-orbital as an acceptor.
The R groups can be used to alter the importance of
-donation and -acceptor behaviour.
This is an electronic effect.
Along the series Me, Ph, OMe and OPh the R group is
increasing in electron withdrawing power.
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Steric effects
The ability of different R groups in PR3 to produce
different steric effects has become very important.
C.A. Tolman was the first to study this extensively.The
cone-angle construction can be used to quantify this property.
This is very useful for fine tuning catalysts
P. McArdle 2009
The phosphine cone angles cover a very wide range with some
taking up more than 180 of the metal’s coordination sphere.
107º
118
122
145
P(OMe)3
PMe3
PMe2Ph
PPh3
P(4-MeC6H4)3
P(3-MeC6H4)3
PtBu3
P(2-MeC6H4)3
145º
165
182
194
Bi and tri-dentate phosphines are also very important
two well known examples are bis(diphenylphosphino)methane
,dppm, and bis(diphenylphosphino)ethane, dppe.
The latter is also called diphos.
(Ph)2P
dppm
P(Ph)2
(Ph)2P
P(Ph)2
dppe (diphos)
P. McArdle 2009
-bonded organic ligands
2 alkenes bond to the metal “side on”
and are two electron donors.
A scheme similar to that used for CO can also be invoked here.
The alkene to M donation is from the filled  C=C orbital and the
back bond from M is into the * C=C orbital
P. McArdle 2009
If the substituents on the olefin are strongly electron
withdrawing (e.g. CN) back-bonding is enhanced to such an
extent that C2(CN)4 complexes are often considered to be
metallacyclopropane complexes.
H
NC
H
CN
M
M
H
H
NC
CN
Zeise’s salt was reported in 1827 (structure in1960s)
K2PtCl4 + ethene or ethanol = K+[(2-ethene)PtCl3]_
Cl
Cl
Cl
H
H
Pt
H
H
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Dienes and trienes may also bond.
(4C4H6)Fe(CO)3
(6-cycloheptatriene)Cr(CO)3
C4H6 = buta-1,3-diene
O
Fe
C C C
O
Cr
O
O
C C C
O
O
(3-allyl)(4-C4H6)(5-Cp)Mo
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Molecular orbitals of ethene
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Molecular Orbitals of Buta-1,3-diene
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Benzene
Molecular
Orbitals
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Odd numbered -systems have a non-bonding
orbital in the middle of the energy range
with a node in the middle.
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18-electron rule
Metal complexes in low oxidation states with high field
ligands tend to have 18-valence electrons on the metal.
[The 18 electron rule does not work for high oxidation states
(>2) and weak field ligands, e.g. [Cr(H2O)6]3+].
To simplify electron counting the metal is always in oxidation
state 0 sharing 1e in covalent bonds and accepting 2e
in donor bonds.
Some 16e compounds are stable at the RHS of the
transition series Rh, Pd, Pt, Ir.
M .. Cl
M
..
PR3
In M-Cl the metal gains one electron from the Cl
And in MPR3 the metal gains 2e from PR3
P. McArdle 2009
Listing ligands by number of electrons they provide
1e
M-H, M-Cl, M-Br, M-I, M-R, bent M-NO and M-M
2e
CO, PR3, R2C=CR2, R2C (carbene)
3e
3-allyl, NO (linear), RC (carbyne), -X (Cl, Br, I)
4e
4-diene
5e
5 -dienyl
6e
6 -C6H6
P. McArdle 2009
Example
(6-C6H6)Cr(CO)3
Cr(0) group 6
6e
6-C6H6
6e
3CO 3x2
6e
18e
This complex would be expected to be stable.
P. McArdle 2009
((2 ethene)2RhCl)2 has a -Cl structure
Cl
Rh
Rh
Cl
Rh(0) group 9
Cl –Rh
ClRh
2 x 2-ethene
9e
1e
2e
4e
16e
16e compounds are often stable for Rh
as it is at the RHS of the d-block.
P. McArdle 2009
Formulae of the binary metal carbonyls M(CO)x
Using the 18 electron rule the formulae of the simplest
binary carbonyls can be predicted.
Remember the 18 electron rule is a principle of maximum
orbital use.
It is not easy to increase a metals valence electron count
beyond 18 as all s, p and d-orbitals are filled by 18e.
The simplest possible formulae are;
M(CO)x when M has an even number of valence electrons
and
M2(CO)x when M has an odd number of valence electrons
P. McArdle 2009
Metal valence electron count
The number of valence electrons = Group number for d-block
No. 3
4
Sc Ti
5
V
6
Cr
7
8
9
10
Mn Fe Co Ni
11 12
Cu Zn
Example with even number of electrons
Cr
Cr(CO)x
X = no. of CO per M
Cr 6e + (xCO) 2x = 18
x = (18 –6) / 2 = 6
Predicted formula Cr(CO)6
P. McArdle 2009
Example with odd number of electrons
Co
Co2(CO)2x
Cobalt is in group 9
These metals get 18e by forming an M-M bond which gives
each metal 1 extra electron.
The formula is then for each metal
M(e) + 1(from M-M) + 2x = 18
x = (18 –1 – M(e)) / 2
Which for Co is x = (18 – 1 – 9) / 2 = 4
Thus the formula is Co2(CO)8
P. McArdle 2009
Simple Stable Binary Metal Carbonyls
M(e) 6
7
Cr(CO)6 Mn2(CO)10
Mo
W
Tc
Re
8
9
10
Fe(CO)5 Co2(CO)8 Ni(CO)4
Ru
Os
Rh
Ir
While other formulae are known the 2nd and 3rd rows
follow the first row for the most part.
There are no simple carbonyls known for Pd or Pt
What about Ti and V ?
Ti
Group 4 x = (18 – 4) / 2 = 7
Ti(CO)7 does not exist, there are not enough orbitals to bond
7 CO groups. However salts of [Ti(CO)6]2– have been made
P. McArdle 2009
V Group 5
V2(CO)2x
M(e) + 1(from M-M) + 2x = 18
x = (18 – 1 – 5) / 2 = 6
This suggests a formula V2(CO)6
This would be 7-coordinate which would exceed 6
and be too sterically hindered.
Salts of 18e [V(CO)6]– are known and
the very unstable V(CO)6 radical has been isolated.
P. McArdle 2009
Another approach
No. 3
Sc
4
Ti
5
V
6
Cr
7
8
Mn Fe
9
10
Co Ni
11 12
Cu Zn
Remember 1 simple formula e.g. Fe(CO)5 or Ni(CO)4 and
work from there.
Fe(CO)5 is 18e
Mn(CO)5 is 17e + 1 M-M = 18e
Formula is (Mn(CO)5)2
Co(CO)5 is 19e Co(CO)4 is 17e + 1 M-M gives (Co(CO)4)2
Many other formulae are known e.g. In the Fe group Fe forms Fe(CO)5 Fe2(CO)9 Fe3(CO)12
Ru and Os also form M(CO)5 but are much more stable
as M3(CO)12
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Structures of some binary carbonyls
Cr(CO)6 Oh
Mn2(CO)10 staggered
Co2(CO)8 bridged + non-bridged (soln.)
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Fe(CO)5 D3h
Ni(CO)4 Td ED
Synthesis of M(CO)x
Reaction of metal powder (often prepared in situ) with CO
under pressures of up to 200 bar is the general method.
390K 70bar
CrCl3 + LiAlH4 + CO

Cr(CO)6
400K 90bar
RuCl3.xH2O +CO +Zn

Ru3(CO)12
Physical properties
Cr(CO)6, white solid
Mn2(CO)10, yellow solid
Fe(CO)5, orange liquid b.p.103º (toxic),
Co2(CO)8, orange air sensitive solid
Ni(CO)4, colourless liquid b.p. 45º (very very toxic)
P. McArdle 2009
Reactions of Organometallic Compounds
Ligand Substitution
Carbonyl substitution
h or 
M(CO)x-1L + CO
M(CO)x + L
M(CO)x
18e
h or 
slow
M(CO)x-1
16e
L
fast
M(CO)x-1L
18e
The substitution mechanism for 18e complexes
is dissociative.
The reaction rate does not depend on the concentration
of L.
P. McArdle 2009
Oxidative Addition
A reaction in which the metal is oxidized by two units and
the metal coordination number is increased usually by 2.
Addition of a molecule X-Y to a metal centre.
N
M(L)x +
X
X-Y
N+2
M(L)x
Y
Oxidation state N
Oxidation state N+2
Coordination No. x
Coordination No. x+2
P. McArdle 2009
The best know examples are provide by the complexes of
Rh(I) Ir(I) Pd(0) Pd(II) Pt(0) and Pt(II).
The metal must have 2 stable oxidation states
separated by 2 units.
e.g. Rh(I) and Rh(III) or Pd(0) and Pd(II)
Perhaps the best known is Vaska’s complex [Ir(PPh3)2(CO)Cl]
This complex reacts with a very large number of X-Y molecules.
[Ir(PPh3)2(CO)Cl] + MeI = [Ir(Me)(PPh3)2(CO)(I)Cl]
16e Ir(I)
4 coord.
18e Ir(III)
6 coord.
P. McArdle 2009
Oxidation state of the metal
In the [MLaXb]c+ formalism
a is the number of L type ligands
b is the number of X type ligands
c is the charge on the complex
Oxidation state of the metal = b + c
[ML4X2]+
The metal is in oxidation state III
P. McArdle 2009
Conversion of (n-ligand)M complexes to [MLaXb]c+ formalism
Ligands which donate an even number of electrons are L type
Monodentate neutral ligands e.g. PR3 are L type
In general n-ligands with even n have
La with a = (no. of electrons / 2) e.g. 6-C6H6 = L3
Ligands which donate an odd number of electrons have an
X type interaction and possible L interactions
n-ligands with odd n have
an X interaction and (n-1) / 2 L interactions
Only X type interactions affect oxidation state
P. McArdle 2009
(3-allyl)(4-butadiene)(5-cyclopentadienyl)Mo
(X L) ( L2 ) ( X L2 )Mo
MoL5X2
Mo oxidation state = II
P. McArdle 2009
[Mn(5-C5H5)(CO)3] Mn( X L2 ) ( L3 )
[Co(5-C5H5) (CO)(PPh3)2],
[Mo(CO)3(PPh3) 2I]
CoL5X
MoL5X
[Co(2-Buta-1,3-diene)(CO)4Br]
[(3-allyl)Mo(CO)4I]
[Fe(CN)5NO]2nitroprusside
MoL5X2
MnL5X
Mn(I)
Co(I)
Mo(I)
CoL5X
Co(I)
Mo(II)
[Fe ( X5 ) ( X L )]2-  [FeLX6]2-  Fe(IV)
NO is more easily oxidized than Fe(II) Fe(II)NO+ [FeX5(L+)]2The iron is diamagnetic low spin d6
[NO]+ is isoelectronic with CO
P. McArdle 2009
You can buy [NO]+[PF6]-
Oxidative addition is reversible
the reverse reaction is called reductive elimination
Vaska’s complex is an oxygen carrier
Ph3P
Cl
Ir
CO
PPh3
+
PPh3
OC Ir O
Cl
O
PPh3
O
O
Oxidative addition followed by reductive elimination
is the basic mechanism of many catalytic processes.
O
OC
OC
Me
Co
CO
H
H
H
Co CO
OC
CO
+
O
Me
H
In this example reductive elimination gives a new C-H bond
P. McArdle 2009
Mechanism of oxidative addition – There are 3 established
reaction mechanisms.
1. Concerted addition
M
H
+
H2
MH2
H
H
H
M
H

H
*
M
H
H
filled empty
This is the most likely mechanism when X=Y in X-Y and the
reaction is conducted in a non-polar solvent.
P. McArdle 2009
2. Ionic stepwise
slow
fast
+
M + RX  MR + X  M(R)(X)
A likely mechanism when X≠Y and the reaction takes place in
a polar solvent.
3. Free radical
In the presence of O2 peroxides or other free radical
sources radical mechanisms have been detected. For
example in the case of Vaska’s complex
Init + Ir(I)  Init-Ir(II)
Init-Ir(II) + RX  Init-Ir-X + R
R + Ir(I)  R-Ir(II)
R-Ir(II) + RX  R-Ir(I)-X + R
P. McArdle 2009
Alkyl and hydrogen migrations
OC
OC
Me
Mn
CO
O
CO
CO
+
OC
CO
OC
Mn
Me
CO
CO
CO
This is an example of alkyl migration
This has also been called CO insertion into a metal alkyl
The reaction mechanism suggests the former
The term migratory insertion tries to satisfy everyone
Using 13CO the mechanism can be deduced.
If 13CO is used none of it ends up in the MeC=O group
P. McArdle 2009
Me
OC
OC
Mn
C
CO
Me
O
O
Mn
OC
CO
CO
6-coordinate
When this 13CO labelled
complex is heated it looses
CO and the 13C label
position in the products can
be used to test the
mechanism.
OC
Mn
CO
CO
Me
25%
+
OC
OC
5-coordinate
O
Me
OC
13
13CO
OC
CO
CO
O
OC
OC
OC
OC
C
CO
Mn 13
CO
CO
- CO
CO
Mn
CO
Me
13
CO
25%
+
OC
Me
P. McArdle 2009
Me
C
Mn
CO
O
CO
6-coordinate
CO is lost trans to another CO (trans
effect of CO > COMe) and Me then
moves into the vacant position.
Thus the result supports the alkyl
migration mechanism.
If only CO insertion was involved the
only products would be those in the
box.
CO
Mn
CO
CO
13
CO
25%
+
Me
OC
CO
Mn
CO
25%
CO
13
CO
-Hydrogen elimination
-H elimination, giving an unstable [M(alkene)H] complex, is the
principle decomposition pathway for metal alkyls which have
a -hydrogen.
This is why the most stable alkyl complexes are formed
by Me, Ph, CH2CMe3, CH2SiMe3 and CH2Ph.
Ln M
H


H
H
R
H
H
H
R
Ln M
H
H
H
R
H
H
H
Ln M
After -elimination the olefin may dissociate (L type ligand) leaving a
very reactive metal hydride which itself then reacts/decomposes.
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“Insertion Reactions”
Olefin into metal hydride Metal hydride + olefin  metal alkyl
Ln M H
+
H
H
olefin
insertion
H
H
-elimination
Ln M
H
H
This is really
H migration
H
CO
CO
Ln M
CO
This is really alkyl
migration
R
O
Olefin into M-R
Ln M R
H
Metal alkyl + CO  metal acyl
CO into M-R
Ln M R
H
H
H
R'
H
H
Ln M
R'
R
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This is another alkyl
migration.
It is a mechanism for
olefin polymerization
Reactions involving M-M bonds
Oxidative clevage
M-M + X2  2M-X
(OC)5Mn-Mn(CO)5 + Br2  2 (CO)5Mn-Br
Reductive clevage
M-M + reducing agent  2 M¯
(OC)5Mn-Mn(CO)5 + Na / Hg  2 (OC)5Mn¯
These anions are good nucleophiles.
(OC)5Mn¯ + MeI  (CO)5Mn-Me + I¯
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(OC)5Mn-Mn(CO)5 + Br2  2 (OC)5Mn-Br
 Na / Hg
(OC)5Mn¯
MeMgBr
 MeI
(OC)5Mn-Me
 CO
(OC)5Mn-(CO)-Me
P. McArdle 2009
Metal carbene complexes
M=CR2
There are two types
1. Hetero atom stabilized or Fischer type carbenes
OMe
(OC)5Cr
R
The O atom is the hetero
atom in this case.
2. Unstabilized or Schrock type carbenes
R'
M
R
There is no hetero atom attached
to the carbene carbon.
P. McArdle 2009
Synthesis of metal carbene complexes
The Fischer type are obtained by reaction of Li alkyls
with coordinated CO. Works best for Cr, Mo and W.
(OC)5Cr
+
C O
Me
(OC)5Cr
Li
C O Li+
Me
The anion is then alkylated using Me3O+ BF4¯
OMe
(OC)5Cr
C
R
The hetero atom stabilizes the molecule and polarizes the
Cr=C bond
+
OMe
The carbene carbon is
(OC)5Cr C
attacked by nucleophiles
R
P. McArdle 2009
The Schrock type are synthesised by -H abstraction
Ta(CH2tBu)3Cl2
LiCH2tBu
-LiCl -CMe4
H
(tBuCH2)3Ta
tBu
Schrock type carbenes involve early t-metals in high
oxidation states.
-
(OC)5Cr
+
OMe
C
 +  - R'
M
R
R
Fischer type are polarized M¯C+ and are
not olefin metathesis catalysts,
C attacked by nucleophiles
Schrock type are polarized M+C¯ and
are good olefin metathesis catalysts,
C attacked by electrophiles.
P. McArdle 2009
Metal carbyne complexes
OMe
(OC)5 M
Ph
+
BX3
X(OC)4M C
Ph
+
BX2OMe
Fischer carbenes react with BX3 (loss of OMe¯)
The cation formed is susceptible to X¯ attack and CO loss
+
(OC)5 M C
Ph
+
BX3OMe
X(OC)4M C
Ph
+
M = Cr, Mo, W and X = Cl, Br, I
-H abstraction from a Schrock carbene can also yield
carbyne complexes.
Ta C
Cl
Cl
tBu
H
PMe3 Ph3P=CH2
- Ph3MePCl
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Ta C tBu
Cl
PMe3
CO
+
CO
Bond length and bond order
An M-C single bond should be close to the sum of the
covalent radii.
M-C double and triple bonds should be shorter than M-C
This is the case
M C
>M
M = Cr ~2.10
C
2.04
>M
C
1.69 Å
Metal carbonyls
M-C bond lengths are shorter than M=C in carbenes
M C O
M C O
P. McArdle 2009
A resonance form with
some MC multiple
character is involved
Metallocenes
di-cyclopentadienyl sandwich complexes
Ferrocene is by far the best known and the most stable
[Fe(5-cyclopentadienyl)2]
FeL4X2
Fe 8
L4 8
X2 2
18
Fe( X L2 )2
FeL4X2
Only Ru and Os can also do this
All non-iron group metallocenes
are less stable than ferrocene
Cobaltcene has 19e and is unstable in air
The cobaltcinium cation is 18e and air stable
[Co(5-cyclopentadienyl)2]+ [PF6]¯
P. McArdle 2009
Bonding in 5-cyclopentadienyl complexes
cyclopentadienyl molecular orbitals Chapt 18 box 18.2
Each C is sp2
e2 hybridized and
after the -frame 5
pz orbitals remain
e1
dxz, dyz
a1
dz2
D5d Metal orbitals
P. McArdle 2009
+ - +
+
-
+
+ -
+
+
Chemistry of ferrocene
O
Fe
Ac2O + H3PO4
Fe
mono acetylation
mild conditions
nBuLi
RCOCl
AlCl3
Fe
Me
Me
Fe
O
Me
O
di-acetylation more
vigorous conditions
P. McArdle 2009
Li
Zirconocene derivatives
Zirconium is stable in oxidation state IV and forms the
Zirconocene derivative (5-C5H5)2ZrCl2
Cl
Zr
Cl
This compound
Is 16e Zr Group 4
( X L2 )2 Zr X2
ZrL4X4 = 16e
The analogous W
compound is 18e
as W is in Group 6
Cl
A related Zr system
has led to important
new chiral
Metallocene Ziegler
catalysts
W
Zr
Cl
Cl
P. McArdle 2009
Cl
This type of compound
is sometimes called a
bent metallocene
Questions
What is meant by the term -acid ligand ?
Explain the term "ligand cone angle". Which of the following
have the largest and smallest ligand cone angles, P(Ph)3,
P(o-tolyl)3 and PH3.
Give metal valence electron counts for the following
systems and indicate those which are likely to be stable
and those which are not;
[Mn(5-C5H5)(CO)3], [Co(5-C5H5)(CO)(PPh3)2],
[Mo(CO)3(PPh3) 2I], [Co(2-Buta-1,3-diene) (CO)4Br]
and [Mo(3-allyl)(CO)4I].
Give metal valence electron counts for the following systems and
indicate those which are likely to be stable and those which are not;
[Cr(5-C5H5)(CO)2], [Mn(5-C5H5)(CO)4],[Mo(CO)3(PPh3)I2],
[Co(2-Butene)(CO)3Br] and [Mo(3-allyl)(5-C5H5)(CO)2].
P. McArdle 2009