Download Organometallic Compounds

Organometallic Compounds
By Prof Yeap Guan Yeow
- Organometallic compound –
contains one or more metal-carbon (M-C) bonds.
Complexes:
WMe6, [MoMe7]-, TiMe4 and MeMn(CO)5
(Me = CH3)
M-CMe ≅ Metal with one-carbon
(equivalent to the localized interaction of metal and
η1-cyclopentadienyl)
Fe-CCHO
Fe-CPh
Carbonyl Ligand
- Bonding in M-CO interaction.
- Weak σ-donor and strong π-acceptor.
- Resonance structure
_
+
:M — C ≡O : ↔ M = C = O
- A form of back-donation of electronic charge
- An example of synergy effect
As a result:
(i) Population of the CO π*-Molecular Orbital weakens and lengthens the
C-O bond.
(ii) Enhance the ordering of M-C bond.
Two modes in multinuclear metal species:
CO (terminal)
CO (bridging)
Structure elucidation by spectroscopy techniques (IR, NMR etc) and X-ray
IR
_
IR spectroscopic data: values of νCO (cm-1) for isoelectronic sets of
tetrahedral M(CO)4 and octahedral M(CO)6 complexes
Complexes
Complexes
Ni(CO)4
[Co(CO)4]-
[Fe(CO)4]2-
2060
1890
1790
[Mn(CO)6]+
Cr(CO)6
[V(CO)6]-
2101
1981
1859
_
- The lower the value of νCO , the weaker the C-O bond and this indicates greater
back-donation of charge from metal to CO.
- Additional negative charge delocalized onto the ligands, causing a decrease in
_
νCO (cm-1)
Bridging mode of CO
Type of CO
Approximate range
for ν(CO) in neutral
complexes (cm-1)
____________________________________________
Free CO
2143
Terminal M-CO
1850-2120
Symmetrical µ2-CO
O
M-C-M
1700-1860
Symmetrical µ3-CO
O
M-C-M
M
1600-1700
Question:
1. What is the synergy effect, and how does it relate to metal-carbonyl
bonding?
2. Infrared spectra of [VCO)6]- and Cr(CO)6 show absorptions at 1839 and 1981
cm-1 respectively assigned to νCO, and 460 and 441 cm-1 assigned to νMC.
3. For the following set, which complex would be expected to have the
highest C-O stretching frequency?
[Re(CO)6]+
W(CO)6
[Ta(CO)6]-
Synthesis/Preparation
1. Direct reaction
Mg + CH3I
→
CH3MgI
2. By using akylating agent
PCl3
+
3C6H5MgCl
→
P(C6H5)3
+
3MgCl2
3. Interaction between halide metal with alkene and alkines
4. Insertion
[(CN)5Co-Co(CN)5]4- + HC≡CH → [(CN)5Co-CH=CH-Co(CN)5]4-
Application of Organometallic Compounds
For example, in catalysis.
(i) Upon isomerization
(ii) Upon hydrogenation
(iii) Upon hydroformylation
(iv) Upon polymerization of alkenes (RCH=CH2) by using
Ziegler-Natta catalyst.
Ziegler-Natta catalyst consists of TiCl4 and Et3Al with
Et2AlCl as co-catalyst.
Hydroformylation
Hydroformylation, also known as oxo synthesis, is an
important industrial process for the production of aldehydes
from alkenes.
This chemical reaction entails the addition of a
formyl group (CHO) group and a hydrogen atom
to a carbon-carbon double bond
Hydrogenation by Wilkinson’s catalyst
Wilkinson's catalyst
It is the common name for
chlorotris(triphenylphosphine)rhodium(I)
RhCl(PPh3)3
It is named after the late organometallic chemist and
1973 Nobel Laureate, Sir Geoffrey Wilkinson
who popularized its use.
Wacker Process
The Wacker process or the Hoechst-Wacker process
(named after the chemical companies of the same name)
originally referred to the oxidation of ethylene to
acetaldehyde by oxygen in water in the presence of a
tetrachloropalladate catalyst.
This chemical reaction, a German invention, was the first
organometallic and organopalladium reaction applied on
an industrial scale.
Ferrocene
The cyclopentadienyl (Cp) ligand is a monoanionic
ligand with the formula C5H5.
The first characterized example of a cyclopentadienyl
complex was ferrocene, Cp2Fe, which has an iron atom
"sandwiched" between two planar Cp rings as shown
on the left.
For this reason, bis(cyclopentadienyl)
complexes are sometimes called "sandwich compounds"
or metallocenes.
Bonding in Cp Complexes