Organometallic
MT Complexes
MT Organometallics
Organometallic compounds of the transition
metals have unusual structures, and practical
applications in organic synthesis and industrial
catalysis.
MT Organometallics
One of the earliest compounds, known as Zeise’s
salt, was prepared in 1827. It contains an ethylene
molecule π bonded to platinum (II).
Zeise’s Salt
The bonding orbital
of ethene donates
electrons to the metal.
The filled d orbitals (dxz
or dyz) donate electrons
to the antibonding
orbital of ethene.
Square Planar Complexes
The complexes of platinum(II),
palladium(II), rhodium(I) and iridium(I) usually
have 4-coordinate square planar geometry.
These complexes also typically contain 16
electrons, rather than 18.
The stability of 16 electron complexes,
especially with σ-donor π-acceptor ligands, can
be understood by examining a MO diagram.
Square Planar Complexes
The electron
pairs from the 4
ligands used in σ
bonding occupy
the bonding
orbitals.
Square Planar Complexes
The dxy, dxz,
dyz and dz2
orbitals are
either weakly
bonding, nonbonding, or
weakly
antibonding.
Square Planar Complexes
The dx2 y2
orbital is antibonding, and
if filled, will
weaken the σ
bonds with
the ligands.
-
Square Planar Complexes
As a result, 16
electrons will
produce a
stable complex.
Catalysis of Square Planar
Compounds
Square planar complexes are often involved
as catalysis for reactions. The four-coordinate
complexes can undergo addition of organic
molecules or hydrogen, and then be regenerated
as the organic product is released from
coordination to the catalyst.
Catalysis – aldehyde formation
Pd(II) undergoes
addition of an alkene
which is subsequently
converted to an alcohol.
Addition of a hydrogen
atom to the metal with
subsequent migration to
the alcohol produces an
aldehyde.
Catalysis
Bonding of Hydrocarbons
Hydrocarbons can bond to transition metals
via σ bonds or π bonds. Wilkinson’s catalyst,
[RhCl(PPh3)] is used to hydrogenate a wide
variety of alkenes using pressures of H2 at 1 atm
or less.
During the hydrogenation, the alkene initially
π bonds to the metal, and then accepts a
hydrogen to σ bond with the metal.
Wilkinson’s Catalyst
Hydrogen Addition
Square planar complexes are known to react
with hydrogen, undergoing addition, and
breaking the H-H bond.
Hydrogen Addition
M
The hydrogen
bonding orbital
donates electron
density into an empty
p or d orbital on the
metal.
Hydrogen Addition
M
The loss of
electron density in
the bonding orbital
weakens the H-H
bond.
Hydrogen Addition
The metal can
donate electron
density from a filled d
orbital (dxz or dyz)
to the antibonding
orbital on hydrogen,
thus weakening or
breaking the H-H
bond.
The Template Effect
A metal ion can be used to assemble a group of
organic ligands which then undergo a condensation
reaction to form a macrocyclic ligand. Nickel (II) is
used in the scheme below.
MT Carbonyls
Metal carbonyl compounds were first
synthesized in 1868. Although many
compounds were produced, they couldn’t be
fully characterized until the development of Xray diffraction, and IR and NMR spectroscopy.
MT Carbonyls
Metal carbonyl compounds typically contain
metals in the zero oxidation state. In general,
these compounds obey the “18 electron rule.”
Although there are exceptions, this rule can
be used to predict the structure of metal
carbonyl cluster compounds, which contain
metal-metal bonds.
The 18 Electron Rule
Many transition metal carbonyl compounds
obey the 18-electron rule. The reason for this
can be readily seen from the molecular orbital
diagram of Cr(CO)6. The σ donor and π
acceptor nature of CO as a ligand results in an
MO diagram with greatest stability at 18
electrons.
The eg* orbitals
are destabilizing
to the complex.
Since the 12
bonding orbitals
are filled with
electrons from
the CO
molecules, 6
electrons from
the metal will
produce a stable
complex.
MT Carbonyls
The CO stretching frequency is often used
to determine the structure of these compounds.
The carbon monoxide molecule can be terminal,
or bridge between 2 or 3 metal atoms.
The CO stretching frequency decreases with
increased bonding to metals. As the π* orbital
on CO receives electrons from the metal, the
CO bond weakens and the ν decreases.
MT Carbonyls
As the π* orbital on CO receives electrons
from the metal, the CO bond weakens and the ν
decreases.
MT Carbonyls
Mn2(CO)10
Fe2(CO)9
MT Carbonyls
Co4(CO)12
MT Carbonyls
ν for free CO = 2143 cm-1
MT Carbonyls
ν for free CO = 2143 cm-1
MT Carbonyls
The CO stretching frequency will also be
affected by the charge of the metal.
Compound
ν (cm-1)
[Fe(CO)6]2+
2204
[Mn(CO)6)]+
2143
Cr(CO)6
2090
[V(CO)6]1860
[Ti(CO)6]21750
MT Carbonyls
The IR spectra of transition metal carbonyl
compounds are consistent with the predictions
based on the symmetry of the molecule and
group theory.
The more symmetrical the structure, the
fewer CO stretches are observed in the IR
spectra.
MT Carbonyls
If there is a center of symmetry, with CO
ligands trans to each other, a symmetrical stretch
will not involve a change in dipole moment, so it
will be IR inactive. An asymmetric stretch will
be seen in the IR spectrum. As a result, trans
carbonyls give one peak in the IR spectrum.
MT Carbonyls
If CO ligands are cis
to each other, both the
symmetric stretch and
the asymmetric stretch
will involve a change in
dipole moment, and
hence two peaks will be
seen in the IR spectrum.
MT Carbonyls
Metal carbonyls with a center of symmetry
typically show only 1 C-O stretch in their IR
spectra, since the symmetric stretch doesn’t
change the dipole moment of the compound.
Combined with the Raman spectrum, the
structure of these compounds can be
determined.
Nomenclature for Ligands
The hapticity of the ligand is the number of atoms
of the ligand which directly interact with the metal
atom or ion. It is indicated using the greek letter η (eta)
with the superscript indicating the number of atoms
bonded.
Cyclopentadienyl Compounds
The ligand C5H5 can bond to metals via a σ
bond (contributing 1 electron), or as a π bonding
ligand. As a π bonding ligand, it can donate 3,
or more commonly 5 electrons to the metal.
Cyclopentadienyl Compounds
W(η3-C5H5)(η5-C5H5)(CO)2 has two π bonded
cyclopentadienyl rings. One donates 3 electrons, and
the other donates 5.
Counting Electrons
There are two common methods for
determining the number of electrons in an
organometallic compound.
One method views the cylcopentadienyl ring
as C5H5-, a 6 electron donor. CO and halides
such as Cl- are viewed as 2 electron donors. The
oxidation state of the metal must be determined
to complete the total electron count of the
complex.
Counting Electrons
The other method treats all ligands as neutral
in charge. η5-C5H5 is viewed as a 5 electron
donor, Cl is viewed as a chlorine atom and a 1
electron donor, and CO is a 2 electron donor.
The metal is viewed as having an oxidation state
of zero in this method.
Counting Electrons
In either method, a metal-metal single bond
is counted as one electron per metal. Metalmetal double bonds count as two electrons per
metal, etc.
Ferrocene
Fe(η5-C5H5)2 , ferrocene, is
known as a “sandwich”
compound. In the solid at
low temperature, the rings
are staggered.
The rotational barrier is
very small, with free
rotation of the rings.
Ferrocene
The cyclopentadienyl rings behave as an
aromatic electron donor. They are viewed as
C5H5- ions donating 6 electrons to the metal.
The iron atom is considered to be Fe(II).
Bonding of Ferrocene
Group theory is used to simplify the analysis
of the bonding. First, consider just a single
C5H5 ring. Determine Τπ by considering only
the pz orbitals which are perpendicular to the 5membered ring.
Bonding of Ferrocene
D5h
Τπ
E
2C5 2C52 5C2
σh
2S5
2S53 5 σv
Bonding of Ferrocene
D5h
E
Τπ
5
2C5 2C52 5C2
σh
2S5
2S53 5 σv
Bonding of Ferrocene
D5h
E
Τπ
5
2C5 2C52 5C2
0
σh
2S5
2S53 5 σv
Bonding of Ferrocene
D5h
E
Τπ
5
2C5 2C52 5C2
0
0
σh
2S5
2S53 5 σv
Bonding of Ferrocene
D5h
E
Τπ
5
2C5 2C52 5C2
0
0
-1
σh
2S5
2S53 5 σv
Bonding of Ferrocene
D5h
E
Τπ
5
2C5 2C52 5C2
0
0
-1
σh
-5
2S5
2S53 5 σv
Bonding of Ferrocene
D5h
E
Τπ
5
2C5 2C52 5C2
0
0
-1
σh
2S5
-5
0
2S53 5 σv
Bonding of Ferrocene
D5h
E
Τπ
5
2C5 2C52 5C2
0
0
-1
σh
2S5
-5
0
2S53 5 σv
0
Bonding of Ferrocene
D5h
E
Τπ
5
2C5 2C52 5C2
0
0
-1
σh
2S5
-5
0
2S53 5 σv
0
1
Bonding of Ferrocene
D5h
E
Τπ
5
2C5 2C52 5C2
0
0
-1
σh
2S5
-5
0
2S53 5 σv
0
1
Τπ reduces to: A′1, E′1 and E′2
Group theory can be used to generate drawings of the
π molecular orbitals.
Bonding of Ferrocene
Τπ reduces to: A′1, E′1 and E′2
E′2
E′1
A′1
Bonding of Ferrocene
The totally bonding
orbital (A′1) has no
nodes, and is lowest in
energy.
Bonding of Ferrocene
The middle set of
orbitals (E′1) are
degenerate, with a single
node. These orbitals are
primarily bonding
orbitals.
Bonding of Ferrocene
The upper set of
orbitals (E′2) are
degenerate, with two
nodes. These orbitals are
primarily anti-bonding
orbitals.
Bonding of Ferrocene
Once the molecular orbitals of the
cyclopentadienyl ring has been determined, two
rings are combined, and matched with symmetry
appropriate orbitals on iron.
Bonding of Ferrocene
The A′1 orbitals on the
two cyclopentadienyl rings
have the same symmetry as
the dz2 orbital on iron.
Since the metal orbital is
located in the center of the
C5H5 rings, this is essentially
a non-bonding orbital.
Bonding of Ferrocene
The E′1 orbitals on the
rings have the same
symmetry as the dxz and
dyz orbitals of the iron.
Bonding of Ferrocene
The E′2 orbitals on the
rings have the same
symmetry as the dxy and
dx2-y2 orbitals of the iron.
Bonding of Ferrocene
These are the bonding orbitals of ferrocene.
If the upper cyclopentadienyl ring is flipped
over, a set of antibonding orbitals results.
MO Diagram
The frontier
orbitals are neither
strongly bonding nor
strongly antibonding.
As a result, metallocene compounds
often diverge from
the 18 electron rule.
MO Diagram
If the complex has
more than 18 electrons,
the e1u orbitals, which
are slightly antibonding
(the dxzand dyz), become
occupied. This
lengthens the M-C
distance.
Electron Count and Stability
(η5-Cp)2M e- count
Fe
18
Co
19
Ni
20
M-C(pm)
206.4
211.9
219.6
ΔHdissoc.*
1470 kJ/mol
1400
1320
* ΔHdissoc refers to the complex dissociating to
M2+ and 2C5H5-