coordination chemistry ii (1)
TRANSCRIPT
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Coordination
Chemistry IIBonding, including crystal field theory
and ligand field theory
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Basis for Bonding Theories
Models for the bonding in transition metal
complexes must be consistent with observed
behavior. Specific data used include stability (or
formation) constants, magnetic susceptibility,and the electronic (UV/Vis) spectra of the
complexes.
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Bonding Approaches
Valence Bond theory provides the
hybridization for octahedral complexes. For the
first row transition metals, the hybridization can
be: d2sp3 (using the 3d, 4s and 4p orbitals), orsp3d2 (using the 4s, 4p and 4d orbitals).
The valence bond approach isnt used
because it fails to explain the electronic spectraand magnetic moments of most complexes.
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Crystal Field Theory
In crystal field theory, the electron pairs on
the ligands are viewed as point negative charges
that interact with the dorbitals on the central
metal. The nature of the ligand and thetendency toward covalent bonding is ignored.
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d Orbitals
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Crystal Field Theory
Ligands, viewed as point charges, at the
corners of an octahedron affect the various d
orbitals differently.
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Crystal Field Theory
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Crystal Field Theory
The repulsion
between ligand lone
pairs and the d
orbitals on the metalresults in a splitting of
the energy of the d
orbitals.
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d Orbital Splitting
__ __ __ __ __
Spherical field
__ __
dz2 dx2-y2
__ __ __
dxy dxz dyz
o0.6o
0.4o
Octahedral field
eg
t2g
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d Orbital Splitting
In some texts and articles, the gap in the d
orbitals is assigned a value of 10Dq. The upper
(eg) set goes up by 6Dq, and the lower set (t2g)
goes down by 4Dq.
The actual size of the gap varies with the
metal and the ligands.
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d Orbital Splitting
The colors exhibited by most transition
metal complexes arises from the splitting of the
dorbitals. As electrons transition from the
lower t2gset to the egset, light in the visiblerange is absorbed.
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d Orbital Splitting
The splitting dueto the nature of theligand can beobserved and
measured using aspectrophotometer.Smaller values of oresult in colors in the
green range. Largergaps shift the color toyellow.
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The Spectrochemical Series
Based on measurements for a given metal
ion, the following series has been developed:
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The Spectrochemical Series
The complexes of
cobalt (III) show the
shift in color due to the
ligand.(a) CN, (b) NO2
, (c)
phen, (d) en, (e) NH3, (f)
gly, (g) H2O, (h) ox2, (i)
CO3 2.
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Ligand Field Strength Observations
1. o increases with increasing oxidation number
on the metal.
Mn+2
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Ligand Field Theory
Crystal Field Theory completely ignores the
nature of the ligand. As a result, it cannot
explain the spectrochemical series.
Ligand Field Theory uses a molecular orbital
approach. Initially, the ligands can be viewed as
having a hybrid orbital or ap orbital pointing
toward the metal to make bonds.
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Octahedral Symmetry
http://www.iumsc.indiana.edu/morphology/sym
metry/octahedral.html
http://www.iumsc.indiana.edu/morphology/symmetry/octahedral.htmlhttp://www.iumsc.indiana.edu/morphology/symmetry/octahedral.htmlhttp://www.iumsc.indiana.edu/morphology/symmetry/octahedral.htmlhttp://www.iumsc.indiana.edu/morphology/symmetry/octahedral.html -
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Ligand Field Theory
Oh E 8C3 6C2 6C4
3C2
(=C42) i 6S4 8S6 3h 6d
6 0 0 2 2 0 0 0 4 2
This reduces to A1g+ Eg+ T1u
Consider the sigma bonds to all six ligandsin octahedral geometry.
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Ligand Field Theory
The A1ggroup
orbitals have the same
symmetry as an s
orbital on the centralmetal.
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Ligand Field Theory
The T1u group
orbitals have the same
symmetry as thep
orbitals on the centralmetal.
(T representations
are triply degenerate.)
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Ligand Field Theory
The Eggroup
orbitals have the same
symmetry as the dz2
and dx2-y2 orbitals onthe central metal.
(E representations are
doubly degenerate.)
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Ligand Field Theory
Since the ligands
dont have a
combination with t2g
symmetry, the dxy, dyzand dxyorbitals on the
metal will be non-
bonding whenconsidering bonding.
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Ligand Field Theory
The molecular
orbital diagram is
consistent with the
crystal fieldapproach.
Note that the
t2gset of orbitals is
non-bonding, andthe egset of orbitals
is antibonding.
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Ligand Field Theory
The electrons
from the ligands
(12 electrons
from 6 ligands inoctahedral
complexes) will
fill the lowerbonding orbitals.{
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Ligand Field Theory
The electrons
from the 4s and
3d orbitals of the
metal (in the firsttransition row)
will occupy the
middle portion ofthe diagram.
{
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Experimental Evidence for Splitting
Several tools are used to confirm the
splitting of the t2gand egmolecular orbitals.
The broad range in colors of transition metal
complexes arises from electronic transitions as
seen in the UV/visible spectra of complexes.
Additional information is gained from
measuring the magnetic moments of thecomplexes.
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Experimental Evidence for Splitting
Magnetic susceptibilitymeasurements can be
used to calculate the
number of unpaired
electrons in a compound.
Paramagnetic
substances are attracted
to a magnetic field.
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Magnetic Moments
A magnetic balance can be used to
determine the magnetic moment of a substance.
If a substance has unpaired electrons, it is
paramagnetic, and attracted to a magnetic field.
For the upper transition metals, the spin-
only magnetic moment, s, can be used to
determine the number of unpaired electrons.
s = [n(n+2)]1/2
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Magnetic Moments
The magnetic moment of a substance, in
Bohr magnetons, can be related to the number
of unpaired electrons in the compound.
s = [n(n+2)]1/2
Where n is the number of unpaired electrons
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Magnetic Moments
Complexes with 4-7 electrons in the d
orbitals have two possibilities for the
distribution of electrons. The complexes can be
low spin, in which the electrons occupy the lowert2gset and pair up, or they can be high spin. In
these complexes, the electrons will fill the upper
egset before pairing.
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High and Low Spin Complexes
If the gap between
the dorbitals is large,
electrons will pair up and
fill the lower (t2g) set oforbitals before
occupying the egset of
orbitals. The complexesare called low spin.
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High and Low Spin Complexes
In low spin
complexes, the size
of o is greater than
the pairing energy ofthe electrons.
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High and Low Spin Complexes
If the gap between
the dorbitals is small,
electrons will occupy the
egset of orbitals beforethey pair up and fill the
lower (t2g) set of orbitals
before. The complexesare called high spin.
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High and Low Spin Complexes
In high spin
complexes, the size
of o is less than the
pairing energy of theelectrons.
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Ligand Field Stabilization Energy
The first row transition metals in water are
all weak field, high spin cases.
do d1 d2 d3 d4 d5 d6 d7 d8 d9 d10
LFSE 0 .4o .8 1.2 .6 0 .4 .8 1.2 .6 0
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Experimental Evidence for LFSE
The hydration energies of the first row
transition metals should increase across the period
as the size of the metal ion gets smaller.
M2+ + 6 H2O(l)M(H2O)62+
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Experimental Evidence for LFSE
The heats of
hydration show two
humps consistent
with the expected LFSEfor the metal ions. The
values for d5 and d10 are
the same as expectedwith a LFSE equal to 0.
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Experimental Evidence of LFSE
do d1 d2 d3 d4 d5 d6 d7 d8 d9 d10
LFSE 0 .4o .8 1.2 .6 0 .4 .8 1.2 .6 0
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High Spin vs. Low Spin
3d metals are generally high spin complexes except
with very strong ligands. CN- forms low spin
complexes, especially with M3+ ions.
4d & 4d metals generally have a larger value of o
than for 3d metals. As a result, complexes are
typically low spin.
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Nature of the Ligands
Crystal field theory and ligand field theory
differ in that LFT considers the nature of the
ligands. Thus far, we have only viewed the
ligands as electron pairs used for makingbonds with the metal. Many ligands can also
form bonds with the metal. Group theory
greatly simplifies the construction of molecularorbital diagrams.
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Considering Bonding
To obtain red for bonding, a set of
cartesian coordinates is established for each of
the ligands. The direction of the bonds is
arbitrarily set as theyaxis (or the pyorbitals).The px and pz orbitals are used in bonding.
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Considering
Bonding
y y
y
y y
y
x
x
x
x
x
x
zz
z
z
zz
Oh E 8C3 6C2 6C4
3C2(=C42) i 6S4 8S6 3h 6d
12 0 0 0 -4 0 0 0 0 0
Consider only the px and
pz orbitals on each of
the ligands to obtain .
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Considering Bonding
This reduces to T1g+ T2g+ T1u + T2u. The T2g
set has the same symmetry as the dxy, dyz and dxzorbitals on the metal. The T1u set has the same
symmetry as the px, pyand pz orbitals on the metal.
Oh E 8C3 6C2 6C43C2
(=C42)
i 6S4 8S6 3h 6d
12 0 0 0 -4 0 0 0 0 0
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Considering Bonding
reduces to: T1g+ T2g+ T1u + T2u.
The T1gand T2ugroup orbitals for the ligands dont matchthe symmetry of any of the metal orbitals.
The T1u set has the same symmetry as the px, pyand pzorbitals on the metal. These orbitals are used primarily tomake the bonds to the ligands.
The T2gset has the same symmetry as the dxy, dyz and dxzorbitals on the metal.
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Bonding
The main source of bonding is between
the dxy, dyz and dxz orbitals on the metal and the
d, p or * orbitals on the ligand.
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Bonding
The ligand may have empty d or * orbitals
and serve as a acceptorligand, or full p or d
orbitals and serve as a donorligand.
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Bonding
The empty antibonding orbital on CO can
accept electron density from a filled dorbital on
the metal. CO is api acceptorligand.
empty*
orbitalfilled d
orbital
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Donor Ligands (LM)All ligands are donors. Ligands with filled
p or dorbitals may also serve as pi donor ligands.
Examples of donor ligands are I-, Cl-, and S2-.
The filled p or d orbitals on these ions interactwith the t2gset of orbitals (dxy, dyz and dxz) on
the metal to form bonding and antibonding
molecular orbitals.
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Donor Ligands (LM)The bonding orbitals,
which are lower in energy,
are primarily filled with
electrons from the ligand,the and antibonding
molecular orbitals are
primarily occupied byelectrons from the metal.
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Donor Ligands (LM)The size of o
decreases, since it is now
between an antibonding t2g
orbital and the eg* orbital.This is confirmed by
the spectrochemical series.
Weak field ligands are alsopi donor ligands.
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Acceptor Ligands (ML)Ligands such as CN,
N2 and CO have empty
antibonding orbitals
of the proper symmetryand energy to interact
with filled dorbitals on
the metal.
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Acceptor Ligands (ML)The metal uses the
t2gset of orbitals (dxy,
dyz and dxz) to engage in
pi bonding with theligand. The * orbitals
on the ligand are usually
higher in energy thanthe d orbitals on the
metal.
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Acceptor Ligands (ML)The metal uses the
t2gset of orbitals (dxy,
dyz and dxz) to engage in
pi bonding with theligand. The * orbitals
on the ligand are usually
higher in energy thanthe d orbitals on the
metal.
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Acceptor Ligands (ML)The interaction
causes the energy of the
t2gbonding orbitals to
drop slightly, thusincreasing the size of
o.
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Summary
1. All ligands are donors. In general, ligand that
engage solely in bonding are in the middle of
the spectrochemical series. Some very strong
donors, such as CH3- and H- are found high inthe series.
2. Ligands with filledp or dorbitals can also serve
as donors. This results in a smaller value ofo.
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Summary
3. Ligands with emptyp, d or * orbitals can also
serve as acceptors. This results in a larger
value of o.
I-
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4 Coordinate Complexes
Square planar and tetrahedral complexes are
quite common for certain transition metals. The
splitting patterns of the dorbitals on the metal
will differ depending on the geometry of thecomplex.
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Tetrahedral Complexes
The dz2 and dx2-y2 orbitals
point directly between the
ligands in a tetrahedral
arrangement. As a result, thesetwo orbitals, designated as ein
the point group Td, are lower in
energy.
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Tetrahedral Complexes
The t2set of orbitals,consisting of the dxy, dyz, and
dxz orbitals, are directed more
in the direction of the ligands.
These orbitals will be
higher in energy in a
tetrahedral field due to
repulsion with the electrons
on the ligands.
T
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Tetrahedral Complexes
The size of the splitting,T, is considerable smaller
than with comparable
octahedral complexes. This is
because only 4 bonds are
formed, and the metal orbitals
used in bonding dont point
right at the ligands as they doin octahedral complexes.
T h d l C l
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Tetrahedral Complexes
In general, T 4/9o. Since the splitting
is smaller, all tetrahedral
complexes are weak-
field, high-spin cases.
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Tetragonal Complexes
Six coordinate complexes, notably those of
Cu2+, distort from octahedral geometry. One
such distortion is called tetragonal distortion, in
which the bonds along one axis elongate, withcompression of the bond distances along the
other two axes.
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Tetragonal Complexes
The elongationalong the zaxis causes
the dorbitals with
density along the axis todrop in energy. As a
result, the dxz and dyz
orbitals lower in energy.
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Tetragonal Complexes
The compressionalong the xandyaxis
causes orbitals with
density along these axesto increase in energy.
.
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Tetragonal Complexes
For complexes with1-3 electrons in the egset
of orbitals, this type of
tetragonal distortion maylower the energy of the
complex.
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Square Planar Complexes
For complexes with 2electrons in the egset of
orbitals, a d8 configuration,
a severe distortion mayoccur, resulting in a 4-
coordinate square planar
shape, with the ligands
along the zaxis no longer
bonded to the metal.
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Square Planar Complexes
Square planarcomplexes are quitecommon for the d8 metalsin the 4th and 5th periods:
Rh(I), IR(I), Pt(II), Pd(II)and Au(III). The lowertransition metals have large
ligand field stabalizationenergies, favoring four-coordinate complexes.
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Square Planar Complexes
Square planarcomplexes are rare for the
3rd period metals. Ni(II)
generally forms tetrahedralcomplexes. Only with very
strong ligands such as CN-,
is square planar geometry
seen with Ni(II).
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Square Planar Complexes
The value of sp for agiven metal, ligands and
bond length is
approximately 1.3(o).
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The Jahn-Teller Effect
If the ground electronic configuration of a non-linear
complex is orbitally degenerate, the complex will distort
so as to remove the degeneracy and achieve a lower energy.
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The Jahn-Teller Effect
The Jahn-Teller effect predicts whichstructures will distort. It does not predict the
nature or extent of the distortion. The effect is
most often seen when the orbital degneracy is inthe orbitals that point directly towards the
ligands.
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The Jahn-Teller Effect
In octahedral complexes, the effect is mostpronounced in high spin d4, low spin d7 and d9
configurations, as the degeneracy occurs in the
egset of orbitals.
d4 d7 d9
eg
t2g
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The Jahn-Teller Effect
The strength of the Jahn-Teller effect istabulated below: (w=weak, s=strong)
# e- 1 2 3 4 5 6 7 8 9 10
High
spin* * * s - w w * * *
Low
spin w w - w w - s - s -
*There is only 1 possible ground state configuration.
- No Jahn-Teller distortion is expected.
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Experimental Evidence of LFSE
do
d1
d2
d3
d4
d5
d6
d7
d8
d9
d10
LFSE 0 .4o .8 1.2 .6 0 .4 .8 1.2 .6 0