Transcript 1 0 +1

The Electronic Spectra of
Coordination Compounds
The UV/Vis
spectra of
transition metal
complexes shows
the transitions of
the electrons.
Analysis of
these spectra can
be quite complex.
Electron Spectra
The UV/Vis spectra are used to determine
the value of ∆o for the complex. The spectra
arise from electronic transitions between the t2g
and eg sets if molecular orbitals. Electronelectron interactions can greatly complicate the
spectra. Only in the case of a single electron is
interpretation of the spectrum straightforward.
Obtaining ∆o
For a d1 configuration, only a single peak is seen. It
results from the electron promotion from the t2g
orbitals to the eg orbitals. The “toothed” appearance of
the peak is due to a Jahn-Teller distortion of the excited
state. The energy of the peak = ∆o.
General
Observations
d1, d4, d6 and d9 usually
have 1 absorption, though
a side “hump” results
from Jahn-Teller
distortions.
General
Observations
d2, d3, d7 and d8
usually have 3
absorptions, one is
often obscured by a
charge transfer band.
General Observations
d5 complexes consist of very weak, relatively
sharp transitions which are spin-forbidden, and
have a very low intensity.
Qualitative Explanation
Consider a Cr(III) complex such as
[Cr(NH3)6]3+. The ground state configuration is:
____ ____
A transition from the
dz2 dx2-y2
dxy to the dx2-y2, or the
dyz or dxz to the dz2
____ ____ ____
orbitals involve a relatively
dxy
dyz dxz
minor change in
environment.
Qualitative Explanation
The transition from
the dxz orbitals to the dz2
orbitals involves a
relatively minor change
in the electronic
environment.
Qualitative Explanation
Consider a Cr(III) complex such as
[Cr(NH3)6]3+. The ground state configuration is:
____ ____
A transition from the
dz2 dx2-y2
dxy to the dz2, or the
dyz or dxz to the dx2-y2
____ ____ ____
orbitals involve a major
dxy
dyz dxz
change in environment.
Qualitative Explanation
The transition from
orbitals in the xy plane
to the dz2 orbitals
involves a fairly major
change in the electronic
environment.
Qualitative Explanation
Since the promotion of an electron from the
t2g set of orbitals to the eg set can involve
differing changes in environment, several peaks
will be seen in the spectrum.
3d Multi-electron Complexes
For complexes with more than one electron
in the 3d (and 4s) orbitals of the metal, electron
interactions must be considered.
The electrons are not independent of each
other, and the orbital angular momenta (ml
values) and the spin angular momenta (ms
values) interact.
4d and 5d Metal Complexes
The lower transition metals undergo further
coupling (called j-j coupling or spin-orbit
coupling).
3d Multi-electron Complexes
The interaction is called Russel-Saunders or
L-S coupling. The interactions produce atomic
states called microstates that are described by a
new set of quantum numbers.
ML = total orbital angular momentum =Σml
MS = total spin angular momentum = Σms
Determining the Energy States of an
Atom
A microstate table that contains all possible
combinations of ml and ms is constructed.
Each microstate represents a possible electron
configuration. Both ground state and excited
states are considered.
Energy States
Microstates would have the same energy only
if repulsion between electrons is negligible. In
an octahedral or tetrahedral complex,
microstates that correspond to different relative
spatial distributions of the electrons will have
different energies. As a result, distinguishable
energy levels, called terms are seen.
Energy States
To obtain all of the terms for a given
electron configuration, a microstate table is
constructed. The table is a grid of all possible
electronic arrangements. It lists all of the
possible values of spin and orbital orientation.
It includes both ground and excited states, and
must obey the Pauli Exclusion Principle.
Constructing a Microstate Table
Consider an atom of carbon. Its highest
occupied orbital has a p2 electron configuration.
Microstates correspond to the various
possible occupation of the px, py and pz orbitals.
Constructing a Microstate Table
ml =
+1
Configurations: ___
___
___
0
___
___
___
-1 microstate:
___ (1+,0+)
___ (0+,-1+)
___ (1+,-1+)
These are examples of some of the ground
state microstates. Others would have the
electrons (arrows) pointing down.
Constructing a Microstate Table
ml =
+1
Configurations: ___
___
___
0
___
___
___
-1 microstate:
___ (1+,1-)
___ (0+,0-)
___ (-1+,-1-)
These are examples of some of the excited
state microstates.
Microstate Table for
2
p
For the carbon atom, ML will range from +2
down to -2, and MS can have values of +1 (both
electrons “pointing up”), 0 (one electron “up”,
one electron “down), or -1 (both electrons
“pointing down”).
Microstate Notation
For the carbon atom, ML will range from +2
down to -2, and MS can have values of +1 (both
electrons “pointing up”), 0 (one electron “up”,
one electron “down), or -1 (both electrons
“pointing down”).
Microstate Table for
2
p
MS
ML
+1
0
-1
1+1-
+2:
1+0+
1+01-0+
1-0-
0:
-1+1+
-1+10+0-1-1+
-1-1-
-1:
-1+0+
-1+0-1-0+
-1-0-
+1:
-2:
-1+-1-
The table
includes all
possible
microstates.
Constructing a Microstate Table
Once the microstate table is complete, the
microstates are collected or grouped into atomic
(coupled) energy states.
Constructing a Microstate Table
For two electrons,
L = l1+ l2, l1+ l2-1, l1+ l2-2,…│l1- l2│
For a p2 configuration, L = 1+1, 1+1-1, 1-1.
The values of L are: 2, 1 and 0.
L is always positive, and ranges from the
maximum value of Σl.
Constructing a Microstate Table
For two electrons,
S = s1+ s2, s1+ s2-1, s1+ s2-2,…│s1- s2│
For a p2 configuration, S = ½ + ½ , ½ + ½ -1.
The values of S are: 1 and 0.
Atomic Quantum Numbers
Quantum numbers L and S describe
collections of microstates, whereas ML and MS
describe the individual microstates themselves.
Constructing a Microstate Table
The microstate table is a grid that includes all
possible combinations of L, the total angular
momentum quantum number, and S, the total
spin angular momentum quantum number.
For two electrons,
L = l1+ l2, l1+ l2-1, l1+ l2-2,…│l1- l2│
S = s1+ s2, s1+ s2-1, s1+ s2-2,…│s1- s2│
Constructing a Microstate Table
Once the microstate table is complete, all
microstates associated with an energy state with
specific value of L and S are grouped.
It doesn’t matter which specific microstates
are placed in the group. Microstates are grouped
and eliminated until all microstates are
associated with a specific energy state or term.
Term Symbols
Each energy state or term is represented by a
term symbol. The term symbol is a capitol letter
that is related to the value of L.
L=
0
1
2
3
4
Term
Symbol
S
P
D
F
G
Term Symbols
The upper left corner of the term
symbol contains a number called the
multiplicity. The multiplicity is the number of
unpaired electrons +1, or 2S+1.
Microstate Table for
MS
ML
+1
0
-1
1+1-
+2:
1+0+
1+01-0+
1-0-
0:
-1+1+
-1+10+0-1-1+
-1-1-
-1:
-1+0+
-1+0-1-0+
-1-0-
+1:
-2:
-1+-1-
2
p
Eliminate
microstates
with
ML=+2-2,
with Ms=0.
Microstate Table for
MS
ML
+1
0
-1
1+1-
+2:
1+0+
1+01-0+
1-0-
0:
-1+1+
-1+10+0-1-1+
-1-1-
-1:
-1+0+
-1+0-1-0+
-1-0-
+1:
-2:
-1+-1-
2
p
Eliminate
microstates
with
ML=+2-2,
with Ms=0.
Microstate Table for
MS
ML
+1
0
-1
1+1-
+2:
1+0+
1+01-0+
1-0-
0:
-1+1+
-1+10+0-1-1+
-1-1-
-1:
-1+0+
-1+0-1-0+
-1-0-
+1:
-2:
-1+-1-
2
p
These
microstates
are associated
with the term
1D.
Microstate Table for
MS
ML
+1
0
-1
1+1-
+2:
1+0+
1+01-0+
1-0-
0:
-1+1+
-1+10+0-1-1+
-1-1-
-1:
-1+0+
-1+0-1-0+
-1-0-
+1:
-2:
-1+-1-
2
p
Eliminate
microstates
with
ML=+1-1,
with
Ms=+1-1
Microstate Table for
MS
ML
+1
0
-1
1+1-
+2:
1+0+
1+01-0+
1-0-
0:
-1+1+
-1+10+0-1-1+
-1-1-
-1:
-1+0+
-1+0-1-0+
-1-0-
+1:
-2:
-1+-1-
2
p
Eliminate
microstates
with
ML=+1-1,
with
Ms=+1-1
Microstate Table for
MS
ML
+1
0
-1
1+1-
+2:
1+0+
1+01-0+
1-0-
0:
-1+1+
-1+10+0-1-1+
-1-1-
-1:
-1+0+
-1+0-1-0+
-1-0-
+1:
-2:
-1+-1-
2
p
These
microstates
are associated
with the term
3P.
Microstate Table for
MS
ML
+1
0
-1
1+1-
+2:
1+0+
1+01-0+
1-0-
0:
-1+1+
-1+10+0-1-1+
-1-1-
-1:
-1+0+
-1+0-1-0+
-1-0-
+1:
-2:
-1+-1-
2
p
One
microstate
remains. It is
associated
with the term
1S.
Term States for
2
p
The term states for a p2 electron
configuration are 1S, 3P, and 1D.
The term symbol with the greatest
multiplicity and highest value of ML will be the
ground state. 3P is the ground state term for
carbon.
Determining the Relative Energy
of Term States
1. For a given electron configuration, the term
with the greatest multiplicity lies lowest in
energy. (This is consistent with Hund’s rule.)
2. For a term of a given multiplicity, the greater
the value of L, the lower the energy.
Determining the Relative Energy
of Term States
For a p2 configuration, the term
states are 3P, 1D and 1S.
The terms for the free atom should
have the following relative energies:
3P< 1D
<1S
Determining the Relative Energy
of Term States
The rules for predicting the ground state
always work, but they may fail in
predicting the order of energies for
excited states.
Energy States for a d2 Configuration
A microstate table for a d2 electron
configuration will contain 45 microstates (ML =
4-4, and MS=1, 0 or -1) associated with the
following terms:
1S, 1D, 1G, 3P, and 3F
Energy States for a d2 Configuration

Problem: Determine the ground state of a free
atom with a d2 electron configuration, and place
the terms in order of increasing energy.
1S, 1D, 1G, 3P,
and 3F
Determining the Ground State Term
We only need to know the ground state term
to interpret the spectra of transition metal
complexes. This can be obtained without
constructing a microstate table.
The ground state will
a) have the maximum multiplicity
b) have the maximum value of ML for the
configuration obtained in part (a).
The Splitting of Terms
In an octahedral field, the free ion terms will
split due to their differing spatial orientations.
Term
S
P
D
F
G
# of States
1
3
5
7
9
Terms in Oh Field
A1g
T1g
T2g + Eg
T1g + T2g + A2g
A1g + Eg+T1g+T2g
Correlation
Diagrams
The diagrams
show the free ion
terms on the left,
and the effect of a
strong octahedral
field on the right.
This diagram is
for a d2 ion.
Correlation
Diagrams
The terms converge
on the right side of
the diagram in three
clusters. Each of
these represents the
possible electron
configurations for a
d2 ion in a strong
octahedral field.
Correlation
Diagrams
At the left, the freeion terms (due to L-S
coupling) predominate.
At the right, the
electron configurations
predominate.
The diagram shows
the intermediate cases
in which both factors
need to be considered.
Correlation
Diagrams
This correlation
diagram shows the
ground state and spinallowed transitions in
bold lines.
Selection Rules
There are several selection rules that govern
the intensities of the absorption bands seen in
transition metal complexes.
1. Transitions between states of the same parity
(gg or uu) are forbidden. This is the
Laporte Rule. This rule would forbid
electronic transitions between d orbitals,
since all d orbitals are gerade.
Selection Rules
1. Transitions between states of the same parity
(gg or uu) are forbidden. This is the
Laporte Rule.
This rule is relaxed due to vibrations of the
complex that cause a loss of the center of
symmetry. As a result, molar absorbitivities
of 10-50 L/mol-cm are observed.
Selection Rules
2. Transitions between states of different
multiplicities are forbidden. This is called
the spin selection rule.
This rule can be relaxed very slightly for
the first row transition metals by spin-orbit
coupling. Typical molar absorbitivities are
less than 1 L/mol-cm, with very pale color
observed.
Spin-Forbidden Transitions
Mn2+ (and Fe+3) usually have a high spin d5
configuration. As a result, all electronic
transitions are spin-forbidden. Mn(II)
compounds are sometimes a very pale pink, and
Fe(III) compounds a very pale green due to
relaxing of the selection rule.
Correlation
Diagrams
This diagram
shows the possible
transitions that do
not violate the
spin selection rule.
A d2 complex
should have 3
possible
transitions.
Correlation Diagrams
A non-crossing rule is observed in correlation
diagrams.
Terms or energy states of the same symmetry interact
so that their energies never cross.
Tanabe-Sugano Diagrams
In order to accurately interpret the electronic
spectra of transition metal complexes, a series
of diagrams have been created.
These diagrams are used to assign transitions
(initial energy state and final energy state) to
peaks observed in the spectra, and to calculate
the value of ∆o.
Tanabe-Sugano Diagrams
Tanabe-Sugano diagrams have the lowest
energy state (the ground state) plotted along the
horizontal axis. The energy of excited states can
then be readily compared to the ground state.
Tanabe-Sugano Diagrams
Tanabe-Sugano Diagrams
Many
tables
eliminate, or
use dotted
lines for
excited states
that are spinforbidden.
Tanabe-Sugano Diagrams
Also, since the
d orbitals are
all gerade, the g
subscript is
usually left
off.
Tanabe-Sugano Diagrams
The vertical axis is
E/B, where B is a
Racah parameter. B is
a measure of repulsion
between terms of the
same multiplicity.
Tanabe-Sugano Diagrams
The horizontal axis
is ∆o/B. In order to
determine ∆o, we need
to determine the value
of B, or mathematically
eliminate it.
Tanabe-Sugano Diagrams
The diagrams for
configurations d4-d6
have a vertical “break”
in the middle of the
diagram. This is due to
the shift from a high
spin (weak field)
complex to a low spin
(high field) complex.
Symmetry Labels and Electron
Configurations
At the far right side of the diagrams, at an
infinitely strong octahedral field, the symmetry
labels correspond to the electron configuration
of the complex.
T designates a triply degenerate asymmetrically occupied state.
or
Symmetry Labels and Electron
Configurations
An E label designates a doubly degenerate
asymmetrically occupied state.
or
An A or B label designates a non-degenerate
state.
or
Interpretation of Spectra – d1 &
d9
There is only 1
spin-allowed
transition, with
the energy
absorbed equal to
the value of ∆o.
Interpretation of Spectra – d1 &
d9
The d1 excited state exhibits a
strong Jahn-Teller distortion, as
seen in the UV/Vis spectrum.
Interpretation of Spectra – d1 &
d9
The d9 ground state exhibits a
strong Jahn-Teller distortion.
The result is a “side peak” in the
UV/Vis spectrum.
Interpretation of Spectra – d4 & d6
(high spin)
The Tanabe-Sugano
diagrams show only
one spin-allowed
transition for either
complex. The
frequency of the
absorption equals Δo.
Interpretation of Spectra – d4 & d6
(high spin)
The single peak
shows distortion
from octahedral
geometry due to the
Jahn-Teller effect.
Interpretation of Spectra – d4 & d6
(high spin)
The ground state of Cr2+ (d4) and the excited
state of Fe2+ (d6) should exhibit strong JahnTeller distortions.
Interpretation of Spectra – d3 &
d8
For a d3 ground state, the first
transition, from 4A2g(F) to
4T (F) corresponds to ∆ .
2g
o
LFSE = .6Δo- .8 Δo
= -.2 Δo
∆o
LFSE = -1.2 Δo
Interpretation of Spectra – d3 &
d8
The frequency of
the lowest energy
transition provides
the value of ∆o.
The third peak is
obscured by a very
intense charge
transfer band.
∆o
Interpretation of Spectra – d3 &
d8
The curvature of the
4T states is a result of
1
the non-crossing rule.
Since the terms won’t
cross, they mix, and
curve away from each
other.
Interpretation of Spectra – d3 &
d8
The Tanabe-Sugano
diagram for d8 is the
same as that for d3,
except the multiplicity is
different.
Three peaks are
expected, with the
lowest energy
absorption equal to Δo.
Interpretation of Spectra – d3 &
d8
Δo
The peaks are jagged due to distortions from
octahedral geometry.
Interpretation of Spectra – d3 &
d8
The Tanabe-Sugano
diagram can be used to
assign transitions to each
absorption.
Interpretation of Spectra – d3 &
d8
ν1
The first peak is due to the
4A (F) 4T (F) transition.
2g
2g
ν1
Interpretation of Spectra – d3 &
d8
ν1
ν2
The second peak is due to
the 4A2g(F) 4T1g(F)
transition.
ν2
Interpretation of Spectra – d3 &
d8
ν3
ν1
ν2
The third peak is due to
the 4A2g(F) 4T1g(P)
transition.
ν3
Interpretation of Spectra – d3 &
d8
ν1
The first peak [4A2g(F)
4T2g(F)] has an energy equal
to ∆o.
ν1
Interpretation of Spectra – d2 &
d7
The interpretation of spectra from d2 or d7
(high spin) complexes is the most complicated
due to curvature in the ground state of the
Tanabe-Sugano diagrams.
Since the ground state and an excited state
have the same symmetry (4T1g), they mix and
curve away from each other.
Interpretation of Spectra – d2 &
d7
Interpretation of Spectra – d2 &
d7
∆o
∆o
The repulsion
of like terms
means that the
energy of the
ground state
fluctuates with
field strength.
Interpretation of Spectra – d2 &
d7
∆o
ν1
ν3
∆o
If ν1 and ν3 are
both seen in the
spectrum, the
difference between
the two
absorptions = ∆o.
Interpretation of Spectra – d2 &
d7
∆o
ν1
ν3
∆o
The transition
corresponding to ν3
is often quite weak,
as it involves the
simultaneous
excitation of two
electrons, and is
therefore less
probable.
Interpretation of Spectra – d2 &
d7
It is not easy to assign the absorptions due to
several complications:
1. Lines cross in the Tanabe-Sugano diagram,
making assignment difficult
2. The second and third absorptions may
overlap, making it difficult to determine the
actual position of the peaks
Interpretation of Spectra – d2 &
d7
An additional
problem arises from the
crossing of lines.
Assignment of the
absorptions is not
obvious.
Interpretation of Spectra – d5
(high spin)
There are no spin allowed transitions for d5
high spin configurations. Extinction coefficients
are very low, though the selection rule is relaxed
by spin-orbit coupling.
Interpretation of Spectra – d5
(high spin)
Mn2+ compounds are white to pale pink in
color.
Charge Transfer Spectra
Many transition metal complexes exhibit
strong charge-transfer absorptions in the UV or
visible range. These are much more intense than
dd transitions, with extinction coefficients ≥
50,000 L/mol-cm (as compared to 20 L/mol-cm
for dd transitions).
Charge Transfer Spectra
Examples of
these intense
absorptions can be
seen in the
permanganate ion,
MnO4-. They result
from electron transfer
between the metal
and the ligands.
Charge Transfer Spectra
In charge transfer absorptions, electrons
from molecular orbitals that reside primarily on
the ligands are promoted to molecular orbitals
that lie primarily on the metal. This is known as
a charge transfer to metal (CTTM) or ligand to metal
charge transfer (LMCT). The metal is reduced as a
result of the transfer.
Charge Transfer Spectra
_ _ eg
d_____
_ _ _ t2g
Ligand to metal
charge transfer
______
______
free metal
octahedral complex ligand σ orbitals
Ligand to Metal Charge Transfer
LMCT occurs in the permangate ion,
MnO41-. Electrons from the filled p orbitals on
the oxygens are promoted to empty orbitals on
the manganese. The result is the intense purple
color of the complex.
Ligand to Metal Charge Transfer
LMCT typically occurs in complexes with
the metal in a fairly high oxidation state. It is
the cause of the intense color of complexes in
which the metal, at least formally, has no d
electrons (CrO42-, MnO41-).
Metal to Ligand Charge Transfer
MLCT typically occurs in complexes with π
acceptor ligands. The empty π* orbitals on the
ligands accept electrons from the metal upon
absorption of light. The result is oxidation of
the metal.
Charge Transfer Spectra
______
_ _ _ _ _ _ π*
_ _ eg
Metal to ligand
charge transfer
d_____
_ _ _ t2g
free metal
octahedral complex
ligand π* orbitals
Metal to Ligand Charge Transfer
Examples of LMCT include iron(III) with
acceptor ligands such as CN- or SCN1-. The
complex absorbs light and oxidizes the iron to a
+4 oxidation state.
Metal to Ligand Charge Transfer
The metal may be in a low oxidation state (0)
with carbon monoxide as the ligand. Many of
these complexes are brightly colored, and some
appear to exhibit both types of electron transfer.