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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
1
Chemistry 3820 – Electronic Spectroscopy of Transition Metal Complexes
PART 1: Qualitative Description of UV-Visible Spectra
The following figure shows the UV-Visible absorption spectra for aqueous solutions of first-row
transition metal complexes with the formula [M(H2O)6]x+ (x = 2 or 3).
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
2
Note that in the preceding absorption spectra, the y-axes show molar absorptivity (ε with units of
dm3 mol-1 cm-1) which is calculated from the measured absorbance (A) by the equation
log(Io/I) = A = εcl {Io = incident light intensity; I = emerging light intensity; c = concentration
(mol dm-3), l = pathlength through the test solution (cm)}.
From the UV-Visible absorption spectra on the previous page, it can be seen that certain complexes
have relatively simple spectra (e.g. d1, HS d4, HS d6 and d9 complexes), while others are far more
complex.
First, let us look at the simplest case; the aqueous Ti3+ ion (d1) which displays only one band in the
UV-Visible absorption spectrum. Considering the splitting of the d-orbitals in an octahedral
environment, this band can be assigned to the transition eg1 ß t2g
1 as shown below:
hv ∆o = hc/λmax
eg
t2g
For aqueous V3+ (d2), a similar representation would look like:
hv ∆o
eg
t2g
Since the t2g and eg* sets of orbitals are degenerate, we might expect to see only one absorption in the
spectrum. However, there are clearly two bands in the spectrum of [V(H 2O)6]3+ on page 1. Consider
the relative orientations of orbitals involved in the electronically excited states of V 3+:
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
3 Interelectronic repulsion makes excited state configurations such as those shown below
non-degenerate. As a consequence, more than one absorption band is observed and ∆o cannot be
obtained directly from the position of the maximum in the spectrum.
dz2 dx2-y2
dxy dxz dyz
dz2 dx2-y2
dxy dxz dyz
In order to understand the number and position of peaks in
UV-Visible absorption spectra of transition metal
complexes with more than one d-electron, we need to use
a more rigorous treatment that separates the different
electronic states into groups with the same energy. We
will look at this process in more detail in the next few
pages, but it is worth noting that the final outcome of this
process can be summarized in Tanabe -Sugano diagrams
(plots of the energy of different groups of degenerate
electronic states as a function of ligand field strength) for
each d-configuration with a particular geometry. The
Tanabe-Sugano diagram for an octahedral d2 complex is
shown on the right.
PART 2: Understanding Tanabe Sugano Diagrams
In the next few pages, we will discuss the fundamental principles behind this diagram.
To start with, we will consider the transition metal atom or ion of interest in the gas phase, rather
than as part of a complex. Note that it is only necessary to consider valence electrons in order to
understand the observed UV-Visible absorption spectra.
The following discussion is for a d2 ion in the gas phase :
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
4 A – What different combinations of ml and ms possible for two d-electrons, and how may the
electronic quantum numbers (ml and ms) be combined to give the atomic quantum numbers ML
and MS which describe atomic microstates?
Remember that an electron may be described using four quantum numbers (n = principal q.n.; l =
orbital angular momentum q.n.; ml = magnetic q.n.; ms = spin q.n. ).
For a gas-phase d1 ion, the lone d-electron can take values of ml = +2, +1, 0, -1, -2 and ms = +1/2,
-1/2, which gives a total of 10 possible arrangements of ml and ms. In the absence of an electric or
magnetic field, these 10 arrangements are degenerate (all of the same energy).
For a gas-phase d2 ion, one electron can in principle have ten different arrangements of ml and ms as
shown below , while the other can have nine different combinations (Pauli Exclusion Principle),
giving a total of 90 possible combinations . However, since the two d-electrons are indistinguishable
particles, the number of possible states is reduced to 45.
The two d-electrons are not independent of each other and their orbital angular momenta
(characterized by ml values) and spin angular momenta (characterized by ms values) interact in a
manner called Russell-Saunders coupling. These interactions produce states called microstates that
can be described by the new quantum numbers ML and MS:
Total Orbital Angular Momentum = ML = ? m l
(e.g. for a microstate with two electrons with ml = +2 and +1, ML = +3)
Total Spin Angular Momentum = MS = ? ms
(e.g. for a microstate with two electrons with ms = +1/2 and +1/2, MS = +1)
We need to determine how many possible combinations of ml and ms values there are for a d2
configuration. Once these combinations are known, we can determine the corresponding values of
ML and MS. For shorthand we will designate the ms value of each electron by a superscript + (for ms =
+1/2) or – (for ms = -1/2). For example, an electron having ml = +1 and ms = +1/2 will be written as
1+. One possible set of values for two d-electrons is shown below:
First electron:
Second electron:
ml = +2 and ms = +1/2
ml = 0 and ms = -1/2 Notation: 2+0-
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
5 We have seen that for a d1 configuration, there are 10 possible microstates, and for a d2 ion, there are
45 microstates. In general, for a subshell (of certain l) with n electrons, the equation below can be
used to calculate the total number of states:
Total number of microstates = [2(2l+1)]!
n![2(2l+1) – n]!
B - Tabulation of all the possible microstates.
When tabulating microstates, we must: (1) make sure that no two electrons have identical quantum
numbers (Pauli exclusion principal), and (2) count only unique mic rostates (e.g. 1+0- and 0-1+ are the
same so only one of these should be listed). The 45 microstates possible for a d2 configuration are
tabulated below according to their ML and MS values:
MS = +1 MS = 0 MS = -1
ML = +4 2+ 2-
ML = +3 2+ 1+ 2+ 1- 2- 1+
2- 1-
ML = +2 2+ 0+ 2+ 0- 2- 0+ 1+ 1-
2- 0-
ML = +1 2+ -1+ 1+ 0+
2+ -1- 2- -1+ 1+ 0- 1- 0+
2- -1- 1- 0-
ML = 0 2+ -2+ 1+ -1+
2+ -2- 2- -2+ 1+ -1- 1- -1+ 0+ 0-
2- -2- 1- -1-
ML = -1 -2+ 1+ -1+ 0+
-2+ 1- -2- 1+ -1+ 0- -1- 0+
-2- 1- -1- 0-
ML = -2 -2+ 0+ -2+ 0- -2- 0+ -1+ -1-
-2- 0-
ML = -3 -2+ -1+ -2+ -1- -2- -1+
-2- -1-
ML = -4 -2+ -2-
¦ = 1G
¦ = 3F
¦ = 1D
¦ = 3P
¦ = 1S
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
6
C – Labeling a group of microstates using the quantum numbers L and S.
We have seen how electronic quantum numbers (ml and ms) may be combined into atomic quantum
numbers ML and MS which describe atomic microstates. While quantum numbers ML and MS describe
the microstates themselves, microstates can be organized into groups with a particular energy, and
these groups (called free ion terms) are labelled using symbols called term symbols which take the
form (2S+1)L, where L and S are the total angular momentum quantum number and total spin quantum
number respectively. These are defined below:
For a collection of microstates (we will learn how to group microstates into terms with the same
energy next), S and L are the largest positive values of ML and MS w ithin the collection. For example,
for the group of microstates; {2+ 2-}, {2+ 1-}, {2+ 0-}, {2+ -1-}, {2+ -2-}, {-2+ 1-}, {-2+ 0-}, {-2+ -1-},
{-2+ -2-}, L = 4 and S = 0. While the value of S is represented by a number, the value of L is
represented by a letter (see below). Therefore, for this collection of microstates, the term symbol
(defined by (2S+1)L , where 2S+1 is called the spin multiplicity) is 1G.
L = 0 à S term
L = 1 à P term
L = 2 à D term
L = 3 à F term
L = 4 à G term
L = 5 à H term
L = 6 à I term
The use of a letter (S, P, D , F, G , H, I …) in place of the
numerical value for L is similar to the use of letters to describe
orbitals (l = 0 à s-orbital; l = 1 à p-orbital; l = 2 à d-orbital;
l = 3 à f-orbital; l = 4 à g-orbital)
The number of microstates in a term = (2L+1)(2S+1).
D – Identification of the terms to which the microstates in a microstate table belong.
* Start with the microstate of largest ML in the table, {2+ 2-}. This microstate has ML = 4, so must be
the first microstate of a term with L = 4 (G). This term has MS = 0, and there is no term with ML = 4
and MS ? 0, so this microstate must belong to the 1G term. The 1G term must contain (2L+1)(2S+1) =
(2x4+1)(2x0+1) = 9 microstates. To account for the other 8 microstates, we can pick microstates
with MS =0 and ML = 3, 2, 1, 0, -1, -2, -3, -4. For example: {2+ 1-}, {2+ 0-}, {2+ -1-}, {2+ -2-}, {-2+ 1-},
{-2+ 0-}, {-2+ -1-} and {-2+ -2-}. It is worth noting that there is a rigorous method to identify exactly
which microstate belongs to which term, but this is outside the scope of Chem 3820, and a more
qualitative approach is sufficient in order to identify the terms for a particular d-configuration.
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
7
* Now we consider the remaining microstates. Choose the one with the largest ML (and the largest
MS if there are several microstates with the same maximum ML) that has not yet been assigned: {2+
1+} in this case, which has ML = 3 and MS = 1. This must be the first microstate in the term 3F, which
will have (2L+1)(2S+1) = (2x3+1)(2x1+1) = 7x3 = 21 microstates. After assigning all of these 21
microstates, we are left with 15 microstates. Following the same method we can identify the other
terms of this configuration: 1D , 3P and 1S .
The same procedure can be used to determine the terms for different d-electron configurations, and
the resulting terms for each configuration are shown below:
d1 2D
d2 1S, 1D , 1G, 3P , 3F
d3 2D, 4P , 4F, 2P , 2D, 2F , 2G, 2H
d4 5D, 1S , 1D, 3P , 3F, 3P, 3D, 3F , 3G, 3H , 2S, 1D , 1F, 1G , 1I
d5 2D, 4P , 4F, 2P , 2D, 2F , 2G, 2H , 2S, 2D , 2F, 2G , 2I, 4D , 4G , 6S
d6 5D, 1S , 1D, 3P , 3F, 3P, 3D, 3F , 3G, 3H , 2S, 1D , 1F, 1G , 1I
d7 2D, 4P , 4F, 2P , 2D, 2F , 2G, 2H
d8 1S, 1D , 1G, 3P , 3F
d9 2D
E – Identification of the lowest energy term.
* Since all microstates belonging to the same term are degenerate, the electronic transitions that are
observed spectroscopically occur between terms.
It is important to identify the ground state term because this is the term from which electronic
transitions will occur. This can be done using two of Hund’s rules:
(1) The ground state term is the term with the highest value of S or (2S+1).
(2) If >1 term shares the highest value of S , the lowest energy term has the highest value of L
For the d2 configuration, the ground state term is 3F, and is located at the bottom left hand corner of
the Tanabe-Sugano diagram. With these rules we can identify the microstate with the highest ML and
MS in the ground state term for any electronic configuration (see below):
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
8
Configuration m l ML MS Ground
2 1 0 -1 -2 Term
d1 ? 2 1/2 2D
d2 ? ? 3 1 3F
d3 ? ? ? 3 3/2 4F
d4 ? ? ? ? 2 2 5D
d5 ? ? ? ? ? 0 5/2 6S
d6 ?? ? ? ? ? 2 2 5D
d7 ?? ?? ? ? ? 3 3/2 4F
d8 ?? ?? ?? ? ? 3 1 3F
d9 ?? ?? ?? ?? ? 2 1/2 2D
Note that these simple rules can only be used to identify the ground state term. In order to find the
energy of the higher energy terms, Schrödinger’s equation must be solved, which involves the
evaluation of many “2-electron” integrals. Racah worked out all of the integrals for a d2
configuration, and found that they can be classified into three sets called A, B and C. Even better, he
found that the energy of all terms can be written as simple combinations of these integrals. The
values of A, B and C can be computed, but are more frequently matched to spectroscopic results, and
are thus used as “parameters”. Of the three parameters, A is not meaningful when dealing with
differences in energy between terms rather than actual energies. In addition, it is found that in many
cases C ˜ 4B. This permits fairly simple interpretation of spectra using only parameter B, which is
called the principal Racah parameter.
F – Splitting of the free ion terms in coordination complexes.
The previous discussion applies only to metal ions in the gas phase. To describe the situation in a
coordination complex, we must consider the effects of a reduction in symmetry.
Using either Crystal Field Theory or Ligand Field Theory, confinement of a metal ion in an
octahedral environment results in a splitting of the energy into the t2g and eg* sets. For the
monoelectronic d1 system, there is only one possible electronic transition eg1 ß t2g
1 (see below). The 2D term corresponds to a d2 atom or ion in the gas phase, while the two possible electronic
configurations in an octahedral environment, t2g and eg* result in two terms 2Eg
* and 2T2g. Notice the
distinction of electronic configuration and spectroscopic terms using a different letter case. The
difference in energy between 2Eg* and 2T 2g is also called ∆o, and as ligand field strength increases, ∆o
increases.
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
9
2D
2Eg
2T2g
∆o0.6 ∆o
0.4 ∆o
Ion in the gas phase OhIncreasing Ligand
Field Strength
The figure shown above is called a correlation diagram, and indicates the relationship between the
free ion terms for a gas phase ion and an ion in an octahedra l environment (Oh). Correlation diagrams
depict the energy of the O h terms in relation to the parent free ion term.
For other dn configurations (n > 1), the reduction in symmetry also results in splitting of the terms.
This splitting is formally obtained using group theory, but for our purposes, it is enough to examine
the end result shown in the table below:
Free Ion Term Splitting under Oh symmetry
S à A1g
P à T1g
D à Eg + T2g
F à A2g + T1g + T2g
G à A1g + Eg + T1g + T2g
H à Eg + 2(T1g) + T2g
I à A1g + A2g + Eg + T1g + 2(T2g)
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
10 The correlation diagram below shows the splitting of the important triplet terms (3F and 3P) for the d2
configuration:
3F
3T2g
3T1g
Ion in the gas phase OhIncreasing Ligand
Field Strength
3P 3T1g
3A2g
∆o
∆o
∆o
approx∆o
t2g1eg
1
t2g2
eg2
Field strengths for actual complexes
Infinitely high field strength
The free d2 ion is shown on the left hand side of the diagram, while the right hand side of the
diagram shows the energy levels in an infinitely strong ligand field (not one that could be found in a
real complex). At this “strong field limit”, the effects of the ligands is so strong that it completely
overrides Russell-Saunders coupling, and we are left with the familiar electronic configurations of
t2g2, t2g
1eg1 and eg
2. At this strong field limit, ∆o can easily be marked on the diagram. In actual
compounds, the situation is intermediate between these two extremes. However, by looking at these
extremes, it can be seen that in the central region of the diagram, ∆o is represented by 3T2g ß 3T1g
and 3A2g ß 3T2g transitions. Note: the former of these transitions cannot be used reliably to find Do
because 3T1g does not vary linearly with ∆o (the line is curved – see later).
G - Tanabe-Sugano diagrams.
A Tanabe-Sugano diagram is a special type of correlation diagram that is particularly useful for the
interpretation of electronic spectra for transition metal complexes. In a Tanabe-Sugano diagram, the
ground state term is set as the base line and all other energies are referenced to this ground state. The
energy and ∆o are presented as multiples of the principal Racah parameter B, and energies are
calculated using a value of C (~ 4) which is reported in the diagram.
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
11 It is fortunate that we do not need to carry out detailed analysis for all cases. The Tanabe -Sugano
diagrams have been worked out for most dn configurations with common geometries (e.g. octahedral
and tetrahedral). Tanabe-Sugano diagrams for octahedral complexes are shown below.
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
12
Tanabe-Sugano Diagrams for Octahedral dn Complexes
2
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
13
Tetrahedral Complexes
The same octahedral Tanabe-Sugano diagrams can be used for tetrahedral complexes. However, for a
dn tetrahedral complex, the d(10-n) Tanabe-Sugano diagram must be used (i.e. for d8 Td, use the d2 Oh
T-S diagram). For more details, see Miessler and Tarr, Inorganic Chemistry, 3rd Edition, pg 406.
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
14
Some important general observations from the Tanabe -Sugano diagrams are:
(1) Some lines are curved. This arises because whenever two states of the same symmetry (same
term symbol) are close in energy, they can be combined to yield one state of lower energy and
another of higher energy. In a Tanabe-Sugano diagram, this makes it appear that some lines bend
away from each other to avoid crossing. As a result, this phenomenon is referred to as the “non-
crossing rule”. Note that transitions involving these curved lines must be avoided when determining
∆o from Tanabe-Sugano diagrams.
(2) For some configurations, there is an abrupt change of slope and the ground state term also
changes. This happens when there is a transition from a High-Spin to a Low -Spin state.
H – Selection rules and molar absorptivity.
There are two selection rules which govern the relative intensities of electronic absorption bands:
Spin Selection Rule – Transitions between different spin multiplicities are forbidden. For example,
transitions between 4A2 and 4T1 states are spin-allowed, but between 4A2 and 2A2 are spin-forbidden.
Laporte (Parity) Selection Rule – Transitions between states of the same parity (symmetry with
respect to inversion) are forbidden. For example, transitions between d-orbitals are forbidden (these
are g à g transitions) but transitions between d and p orbitals are allowed (g à u).
These rules would seem to rule out many of the electronic transitions for metal complexes (see
Tanabe-Sugano diagrams). However, many transition metal complexes are famous for their bright
colours; a consequence of various mechanisms by which the above rules can be relaxed. The most
important relaxation mechanisms are:
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
15 * Vibronic Coupling (molecular vibrations ) temporarily changes molecular symmetry, and can result
in temporary loss of the centre of symmetry in an octahedral complex. This provides a way to relax
the Laporte selection rule. As a consequence, d-d transitions often have extinction coefficients (ε ) of
10 to 100 dm3 mol-1 cm-1 which is responsible for the bright colours of many transition metal
complexes.
* Tetrahedral C omplexes absorb more strongly than octahedral complexes. Metal-ligand σ-bonding
in transition metal complexes of Td symmetry, and the lack of a centre of symmetry makes
transitions between d-orbitals more allowed (relaxa tion of the Laporte selection rule ). As a result,
extinction coefficients of around ε = 500 dm3 mol-1 cm-1 are common. As a qualitative example,
[Ni(H 2O)6]2+ is pale green while [Ni(PPh3)4]2+ is deep blue.
* Spin-Orbit Coupling can provide a mechanism for relaxation of the spin selection rule . Such
absorption bands for first-row transition metal complexes are generally weak, with typical extinction
coefficients of around ε = 1 dm3 mol-1 cm-1. For example, octahedral Mn2+ complexes (d5) cannot
undergo any spin-allowed transitions from the 6A1 ground state, so are extremely pale coloured. By
contrast, Spin-Orbit coupling can be more important for 2nd and 3rd row transition metal complexes.
The molar absorptivity (ε , extinction coefficient) is a good indicator of the type of electronic
transition:
Type of transition ε (dm3 mol-1 cm-1) Example
Spin-forbidden < 1 Mn2+ (aq)
d-d Laporte-forbidden 10-100 Most 3d Oh complexes
d-d noncentrosymmetric system ~ 500 Td complexes
π*-π transition, symmetry allowed ~ 10,000 Organic chromophores
Note: while it is important to have an idea of what the different terms on a Tanabe-Sugano diagram
represent, it is most important to be able to use Tanabe-Sugano diagrams and molar absorptivity to
interpret and understand the electronic spectra of transition metal complexes (including finding ∆o or
∆t for a complex).
See annotations to TS diagrams made in lecture for finding ∆o spectroscopically.
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
16
CHARGE TRANSFER ABSORPTIONS [Cr(NH3)6]3+: there is an intense absorption in the UV region of the spectrum. This type of intense absorption (ε ~ 104 dm3 cm-1 mol-1) can be seen more clearly in [CrCl(NH3)5]2+:
Note: The lowest energy dßd transition in [CrCl(NH3)5]2+ is significantly lower in energy than that
of [Cr(NH3)6]3+ due to a lower ∆o (Cl- is lower in the spectrochemical series than NH3). Note: For [CrCl(NH3)5]2+, there are shoulders on the dßd transitions due to reduced symme try
(C4v instead of Oh). Both peaks are examples of LMCT (Ligand to Metal Charge Transfer), which involves the transfer of electrons from molecular orbitals that are primarily ligand based to orbitals that are primarily metal based.
MLCT = Transfer of electrons from the eg and t2g ligand sets to low lying π* orbitals on the ligands. LMCT = Transfer of electrons from filled ligand based orbitals to the eg and t2g sets.
[CrCl(NH3)5]2+ [Cr(NH3)6]3+
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
17 Other examples of complexes with prominent LMCT absorptions are shown below (relative energies of the LMCT absorptions are also shown using < and >). These are all d0 complexes, so cannot undergo dßd transitions:
3d 4d 5d
MnO4- < TcO4
- < ReO4-
M7+
^ ^ ^ CrO 4
2- < MoO42- < WO4
2- M6+
^ ^ ^
VO43- < NbO4
3- < TaO43- M5+
In complexes containing ligands which have low-lying empty π*-orbitals, MLCT (Metal to Ligand Charge Transfer) can be observed. This is often found for diimine type ligands (bipyridine, phenanthrolene) as well as a variety of other soft ligands such as S2C2R2
x-, S2CNR2-, CO, CN- and
SCN-. See molecular orbital diagram above. Examples of molecules which show strong MLCT bands are [M(bipy)3]2+ (M = Fe or Ru), [W(CO)4(phen)] and [Fe(CO)3(bipy)]. [Cr(CO)6] shows both LMCT and MLCT absorptions. Both MLCT and LMCT results in a dramatic redistribution of electron density between the metal and the ligands (hence the name “charge transfer”). This redistribution of electron density results in differences in the solvation for the ground state and excited state molecules. As a result, the energy of charge transfer bands is highly dependent on solvent (SOLVATOCHROMISM).
----------------------------------------------------------------
higher oxidation state metal is easier to reduce
3d easier to reduce
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
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LUMINESCENCE A complex is luminescent if it emits radiation after it has been electronically excited by the absorption of radiation. Luminescence competes with non-radiative decay by thermal dissipation of energy (heat loss to the surroundings). Luminescence can be divided into two categories: (1) Fluorescence à Radiative decay from an excited state with the same multiplicity as the ground
state. This process is fast (nanoseconds). (2) Phosphorescence à Radiative decay from an excited state with different multiplicity from the
ground state. This process is spin-forbidden, so is usually slow (microseconds to seconds). The mechanism of phosphorescence involves intersystem crossing (the non-radiative conversion of the initial excited state into another excited state of different multiplicity). This second excited state acts as an energy reservoir because radiative decay to the ground state is spin forbidden.
Ruby is an important example of a phosphorescent compound. Ruby absorbs green and violet light so appears red. Red phosphorescence helps to enhance the red colour and lustre of ruby (see below). Ruby = Al2O3 with partial replacement of Al3+ by Cr3+ - the Cr3+ ions are “dilute” and are surrounded by an octahedral arrangement of oxide ligands.
Molecular examples of phosphorescent complexes include [Cr(bipy)3]3+ and [Ru(bipy)3]3+. The first laser (1960) was based on ruby.
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Chemistry 3820 (Fall 2006)
© 2006 - Dr. Paul G. Hayes – University of Lethbridge
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LASERS
• A synthetic ruby rod is irradiated using a powerful xenon lamp (see below).
• The long lifetime of the metastable 2Eg level (~ 4 ms) means that with suitably intense irradiation, more than half of the chromium ions can be excited to the 2Eg level at one time. This is called a population inversion.
• Decay of these excited state chromium ions results in a photon at 694 nm.
• The ruby rod is housed between 2 mirrors (one fully reflecting and one only partially reflecting).
• Therefore, if the 694 nm photon is emitted in the correct direction, it bounces back and forward between the two mirrors. When it encounters another excited state Cr3+ ion, it can cause decay by ‘stimulated emission’. This stimulated emission results in production of a photon that relative to the photon that stimulated the emission, is at exactly the same wavelength, is in phase and is travelling in the same direction (i.e. the light produced is monochromatic, coherent and collimated).
• The two photons now move back and forwards between the two mirrors in the resonant cavity, which results in more stimulated emission. As a result, the intensity of the beam of photons increases with each pass through the cavity until it reaches a ste ady state (see figure below).
• Remember that one of the mirrors at the end of the resonant cavity is only partially reflecting. A portion of the coherent, collimated and monochromatic light is therefore able to escape through this output mirror, resulting in a laser beam (see below).
The figure on the left is an illustration of the gain or amplification that occurs with increased path length in the resonant cavity due to the mirrors at each end. Figure 1(a) shows the beginning of stimulated emission, which is amplified in Figure 1(b) through Figure 1(g) as the light is reflected from the mirrors positioned at the cavity ends. A portion of light passes through the partially reflected mirror on the right-hand side of the cavity (Figures 1(b, d, and f)) during each pass. Finally, at the equilibrium state (Figure 1(h)), the cavity is saturated with stimulated emission.
• Other common types of laser include: other solid state lasers (e.g. Yttrium Aluminium Garnet
(YAG) doped with Nd3+, Ho3+ or Er3+; YAG = Y3Al5O12), dye lasers (e.g. a dilute solution of rhodamine or fluorescein ), gas lasers (e.g. He/Ne, Ar, Ar/Kr, CO2) and semiconductor lasers (e.g. InGa/AlP). Solid state and dye lasers are generally excited using optical pumping. Gas lasers and semiconductor lasers generally use electrical pumping.