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EXERCISE 3 – SPECTROSCOPY OF COMPLEXES
Correlation diagramsCharge Transfers
Jahn-Teller Distortion
Correlation Diagram - Definition
Relates the electronic spectra of transition metal complexes to the ligand field splitting for inorganic complexes.
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Figure 3.1. A correlation diagram for d2 in an octahedral ligand field (Miessler and Tarr, 2000).
Correlation Diagrams - Characteristics
The free-ion states are shown on the far left.
The extremely strong-field states are shown on the far right.
Both the free-ion and strong-field states can be reduced to irreducible representations.
Correlation Diagrams – Irreducible Representations
Free ion terms as well as the strong-field states can be expressed using irreducible representations
Irreducible representation entails degeneracy and the symmetry of the terms
Correlation Diagrams – Irreducible Representations
Correlation Diagrams – Irreducible Representations
Each free-ion irreducible representation is matched or correlated to a strong-field irreducible representation having the same symmetry (same label).
Correlation Diagram - Characteristics
States are shown in order of energy
Lines connecting states of the same symmetry designation do not cross; also known as the non-crossing rule
Correlation Diagrams – Noncrossing rule
Noncrossing rule: Strong interaction and repulsion of states having the same representations.
Orgel Diagram - Definition A correlation diagram developed by
Leslie E. Orgel
Illustrates energetic effects of the loss of degeneracy generated by symmetry lowering
A tool in studying splitting patterns of spectroscopic states
Figure 3.2. An Orgel diagram for metal ions having a D spectroscopic ground state (House, 2008).
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Figure 3.3. An Orgel diagram for metal ions having a F spectroscopic ground state (House, 2008).
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Orgel Diagrams – Basis
Hole Formalism
dn (n < 5), OCT d2
Same as d(5+n), OCT Same as d7, OCT
Opposite d(10-n), OCT Opposite as d3, OCT
Orgel Diagrams - BasisTable 3.1. States of different dn configurations (House, 2008).
Orgel Diagrams – Characteristics
Show the symmetry states of the highest spin multiplicity
Show the number of spin allowed transitions, along with their respective symmetry designations
Tanabe-Sugano Diagram - Definition
Developed by Yukito Tanabe and Satoru Sugano
Used Hans Bethes' crystal field theory and Giulio Racah's linear combinations of Slater integrals, now called Racah parameters
Plots of the energies calculated for the electronic states of each electron configuration based on spectral energies and Δ
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Figure 3.4. A Tanabe-Sugano diagram for d2 metal ion in an octahedral field (House, 2008)
Tanabe-Sugano Diagrams - Anatomy
x-axis = ligand field splitting parameter y-axis = in terms of energy with B One line corresponds to each electronic
state, which is given by a symmetry label (all states are gerade)
Term symbols are included in increasing energy wherein the energy is determined by Hund’s rule
Tanabe-Sugano Diagrams – Rise of Racah parameters
There is a maximum of three spin-allowed transitions regardless of the dn configuration of the metal ion.
Tanabe-Sugano Diagrams – Rise of Racah parametersTable 3.2. Allowed transitions for octahedral and tetrahedral complexes (House, 2008).
Tanabe-Sugano Diagrams – Rise of Racah parameters
The interelectronic repulsion is different for the various spectroscopic states in the ligand field due to spin-orbit coupling resulting into different energy values.
Tanabe-Sugano Diagrams – Rise of Racah parameters
A = an average total interelectron repulsion
B and C = individual d-electron repulsions
Tanabe-Sugano Diagrams – Rise of Racah parameters
Usual values of the Racah parameter, B (free ion).
Metals in the first row:
+2 oxidation state = 700 – 1000 cm-1
+3 oxidation state = 850 – 1200 cm-1
Metals in the second and third row:
600 – 800 cm-1
Tanabe-Sugano Diagrams - Nephelauxetic Effect B is a function of the ligands which
surround the metal ion, and that is always lower in a complex than in the free metal ion due to more delocalized electrons in the complex than the free ion
Results to spreading out of electron cloud Decrease of energy required for pairing
depends on the ligand Lower energy, greater covalency
Tanabe-Sugano Diagrams – Nephelauxetic Series
Figure 3.5. The nephelauxetic series of ligands (Constable, 1996)
Figure 3.6. The nephelauxetic series of metals (Housecroft, 2005)
Tanabe-Sugano Diagrams – Nephelauxetic SeriesFigure 3.3. The nephelauxetic parameters of ligands and metals (House, 2008)
Tanabe-Sugano Diagrams – Basis of Nephelauxetic Series
The change in B from the free ion value is expressed as the nephelauxetic ratio, β , which is given by:
β = B’/B
Where: β = nephelauxetic ratio
B = Racah parameter of the metal ion
B’ = Racah parameter of the metal ion in the complex
Tanabe-Sugano Diagrams - Observations
Some TS diagrams (d4, d5, d6, and d7) have a vertical line drawn at a specific Dq/B value.
Left side: high-spin configuration
Right side: low-spin configuration
Tanabe-Sugano Diagrams - Observations
Figure 3.6. Tanabe-Sugano diagram for d5 configuration (House, 2008).
Tanabe-Sugano Diagrams - Observations
NO Tanabe-Sugano diagram for d1, d9, d10!!!
Tanabe-Sugano Diagrams – Observations d1
no electron repulsion in a d1 complex!!single absorption band
d9
same with d1 configuration (hole formalism) d10
no d-d interactions due to completely filled orbitals
Tanabe-Sugano Diagrams – Calculation of Energy
Figure 3.7. Absorption spectra of [Ni(H2O)6]2+.
Wavenumber, m-1 (x106)
Transition
2.5348521246 3A2g → 3T2g
1.522070015 3A2g → 3T1g
1.386001386 3A2g → 3T1g(P)
STEP 1: Assign transitions based from the obtained wavenumbers.
Tanabe-Sugano Diagrams – Calculation of Energy
Step 2: Get the ratio of the two wavenumbers.
Obtained ratio from TS diagram:
1.098039216
Tanabe-Sugano Diagrams – Calculation of Energy
Step 3: Determine the corresponding E/B from the Tanabe-Sugano diagram
Step 4: Obtain the B value
Step 5: Determine the corresponding ∆o
from the Tanabe-Sugano diagram
Step 6: Obtain the value of ∆o
Tanabe-Sugano Diagrams – Calculation of Energy
B = 16305.89866
Orgel vs. Tanabe-Sugano Diagram
Orgel Diagram Tanabe-Sugano Diagram
No calculation of splitting energy
Spin-forbidden terms are not shown
Limited to high-spin complexes only
There is calculation of splitting energy via Racah parameters
Spin-forbidden terms are shown
Applicable to both high-spin and low-spin complexes
Charge Transfers – Definition Movement of electron density from metal
orbitals to ligand orbitals and vice versa
Capable of masking d-d transitions in spectrum due to formation of intense bands
Very intense signals due to spin-allowed transitions
Usually observed in the UV region and sometimes in the visible region
Charge Transfer Complex Also known as electron-donor-acceptor
complex Result of an electron transition Association of two or more molecules
such that a fraction of electronic charge is transferred between the molecular entities
Produces an unstable chemical bond
Charge Transfers - Detection Color
reflective of the relative energy balance resulting from the transfer of electronic charge from donor to acceptor.
Solvatochromismchange in energy (and color) as the solvent
polarity is varied Intensity
highly intense since transitions are both Laporte and spin allowed
Charge Transfers – Types
Ligand-to-Metal Charge Transfer (LMCT) arises from transfer of electrons from
MO with ligand like character to those with metal like character
transfer is predominant if complexes have ligands with relatively high energy lone pairs (or if the metal has low lying empty orbitals
halides promotes LMCT
Charge Transfers - LMCT
-donor ligands in a complex stabilizes the t2g level and destabilizes the t2g* resulting to a decrease in the splitting energy
Examples:
KMnO4
K2Cr2O7
complexes having O2-, S2-, and halide ions as ligands
Charge Transfers – LMCT
Figure 3.8. Illustration of LMCT (Miessler and Tarr, 2000).
Charge Transfer – Types
Metal-to-Ligand Charge Transfer (MLCT) arises from transfer of electrons from
MO with metal like character to those with ligand like character
transfer is predominant if complexes have ligands that are
Charge Transfer – MLCT
Metal-ligand interaction leads to the modification of the splitting energy since it now involves the t2g and eg* levels (greater splitting energy)
Examples:
complexes having ligands CO, CN-, SCN-, bipyridine, and
dithiocarbamate (S2CNR2-)
Charge Transfers – MLCT
Figure 3.9. Illustration of MLCT (Miessler and Tarr, 2000).
Jahn-Teller Distortion – Definition
In 1937, Jahn and Teller showed that if a nonlinear molecule having an unequally populated degenerate orbitals, the electronic state should distort to lower the symmetry of the molecule and to reduce the degeneracy
Distortion leads to lower energy Can be in the form of elongation or
compression
Jahn-Teller Distortion
Figure 3.10. Energy of the d orbitals of a d9 ion in a field with z elongation produced by Jahn-Teller distortion (House, 2008).
dn configuration Jahn-Teller distortion(+)-present (-) absent
d1 +d2 High spin + Low spin +
d3 High spin - Low spin -
d4 High spin + Low spin +
d5 High spin - Low spin -
d6 High spin + Low spin -
d7 High spin + Low spin +
d8 -d9 +
Table 3.4. Configurations exhibiting Jahn-Teller distortion.
Jahn-Teller Distortion - Manifestation
shouldering in a peak or separation of a peak into two bands due to a reduction in symmetry
Examples:
[Ti(H2O)6]3+ and [Cu(H2O)6]2+
Jahn-Teller Distortion - Manifestation
Figure 3.11. Absorption spectrum of [Ti(H2O)6]2+ exhibiting Jahn-Teller distortion (House, 2008).
END