spectroscopy of complexes - ts diagrams, ct, and jt distortion

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


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).


Figure 3.3. An Orgel diagram for metal ions having a F spectroscopic ground state (House, 2008).


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


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).


The interelectronic repulsion is different for the various spectroscopic states in the ligand field due to spin-orbit coupling resulting into different energy values.


A = an average total interelectron repulsion

B and C = individual d-electron repulsions


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


Figure 3.6. Tanabe-Sugano diagram for d5 configuration (House, 2008).


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.


Step 2: Get the ratio of the two wavenumbers.

Obtained ratio from TS diagram:

1.098039216


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


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).

spectroscopy of complexes - ts diagrams, ct, and jt distortion

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