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Electronic spectra of metal complexes, Laporte rule, Orgal diagram, Tanabe-Sugano(TS) diagram, Term Symbol
Electronic spectroscopy arises due to the quantized nature of energy states of electrons in an atom or molecule. Given enough energy, an electron can be excited from its initial ground state to an excited state.
Electronic transitions involve exciting an electron from one principle quantum state to another in an atom or molecule.
Once it is in the excited state, it will relax back to it’s original more energetically stable state, and in the process, release energy as photons.
Consider a typical low energy electronic spectrum of a transition metal complex given below.
The spectrum shows as much as six peaks which need to be assigned and understood.
(A).”TERM SYMBOL”-(LINK-1 Below)
d-d Transition
The transitions in metal complexes appear as weak spectrum because they are Laporte forbidden.
The Laporte rule is a spectroscopic selection rule that only applies to centrosymmetric (those with an inversion centre) molecules and atoms.
It states that electronic transitions that conserve parity, either symmetry or antisymmetry with respect to an inversion centre — i.e., g (gerade = even → g, or u (ungerade) → u respectively are forbidden.
Allowed transitions in such molecules must involve a change in parity, either g → u or u → g. As a consequence, if a molecule is centrosymmetric, transitions within a given set of p or d orbitals (i.e., those that only involve a redistribution of electrons within a given sub-shell) are forbidden.
Due to vibronic coupling; however, they are weakly allowed. Many transition metal complexes are colored. The molar extinction coefficients for these transition hover around 100.Hence there is a need to understand the electronic state of the electrons as described the term symbol in LINK-1 given below
LINK-1: SP-1(A) Term Symbol
(B).”ORGEL DIAGRAM”-(LINK-2 below)
Orgel diagrams (given in LINK-2 below) are correlation diagrams showing the relative energies of electronic terms in transition metal complexes.
Orgel diagrams are restricted to only weak field (high spin) cases, and offer no information about strong field (low spin) cases.
LINK-2: SP-1(B) Orgel diagram
(C).”TANABE-SUGANO (TS) DIAGRAMS”-(LINK-3 Below)
An alternative method is to use Tanabe Sugano diagrams, which are able to predict the transition energies for both spin-allowed and spin-forbidden transitions, as well as for both strong field (low spin), and weak field (high spin) complexes.
From this spectra of octahedral chromium complex, the d-d transitions are far weaker than the LMCT. Since chlorine is a pi donor ligand, we can label the CT band as LMCT since we know the electron is transitioning from a MO of ligand character to a MO of metal character.
The Laporte forbidden (symmetry forbidden) d-d transitions are shown as less intense since they are only allowed via vibronic coupling.
In addition, the d-d transitions are lower in energy than the CT band because of the smaller energy gap between the t2g and eg in octahedral complexes (or eg to t2g in tetrahedral complexes) than the energy gap between the ground and excited states of the charge transfer band.
We will use the [CrCl(NH3)5]2+; d3-ion as an example for determining the types of transitions that are spin allowed.
The Tanabe-Sugano(TS) diagrams for transition many metal complexes can be a guide for determining which transitions are seen in the spectrum as given in LINK-3.
LINK-3: SP-1(C) TS Diagram
(D).”TS DIAGRAM-color format”(LINK-4 Below)
The Tanabe-Sugano diagrams for transition many metal complexes in LINK-3 are repeated in different color for more clarity in the LINK-4 given below .
LINK-4: SP-1(D) TS-Diagram-Color
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