Uv/Vis Flashcards
(24 cards)
What is crystal field splitting?
descries the breaking of bond degeneracy in transition metal complexes
What are the three influencing factors of DeltaO? explain
identity of ligand -using the spectrochemical series CN, CO,PR3 give large DeltaO and a low spin complex Cl-,Br-,OH- give small DeltaO and high spin complexes
coordination number- octahedral vs tetrahedral, tetrahedral complexes have less ligands so less overlap of ligands and the metal centre by considering oxidation state and period of metal, larger O.S or larger radius(3d-4d-5d) sees increase in DeltaO
What are the three arrangements of electrons? which are which?
A,E or T ground state.
in A ground state there are no degenerate ways of rearranging electrons, in E ground state there are two degenerate ways to arrange electrons, in T ground state there are three degenerate ways to arrange electrons
What region of the spectrum is visible light?
wavelength between 400-800 nm
What does UV/Vis absorption spectroscopy tell us?
tells us where the light is absorbed and how much light is absorbed at a particular wavelength
What is the Beer-Lambert Law?
Absorbance=epsilon x conc x path length
why do transition metal complexes have colour?
arises from electron transitions between d-orbitals that become split in a crystal field. from a ground state to an excited state
What is epsilon in the Beer-Lambert law? what is the value range for epsilon?why?
epsilon is the extinction coefficient, which determines the intensity
the value range is 1-50,000 which is very large due to the selection rules or possibility of other types of electronic transitions(not only d-d)
what are the selection rules of UV/Vis transitions?
spin selection rule-during a transition, the spin of the electron cannot change
parity selection rule- electron must move from g to u orbital or vice versa
orbital selection rule- orbital angular momentum QN (L) must change by +-1
we see absoption in [Ti(OH2)6]3+ but the selection rules are not obeyed, why?
the bonds are vibrating slower than the electron transition happens so the octahedral complex may not be perfectly octahedral so the parity rule doesnt fully apply so a weak absorption is seen
What are the three other types of UV/Vis transitions
charge transfer transitions
-ligand based transitions of ligand HOMO excited to ligand LUMO
-Ligand-To-Metal Charge transfer where the ligand HOMO is of appropriate energy where an electron can be excited from HOMO to into metal D-orbitals (often with high O.S)
-Metal-To-Ligand Charge transfer where the ligand LUMO is of appropriate energy to accept an electron from the metal D-Orbitals (often with low O.S)
How to find multiplicity of an electronic term symbol
find S by the sum of all mS, mult=2S+1
with what electronic configurations does Orgel theory not work for and why?
orgel theory will not work for d10 or d5 configurations as they have no vacancies for transitions without a change of spin
What are Hunds rules?
- Maximise s
- for a given S, maximise L
- for a given S
What is the problem we bump into when a TM complex has multiple d-electron transitions?
most TM complexes have more than 1 d-electron transition which makes assignment of spectra harder as transitions are split by electron-electron repulsions
how do we combine crystal field and electron repulsion?
weak field approximation where e-e repulsion (B) is greater than DeltaO by only considering high spin complexes
you start with the free ion terms and consider energy changes due to interactions of ligands, these are the irreducible representations seen in group theory
How do you find the crystal field term?
1st determine oxidation state and dn configuration
2nd using expected geometry determine crystal field config
3rd determine degeneracy arrangements (A/E/T)
4th determine spin multiplicity
What can orgel diagrams be used for? what are the limitations?
used to assign transitions in a UV/Vis spectrum for a tetrahedral or octahedral complex assuming:
-considering high spin complexes only
-only spin-allowed transitions considered
-qualitative rather than quantitative
What are the differences between orgel diagrams and Tanabe-Sugano diagrams?
Tanabe-Sugano diagrams take into account if DeltaO is greater than B (e-e repulsion) which cant be accounted for in orgel diagrams, they also consider transitions for d5 high spin complexes which are spin forbidden, unlike orgel diagrams
How do you work out DeltaO from a Tanabe-Sugano diagram
1-determine O.S and dn count and ground state config in crystal field
2-locate relative T-S diagram
3-determine GS free ion and crystal field terms for metal in complex
4-evaluate spectroscopic info to assign transition to bands
5-calculate ratio between Vn/V1 assigned bands
6-use info on diagram to find corresponding value of DeltaO
What are some trends in electron-electron repulsion (B)
B tends to increase along the period and with charge as the ions get smaller
More polarisable and softer ligands tend to give lower B parameters
What is the nephelauxetic effect?
when the metal d-electrons delocalise onto the ligands as the metal-ligand bond is partially covalent
How is the nephelauxetic effect quantified?
usually by ratio B/Bfree = Beta
can be approximated by average environment by taking values of a given metal ion(hm) and ligands (Fligand)
1-Beta=hm x sum of Fligand
What factors need to be considered when interpreting TS Diagrams?
-spin-allowed vs spin-forbidden transitions
-energy of transition
-details of transition (probability)
-possibility of charge transfer transitions
