Studying the Spectrochemical Series: Crystal Fields of CrIII

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Studying the Spectrochemical Series Crystal
Fields of Cr(III)
Objectives --For a series of Cr(III) compounds
calculate crystal field splitting energies
using electronic spectroscopy --Find true
ligand-field strength of five ligands --Compare
to qualitative predictions of spectrochemical
series
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The d-Orbitals
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Octahedral Crystal Field Splitting
eg
Do
t2g
Magnitude of Do strength of interaction with
ligands
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• Spectrochemical Series
• Halides lt OH- lt C2O42- lt H2O lt NCS- lt py lt NH3 lt
  en lt NO2- lt CN- lt CO

Weak Field Ligands Strong Field Ligands Weak
Metal Interactions Strong Metal
Interactions Small Do Large Do
High-Spin Complexes
Low-Spin Complexes
Qualitative Theory on Magnitude of Do
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Tetrahedral Crystal Field Splitting
DT
DT 4/9 DO
All Tetrahedral Complexes Are High-Spin
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Electronic Spectroscopy
--Quantitative Measure of Do
--Energy needed to promote one electron from t2g
to eg is Do
--Measure magnitude of Do by wavelength at which
electronic transitions occur
Which electron promotions occur? Which
corresponds to energy of Do?
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Tanabe-Sugano Diagram
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Absorbance at Least Energy Wavelength
Proportional to Do
1) Longest Wavelength, lmax
2) Convert l units (nm) to energy units, find Do
3) Arrange five ligands in order of ligand field
splitting strength acac-, en, H2O, Cl-,
NH3
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Rule of Average Environments
For mixed-ligand compounds DoMAnBm (1/6)
nDoMA6 mDoMB6
Ex. True ligand-field strength of NH3 Do of
Cr(NH3)63
DoCr(NH3)5Cl2 1/6 5 DoCr(NH3)63
DoCrCl6-3 known known
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Summary
--Determine Dos for octahedral Cr3 complexes by
electronic spectroscopy Find true ligand
field strength for five ligands.
--Compare experimental ligand-field strength with
that predicted by the spectrochemical series.
---Use ligand-field strength of ligands to
control/predict color, energy of electronic
transitions, geometry of complexes, magnetism.