Studying the Spectrochemical Series: Crystal Fields of Cr(III)

1. 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

2. The d-Orbitals

3. Octahedral Crystal Field Splitting eg Do t2g Magnitude of Do: strength of interaction with ligands

4. Spectrochemical Series Halides < OH- < C2O42- < H2O < NCS- < py < NH3 < en < NO2- < CN- < 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

5. Tetrahedral Crystal Field Splitting DT DT ≈ 4/9 DO All Tetrahedral Complexes Are High-Spin

6. 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?

7. Tanabe-Sugano Diagram

8. 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

9. Rule of Average Environments For mixed-ligand compounds: DoMAnBm = (1/6) {nDoMA6 + mDoMB6} Ex. True ligand-field strength of NH3: Do of [Cr(NH3)6]+3 Do[Cr(NH3)5Cl]+2= 1/6 {5 Do[Cr(NH3)6]+3+ Do[CrCl6]-3} known known

10. Summary --Determine Do’s for octahedral Cr+3 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.